roofing in sudan

8/3/2019 Roofing in Sudan

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RoofingSystems in Sudan

Doctor E.A.Adam

in collaboration with

Professor A.R.A.Agib

United Nations Educational, Scientific and Cultural Organization


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RoofingSystems in Sudan

Doctor E.A.Adamin collaboration with

Professor A.R.A.Agib


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The authors are responsible for the choice and the presentation of the facts

contained in this book and for the opinions expressed therein, which are not

necessarily those of UNESCO and do not commit the Organization.

The designations employed and the presentation of material throughout this

publication do not imply the expression of any opinion whatsoever on the part

of UNESCO concerning legal status of any country, territory, city or area or of its

authorities, or concerning the delimitation of its frontiers or boundaries.

M. Asghar Husain : Director of the Division for Educational Policies and Strategies

Alfeo Tonellotto : Chief of the Architecture for Education Section

Authors : Dr. E. A. Adam in collaboration with Prof. A. R. A. Agib

Text editing and graphic design : Barbara Brink

Printed by : Graphoprintfor the

United Nations Educational, Scientific and Cultural Organization

7 Place de Fontenoy, 75352 Paris 07 SP, France

Paris June 2002

UNESCO


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PrPrefefaceaceThis publication has been prepared within the framework of the UNESCO project

Improvement of Educational Facilities in the Least Developed Countries of the Arab

States, 522/RAB/11.

Co-financed by AGFUND and AFESD, the main component of the project was to

construct the El Haj Yousif School in the Khartoum area using compressed

stabilised earth block building technology, a practical way of demonstrating the

potential of earth as a viable and desirable construction material.

In order to make the knowledge acquired during the construction process of El Haj

Yousif school widely available, this publication Roofing Systems in Sudanand a twin

publication Compressed Stabilised Earth Block Manufacture in Sudanhave beenprepared.

The roof is an essential part of any building. It is both an esthetic and structural

element, the shape and materials used determine the style of the entire

construction. Its main function, however, is to enclose a given space and provide

protection of this space from the weather. A weather tight roof is essential for

preserving the structure of the entire building. If the roof is badly constructed with

inappropriate poor quality materials, heat loss or gain will result as well as water

penetration and consequently the rest of the building will suffer damage.

In general a roof should provide adequate strength and stability, weather resistance,

thermal insulation, fire resistance and sound insulation. This can be achieved in a

number of ways depending on availability of materials and expertise.

In recent years the potential for earth as a valuable and desirable construction

material is being rediscovered. Methods derived from the traditional techniques are

being developed to improve the quality of earth construction and broaden the

potential for its application. Earth is primarily used for the construction of walls but

it can also be used for the construction of roofs where its good insulation

properties are particularly advantageous, especially in hot dry climates.

PREFACE


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CHAPTER 1 - Introduction1.1 Why the Roof is so Important

1.2 Different Roof Systems used in Sudan

1.3 Choosing a Roof System

CHAPTER 2 - Flat Roofs2.1 Traditional Earth Roof

2.2 Improved Traditional Earth Roof

2.3 Shagig Roof

2.4 Timber Board Roof

2.5 Precast Concrete Element Roof

CHAPTER 3 - Truss and Lean-to Roofs3.1 Truss and Lean-to Roofs

3.2 Thatched Roof

3.3 Fibre Reinforced Roof

CHAPTER 4 - Vaults4.1 Vault Roofs

4.2 Jack Arch Roofs

CHAPTER 5 - Domes5.1 Thatched Domes

5.2 Brick Domes

CHAPTER 6 - Comparative Costs6.1 Cost Comparison of Various Types of Roofs Used in Sudan

CHAPTER 7 - Some Conclusions7.1 The Roof - weighing up the options

GLOSSARY

1

3

3

3

5

7

8

10

11

12

15

17

18

20

23

25

26

29

31

32

33

35

3739

41

CONTENTS


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C

HAPTER

Introduction

1


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1.11.1 WhWhy the Roof is so Impory the Roof is so ImportanttantThe roof is one of the most essential components of a building. Its most important

function is to protect the rest of the building from the weather. Without a roof a

building not only loses its esthetic identity but the structure of the building willrapidly deteriorate with the inevitable exposure to the elements - wind, rain, snow

and sun.

1.2 Diff1.2 Differerent Roof Systems used in Sudanent Roof Systems used in SudanRoofs commonly used in Sudan may be classified in four main categories:

Flat roofs,

Truss and lean-to roofs,

Vaults,

Domes.

Selected types of roof systems of the four main categories commonly used in

Sudan are described in this publication. Examples are given of some of the recent

developments made in roof construction, designed to improve the performance of

traditional systems, concentrating in particular on structural rather than thermal

improvements.

1.3 Choosing a Roof System1.3 Choosing a Roof SystemAs a general rule the choice of which type of roof to use is governed by: Availability of materials,

Economic feasibility,

Climatic performance,

Ease of construction.

These criteria are closely related to the main functional properties that a roof

should meet i.e.:

Structural resistance to loads and stresses,

Provision of thermal comfort,

Effective protection against weather, i.e. rain and wind,

Fire resistance,

Sound insulation.

The types of roofs most commonly used in Sudan are flat roofs or jack arch roofs,

mostly used in areas where the rainfall is low or medium. Pitched roofs are

frequently used in areas where rainfall is heavy as they are effective at sheddingrainwater without any damage to the roof structure.

CHAPTER

Introduction

1

3


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C

HAPTER

Flat Roofs

2


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CHAPTER

Flat Roofs

2

7

2.12.1 TTrraditional Earaditional Earth Roofth RoofTraditional earth roofs are still used in various

parts of Sudan, especially in areas of low to

medium rainfall. They are popular because of their low cost, ease of construction, and high

resistance to heat penetration. Construction

materials required to build such roofs are also

readily available.

The traditional earth roof can be laid directly on

top of earth walls. The roof for a room of about

4 x 4 metres is constructed as follows:

A central timber joist (mirig), about 180 mm indiameter, is laid on the walls across the

centre of the space to support the whole

roof structure,

Round wooden rafters, 100 - 120 mm in

diameter, or rectangular timber rafters (100 x

40 mm) are laid on the walls over the mirig

at 500 mm centres. They are laid at a slight

incline to facilitate water runoff,

Reeds (hassir) are laid over the rafters and

fixed firmly with thatch rope,

Dry thatch or hay (humra) is laid over the

reeds to give more insulation,

Stabilised earth is laid over the hay cushion to

an average thickness of 100 mm. The dry

thatch or hay provides protection against

earth penetration through the sofit of the

roof,

Rainwater spouts are embedded in the

stabilised earth cover,

Upstanding parapet walls are built on all

sides,

After the earth layer is completely dry the

surface is plastered withzibla, a mixture of

earth, straw, animal dung and water, cured

for about 3 - 4 days. Thezibla is thenmixed thoroughly into a workable slurry and

used to plaster the roof surface in a layer

zibla

stabilised earththatch or hay(humra)

reeds (hassir)

mirig

stabilised earth

thatch or hay(humra)

reed (hassir)

mirig 100 mm

rafter

rafter

500mm


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CHAPTER

Flat Roofs

2

8

about 20 mm thick. This plaster serves as a

good sealant against water penetration and

with proper treatment of the parapet wall,

under and around the rain water outlet, this

plaster can last for about two rainy seasons

without a need for maintenance. After two

years, however, a new plaster coat needs to

be applied.

Calculation of heat flow through traditional earth

roofs shows that the internal surface temperature

reaches a peak of about 34C - 35C. This

corresponds with the lower temperature limitrecommended for thermal comfort. These peak

temperatures occur very late in the afternoon, at

about 17.00 hours, when the dry bulb

temperature has dropped considerably and

people are normally outdoors.

This type of roof construction has an initial low

capital cost, affordable for low-income groups.

However, the cost of frequent maintenance i.e.

zibla plastering about once every two years and

repair to internal wall plaster affected by possible

rain water leakage, can raise the long term costs.

2.2 Impr2.2 ImproovedvedTTrraditional Earaditional EarththRoofRoof

Attempts by local builders and engineers have

been made to improve the structural, thermal

and water resistance properties of the traditional

earth roof. These improvements are described

below:

The main roof structure is constructed of local

round wooden sections with a diameter of

approximately 120 mm, or alternatively using

wooden sections, 100 x 50 mm, laid at 500 mm

centres. These are supported on a central roundwooden section of 180 mm diameter, or a rolled

steel joist, 140 x 60 mm, and laid to a fall a little


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Flat Roofs

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9

steeper than the fall of the traditional earth roof,

to facilitate water runoff. In a few cases, imported

timber rafters 100 x 100 mm are used in place of

local timber.

The roof cover is laid as follows:

Bamboo poles approximately 25 - 30 mm

diameter are laid adjacent to each other,

covering the whole of the roof area in a

solid mat,

Reed (hassir) mats are laid over the

bamboo mat,

A plastic lateritic soil mixed with hay and

cured for approximately 4 days is applied

over the reed mat in a layer approximately

70 mm thick and left to dry completely. The

reed matting provides protection against

mud penetration through the sofit of the

roof.

Any cracks appearing in the surface of the

lateritic soil coat are filled with very fine

sand,

The surface is sprayed with water until

slightly wet and plastered with a mix of soil

lime cement (8:1:1) in a smooth finish right

up to the parapet up-stand and under and

around the rainwater outlet.

For more effective and durable protection

against rainwater penetration the surface may be

further sealed by the application of two coats of

flinkote. Flinkote is not available in Sudan and

consequently needs to be imported thus raising

costs.

Some alternative improved roofing materials and

methods have also been developed locally

through a research project sponsored by local

research communities. Compressed stabilisedearth bricks have been proposed as alternative to

the stabilised soil mix.

reed (hassir) mat

sand lime cement plaster

plastic lateritic soil

bamboo

RSJ

reed mat

gutter

1:80

sand lime cement plaster

plastic lateritic soil

bamboo

RSJ


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Flat Roofs

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10

The improved traditional earth roof has been

found to be far more effective than the

traditional earth roof. It has a higher resistance to

rain water penetration due to the steeper slope

of the roof and the soil cement lime plaster. In

addition heat insulation properties are slightly

better than the traditional earth roof.

Due, primarily to additional materials and the

slightly more complex structure the cost is about

50% more than the cost of the traditional mud

roof. This increase can be well justified,

however, by the relative improvement in

performance.

2.3 Shagig Roof2.3 Shagig RoofPalm purlins (shagig) are used in the construction

of the shagig roof. This type of roof has been

used successfully in eastern Sudan where palm

trees and consequently palm purlins are available

in abundance. A shagigroof is constructed in the

following way:

Palm purlins, 75 x 100 mm and 4 to 5

metres long are laid across the tops of the

walls at 400 mm centres,

After the shagigare arranged, special earth

blocks (400 x 200 x 120 mm) are laid flat on

top of the supporting shagigpurlins,

Earth or a mixture of soil/lime mortar is

used to fill the gaps between the blocks,

A layer of soil or soil/lime screed,approximately 80 mm thick, is applied on

the top of the blocks. It is laid to a fall to

allow water to drain from the roof,

The roof is finished externally withzibla and

white wash.Zibla is mixture of earth and

animal dung used to protect the exterior of

earth buildings. Internally, the blocks are

rendered with a soil/lime mortar and

finished with a lime wash giving an excellent

internal finish.

earth or soil:lime mortar

palm purlins (shagig)

earth blocks

screed

zibla

earth orsoil:lime mortar

palm purlins(shagig)

RSJ

earthblocks

screed

zibla


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11

2.42.4 Timber BoarTimber Board Roofd RoofThis roof type was very popular in most parts of

Sudan between the 1940s and 1960s. At this

time the cost of imported timber was muchlower and affordable to most income groups.

Nowadays it is very rarely used because of the

high cost of timber. The timber board roof is

normally used on earth or brick constructions:

The top 300 mm of the supporting walls is

usually built of fired clay bricks laid with an

earth or cement mortar,

A rolled steel joist (RSJ) spans across the

centre of the space (when the roof span is

over 4 metres) to support the whole roof

structure,

100 x 100 mm timber rafters are laid over

the RSJ and the walls at approximately

400 mm centres,

Timber boards 230 x 12 mm, planed and

chamfered on both edges to ensure a firm

overlap between adjacent boards, are laidover and nailed firmly to the rafters covering

the full area of the roof. They should

overhang the inside edge of all four walls by

about 50 mm. The rafters and boards are

laid to a fall of about 1:80 to ensure

rainwater runoff,

The boards are covered with thatch mats

(bursh orhassir),

Common fired clay bricks are laid flat on a

layer of earth mortar over the whole area

of the roof,

The bricks are plastered with sand cement

mortar (8:1) and finished smooth to ensure

satisfactory rainwater runoff,

Rainwater is shed through tin or fibre

cement spouts fixed over the brick paving

and projected about 300 mm over theedges of the walls,

zibla or sand cement plaster

rafters RSJ

thatch mats

clay bricks

sand cement plaster

timber boards

timberboards

thatch mats

clay bricks

sand cement plaster

rafter

RSJ

timber boards

thatchmats

clay bricks

sand cementplaster

rafters

RSJ


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12

A parapet wall is built on all sides of the

roof with a minimum height of 300 mm

above the roof surface.

Water is successfully shed off timber board roofs

of this type and consequently these roof

structures have good resistance to rainwater

penetration.

Heat insulation properties of the timber board

roof are very good because of the thickness of

the roof structure and the materials used i.e.

earth, thatch, and timber, which are all materials

with a high heat absorption capacity. The capital

costs, however, are rather high compared with

other traditional roof structures, such as the

traditional or the improved traditional earth

roofs which both have similar heat insulation

properties.

2.5 Pr2.5 Precast Concrecast ConcreteeteElement RoofElement RoofThis type of roof was introduced by the Council

of Scientific and Technological Research in the

early 1980s and has been used for the

construction of some of their office buildings.

Consultants, ALAGIB Group, used this type

roofing technique for the construction of 50

houses in a housing scheme sponsored by the

National Social Security Fund. The cost of this

roof is about 70%. of the cost of traditional

reinforced roofs.

The roof is constructed as follow:

The slope of the tops of two parallel and

opposite walls which will carry the concrete

joists are finished with a longitudinal fall of

about 1:80,

precast or reinforced joists

precast concrete cover slab

khafgior stabilised soil cement

zibla or sand cement plaster


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13

Pre-cast reinforced concrete joists, 300 mm x

200 mm, and 4.5 metres long for a

maximum span of 4 metres are laid at 1.3

metre centres following the slope of theroof. The joists are reinforced with three 16 mm

bottom, two 12 mm top and 6 mm stirrups

at 200 mm centres,

Precast concrete cover slabs, 300 mm wide

x 7 mm thick x 1400 mm long, reinforced

with 3 x 8 mm bars lengthwise, and 8 x 8 mm

bars width wise are placed on top of the

joists butting against each other with a

rebated joint. The joints between the

rebated ends are sealed with a 6:1 sand

cement mortar,

The reinforced slab is covered bykhafgi to

an average thickness of 7 mm (see khafgi

preparation on page 27), or with a special

stabilised soil cement laid to an

average thickness of 7 mm,

This type of roof has good thermal and

insulation properties as a result of the stabilised

soil orkhafgi but its thermal properties are still

slightly lower than that of improved traditional

roof. In terms of cost this roof is almost twice as

expensive as the improved traditional earth roof,

and about 65-70% of the cost of a solid

reinforced concrete roof.

precast or reinforced joists

precast concrete cover slab

khafgior stabilisedsoil cement

zibla or sandcement plaster


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C

HAPTERTruss and

Lean-to Roofs

3


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CHAPTER

Truss and

Lean-to Roofs

3

17

3.13.1 TTruss and Lean-to Roofsruss and Lean-to RoofsTruss and lean-to roofs are capable of shedding

rainwater very efficiently off the roof and away

from the walls. Roofs like these have been usedin many regions of Sudan especially in the south

where rainfall can be heavy.

Generally the truss is made of timber or steel. In

some parts of the country large size bamboo

with a diameter of between 75 mm and 100 mm

is used.

The trusses are constructed on the ground and then lifted and fixed on top of the supporting

walls, spaced at about 2.5 - 3.0 metre centres

depending on the span and length of the space

to be roofed. The truss is fixed firmly to the

walls starting at the gable ends. These are

temporarily supported using long ties that rest on

the floor or walls. Purlins are then fixed to span

between the trusses at about 1.0 - 1.5 metre

centres depending on the material and weight ofthe roof covering. For example, if zinc sheets are

used as the covering material the spacing can be

as large as 1.5 metres while thatch requires closer

spacing of approximately 0.3 metres.

The most common truss roof covering used in

Sudan is zinc or fibre-cement sheeting.

Lean-to roofs have similar properties to those of

trusses i.e. they are good at shedding water off

the roof and away from the walls.

Lean-to roofs can be built with pitches varying

between 30 and 45. They are triangular in

elevation with a rise of the inclined surface of up

to 1/4 of the span. The roof cover can be of zinc

or fibre-cement sheets or timber boarding

covered with rubberoid sheets which serve as aninsulating material and seal the timber surface

against rain water penetration.


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Lean-to Roofs

3

18

Trusses are fixed on the walls by anchoring to a

continuous wall tie beam of timber or concrete.

Where a timber beam is used it must be fixed

firmly to the wall. The truss tie beam is then fixed

firmly to the timber beam using steel bolts or

steel tie straps. When a concrete wall tie beam

is used the truss is firmly fixed to steel straps

which are in turn firmly embedded in the

concrete beam. The efficiency of this roof type

depends on the pitch angle and the roof cover,

the materials used and the construction method.

Roof covering materials typically used for trussand lean-to roofs in Sudan are:

Corrugated zinc sheets,

Corrugated fibre-cement sheets,

Timber boarding,

Thatch,

Fired clay tiles.

3.23.2 ThatcThatched Roofhed RoofThatch is a very effective roof cover often used

in various parts of Sudan, especially in southern,

western and eastern areas of the country where

there is an abundance of ideal materials available,

and where local labour is skilled in this type of

roofing technique.

The roof slope of thatched roofs varies according

to the needs as well as the weight of the material

being used. The slope should always be steep

enough so that the rainwater can run off from

the roof surface with minimum penetration into

the thatch. To ensure this, the thatch is normally

laid on timber trusses or timber pyramids with a

pitch of at least 45. The weight and properties

of the finished thatch varies according to the type

and thickness of the material used. Newly cut

thatch may be rather heavy as it is still wet and

partially green. It is therefore always better to use

bamboo poles

thatch covering- slope at least 45

wall tie beam


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Lean-to Roofs

3

19

materials that have been fully dried out. A

thicker layer of thatch also adds to the weight but

improves the sealing and insulation properties.

Thatch Construction

Grass is collected from the fields and cleaned,

removing all leaves, roots and other foreign

materials. It is then laid on the roof structure,

usually a timber truss or timber pyramid

constructed from round wooden poles and

bamboo.

The grass is stitched to the truss members usinglocal ropes also made of grass. The finished

thickness of the thatch cover is usually 300 mm.

The weight of thatch on the roof of a room

4 x 4 metres is approximately one tonne. This

type of roof is estimated to last for

approximately 20 years with minimal

maintenance. Thatching always begins at the

eaves, starting at one of the corners. This first

course should have a good overhang over thewalls and be firmly fixed onto the truss member

so as to avoid damage by strong winds. The

courses are built of bundles of grass of

approximately 200 - 300 mm thick and 1 - 1.5

metres long, secured to the truss member and to

the rafters using thin wire or grass ropes at

300 mm centres. A second course of thatch is laid

over the first eaves course to the same thickness

with an overlap of approximately 300 mm. It is

fixed to it and to the roof member with wire or

grass rope. The laying and fixing of subsequent

courses of thatch continues upwards to the ridge

of the roof until the whole of the roof structure

is covered. The apex or ridge cover is usually

formed with a round ridge roll of selected thatch

about 150 mm in diameter. Two of these rolls

are fixed on top of each other over the ridge,

and firmly fixed to it using wire every 300 mm.

ridge pole

thatch

300mm

ridge cover - sandcement mortarreinforced withthin wire mesh

bamboo poles boundtogether using locallymade grass ropes

wall tiebeam

thatch

ridge cover - sandcement mortarreinforced withthin wire mesh


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Lean-to Roofs

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20

The ridge is sealed using any of the following

methods:

A coating of sand cement mortar (6:1) laid

to a thickness of approximately 75 mm,

reinforced with thin wire mesh, applied over

the ridge and extended down both sides of

the ridge by approximately 300 mm,

A metal sheet, shaped to cover the ridge,

extended about 300 mm down each side of

the slope of the roof. It is secured to the

slope on each side with firm wire stitching,

Well packed selected thatch is fixed around

the apex approximately 250 mm thick.

Thatched roofs have two main disadvantages:

(i) The dry thatch is highly combustible and will

always present a potential fire hazard.

Applying a coat ofzibla onto the thatch roof

will increase its fire resistance, providing

protection from external fire hazards but not

from internal ones. In Kenana district, for the

construction of the largest sugar factory in

Sudan, a sand cement slurry was used to

coat the surface. It penetrated into the

thatch and proved to be more effective than

zibla.

(ii)Thatched roofs are also susceptible to

damage from insects. Building ceilings will

help to reduce effects of insect damage.

3.3 Fibr3.3 Fibre Reinfe Reinforced Rooforced RoofKenafsand cement corrugated sheets were first

developed by the Intermediate Technology

Group, U.K. and have been used with success for

roofing in several overseas countries.

This roof construction technique was used for

the first time in Sudan in 1985. It was used forroofing classrooms in of the El Haj Yousif School,

a pilot project, co-financed by AGFUND and

Kenafsand cementcorrugated sheets(1.2 x 0.8 metres)


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Lean-to Roofs

3

21

AFESD, to build a primary school using local

building materials and innovative construction

techniques, (See Compressed Stabilised Earth

Block Manufacture in Sudan, Adam and Agib,

2002).

Based on results found during research work and

implementation of a pilot project to roof a

classroom 8 x 6 metres, it was decided to use

this method of construction to build the roof of

the model El Haj Yousif School, completed in

1991. The roof is still in good condition and no

further development was necessary as the

technique was found to be both economically

and technically viable.

This type of roof does have a slight disadvantage.

Because of the small sheet size (only 1.2 metre

lengths) the roof construction needs almost

double the quantity of bearers as those needed

for the normal corrugated zinc sheet roofs.

However, the thermal properties of the sheets,

are almost certainly better than those of the zincsheeting and can therefore be used without a

ceiling, thus reducing costs.

Further exploration is needed into the potential

of going into large-scale commercial production

to enable the use of this product as a viable and

low cost roofing material for a wider market.

Sheet Type

Dimension of a

single sheet

(metres)

Cost in Sudanese

Pounds - SDP

(1996 rate)*

Cost comparison of different types of roof sheet.

* in May 1996 one United States Dollars was equivalent to approximately 1000 Sudanese Pounds

(SDP), (OANDA.com the currency site).

Kenafsand cement corrugated sheets 1.2 x 0.8(dry weight of 24 kg) 1350

Corrugated sand cement sheet 1.0 x 0.8 1100

Zinc sheet 1.0 x 0.8 2300

Cost in United States

Dollars - USD

(1996 rate)*

1.35 USD

1.00 USD

2.30 USD


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C

HAPTER

Vaults

4


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CHAPTER

Vaults

4

25

4.14.1 VVault Roofsault RoofsSemi circular vaults with a half-span rise have

been used in Sudan for low cost residential

buildings with spans of up to 3.5 metres.

Fired clay bricks are used for construction of such

vaults. A course of bricks resting on their edges

i.e. 100 mm thick, is used to construct the vault.

Slightly thicker stabilised earth blocks (140 mm

thick, 1 part cement to 16 parts soil) have also

been tried with success.

The bricks are laid using a 6:1 sand cement

mortar for fired bricks and up to 10:1 sandcement mortar for stabilised earth blocks.

The top surface of the vault is covered with a soil

cement screed to an average thickness of 60 mm.

When stabilised earth blocks are used a sand

cement screed (6 sand : 1 cement) is also applied

in a layer 60 mm thick.

Rainwater is disposed of along the valley

between the vaults when several consecutive

vaults are constructed. In the case of one vault

the rainwater runs along gutters on the two

supporting ends of the vault. Roofs of this type

used by the Ministry of Housing in Khartoum and

the Sudan Armed Forces in the Khartoum area

have performed very well for over 15 years with

respect to their resistance to rainwater

penetration and their heat insulation capacity.

In a UNESCO supported project to construct an

experimental low cost school building, the El Haj

Yousif School, a form of in-situ vault roof was

introduced. This type of roofing technique was

first used for building an agricultural centre in

Senegal.

A stable and reliable vault roof is constructed as

follows:

A steel formwork, circular in section, with a2.75 metre span and a 0.7 metre rise is

fixed on opposite supporting walls,

soil cement (60 mm) fire clay bricks orcompressedearth blocks)

up to 3.5 metres


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Vaults

4

26

A reed mat is laid over the formwork. A

sand cement mix of 4 - 6 soil : 1 cement is

spread over the reed matting, 20 mm thick

at the apex and gradually increasing to 30 mm

at the spring of the vault, A fine wire mesh reinforcement is laid over

the soil cement layer extending approximately

600 mm either side of the apex,

Another layer of the same sand cement

mix, about 30 mm thick, is applied over the

whole section of the vault,

After seven days the formwork is removed.

The vault is cured with water for at least 4

days starting 24 hours after casting.

Owing to the high horizontal thrust of the vault

on the supporting walls once the formwork has

been removed, it is necessary to build three

vertical support buttresses against each side wall,

two at the end walls and one in the centre. It may

also be necessary to use horizontal steel tie bars.

4.2 Jac4.2 Jack-Arck-Arch Roofh Roof

This type of roof is popular in Sudan despite thecomparatively high capital costs that result from

needing to import the principal structural roof

component, the rolled steel joists (RSJs). This

roof type has very good heat insulation properties

and good resistance to rainwater penetration.

The jack-arch roof is constructed in the following

way:

The roof is composed of rolled steel joists

(RSJs) with a maximum size of 160 x 60 mmfor spans of up to 5m, or 140 x 60 mm for

spans of up to 4m. The joists are laid over

the brick walls at 800 mm centres (maximum),

Timber or steel formwork shallow arches

shaped with a maximum rise of 200 mm are

supported between the adjacent joists. Each

form is approximately 400 - 500 mm wide

and spans 800 mm. i.e. equal to the space

between centre lines of the joists,

Fired clay bricks are laid over the formwork.The bricks are laid on their edges to give a

shallow arch. The arch is approximately 100 mm

stabilised earthblock wall

horizontaltie bars

horizontaloutward thrust

of the vault

wooden orsteel formwork

reed matting

sandcement mix

wire mesh

sandcement mix


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CHAPTER

Vaults

4

27

thick i.e. the thickness of a brick. The bricks

are laid using a 6:1 sand cement mortar,

The formwork is removed after 24 hours

and carried forward along the length of the

supporting walls to continue the arched roof

construction,

The brickwork is cured by spraying with

water for at least 5 days starting 24 hours

after construction is completed,

On the sixth day a special mix known as

khafgi is applied over the jack-arch structure

to give a firm cover and good sealant against

rainwater penetration and to improve the

structural and heat insulation properties of

the roof. The khafgi mix is laid to a thickness

of approximately 80 mm over the arch and

finished with a smooth topcoat with an

adequate fall towards the rainwater outlet. A

roof of approximately 20 m2 surface area

requires at least three rainwater outlets (fibre

cement or zinc spouts, 100 mm diameter).

This roof has a parapet wall approximately

400 mm high built of fired clay brick in 8 : 1 sand

cement mortar.

The khafgi mix is prepared as follows:

3 parts of hydrated lime,

4 parts of course sand,

8 parts of fired clay brick crushed into

particles of approximately 20 mm diameter.

The above elements are mixed with water and

kept in a pile to cure for about 7 days. The khafgi

is then finally prepared as follows:

8 parts of the khafgi are combined with 1

part of ordinary cement,

The resulting mixture is then mixed

thoroughly with water to give a workable

paste that is applied over the top surface

of the roof as described above.

This type of roof has good heat insulation and

water resistance properties. It is very durable andcan last for over 30 years with minimal

maintenance work.

fired clay bricks

RSJs

800 mmmax

khafgi

fired clay bricks

RSJ

khafgi


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Domes

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CHAPTER

Domes

5

31

5.15.1 ThatcThatched Domeshed DomesDome roofs are still used in parts of Sudan,

mainly in the south and west of the country.

The most common dome roofs are constructed

using bamboo or thin wooden members

covered with thatch. In some come cases the

thatch is covered with a layer of earth to provide

additional protection against water and heat

penetration.

The dome construction used nowadays is

generally a monolithic structure, built by handusing bamboo or thin wooden members. The

structure consists of a network of these flexible

members built upwards from the ground, starting

as vertical walls and then closing in towards the

apex of the roof, at a height of about 2.5 metres.

The network of wooden or bamboo members is

covered with thatch about 200 mm thick. It is

fixed firmly to the members using thin wire or

thatch rope.

In some cases the outer surface of the structure

is plastered with clay and animal dung. The

internal surface may also be treated in the same

way.

Another type of dome roof is one where the

dome is built onto the wall of a circular room,

approximately 3-4 metres in diameter (maximum).

The wall is built of mud or wooden members

and covered with thatch and animal dung.

The eaves should extend over the edges of the

wall by approximately 400 - 500 mm so as to

shed rainwater away from the walls.

Nomads often use this type of roof construction.The cost of construction is negligible as all the

materials required for construction such as

bamboo orthin woodenmembers

thatch~ 20 cmthick

bamboo orthin woodenmembers

thatch~ 20 cmthick

monolithicdomeconstruction


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CHAPTER

Domes

5

32

bamboo and wooden members are to be found

locally. The construction does not require

sophisticated materials and equipment and can

be carried out by people not necessarily

specialised in building construction.

5.2 Br5.2 Bricick Domesk DomesIn the early 1930s the Sudanese railway

authorities introduced dome and pyramid

structures built entirely of brick with a sand/lime

mortar or cement mortar. These huts were

mainly used as residences for the railway

stationmasters and staff. This type ofconstruction was largely abandoned in the 1970s

and is rarely used for housing now.

Today brick domes are generally only used in

religious shrines, Mosques and other

monumental buildings.

brick dome


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Comparative Costs

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6.1 Cost Compar6.1 Cost Comparison ofison of VVarariousious TT ypes of Roof in ypes of Roof inSudanSudan

The above costs were prepared in consultation with architects, building contractors

and suppliers in Sudan.

* in May 1996 one United States Dollars was equivalent to approximately 1000

Sudanese Pounds (SDP), (OANDA.com the currency site).

CHAPTER

Comparative Costs

6

35

Type of roof

Cost of construction / m2

Sudanese pounds

SDP (1996 rate)*

United States Dollars

USD (1996 rate)*

Traditional earth roof

Improved traditional earth roof 8700 8.70

Timber board roof 27000 27.00

Precast reinforced concrete roof 26500 26.50

Thatched roof 7300 7.30

Fibre reinforced sand cement roof 20000 20.00

Corrugated zinc sheets 30000 30.00

Jack-arch roof 19375 19.40

Vault roofs (fired brick) 17875 17.90

Vault roofs (in-situ stabilised earth) 10500 10.50

Truss and lean-to Roofs

Vault Roofs

5.505500

Flat Roofs


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Conclusions

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7.17.1 The Roof - weighing up the optionsThe Roof - weighing up the optionsThe roof is the most essential part of a building. It encloses the space within the

building providing vital protection from the elements. Given that the roof is the part

of the building that costs the most to construct, it is very important to carefullydesign and construct it, not necessarily always choosing the cheapest option

available but one that takes into account the prevailing local climatic conditions as

well as the availability of local materials and technical know-how.

In the context of Sudan keeping out the heat is of primary importance, which is

why using earth as a construction material is ideal with its excellent thermal

properties. In addition Sudan is not an earthquake zone and it rains very little in

most parts of the country making it such a viable construction material.

Of the construction techniques described in this publication there is no single

method that that stands out as being much better than the others. At 5500

SDP/m2 or 5.5 USD/m2, (May 1996 rate, OANDA.com, the currency site), the

traditional earth roof is the cheapest to construct and can easily be built using local

materials and labour with comparatively simple equipment. However, it requires

more frequent maintenance thus increasing long term costs.

The vaulted roof is the most expensive to construct but requires less maintenance

and has a longer life span.

When deciding what method to use a number of points need to be considered

and weighed up against each other in the real context of the project i.e. what is the

building for and who will be using it? Who will construct the roof? What materials

and equipment are required and which are easily available? What are the local

climatic conditions etc.?

The main points to consider are:

Capital costs,

Maintenance costs (this will vary according to maintenance work required and

frequency with which it needs to be applied),

What is the building to be used for and by whom? (This is particularly relevant

when considering potential maintenance requirements. For small domestic

buildings it is easier to organise regular maintenance work than for bigger

institutional buildings),

Ease of construction,

Availability of materials and equipment, Availability of labour and skills,

CHAPTER

Some

Conclusions

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39


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Thermal properties i.e. ability to keep out the cold or keep in the warmth,

Water resistance,

Wind resistance (hurricanes, cyclones etc.),

Resistance to earthquakes.

Once all the above points have been considered a decision can be made as to

which is the most suitable roofing system to choose as a function of the real

context of the project.

CHAPTER

Some

Conclusions

7

40


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GlossarGlossaryyHassir Reeds.

Humra Thatch or hay.

Kenaf Kenafis an annual fibre crop native to central Africa and closelyrelated to cotton, okra, hibiscus and hollyhock. Kenaf grows in

tropical and temperate climates and thrives with abundant solar

radiation and high rainfall.

Khafgi Stabilised soil cement (see pages 27-28 for details of composition).

Mirig Round timber joist

Shagig Palm purlins

Zibla A mixture of earth, straw, animal dung, cured for 3-4 days and

used as a plaster finish.

GLOSSARY

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roofing in sudan

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