re-roofing and structural repairs - national trust · 2020-03-13 · background re-roofing and...

Ashdown House, Oxfordshire

Building design guide

Structural repairs and conservation of chalk block elevations August 2012

n structural investigations and repairs

n quarrying chalk block

n volunteer-led scaffold tours

n contractor ceased trading

Re-roofing and Structural Repairs

Background

Re-roofing and Structural RepairsAshdown House, Oxfordshire

As a result of structural alterations in the twentieth century, Ashdown House has suffered from significant, progressive structural movement. Whilst previous phases of work had addressed much of this movement, the lintels and deteriorating chalk elevations had not been tackled. In addition, the roof coverings had reached the end of their useful life, resulting in water penetration and deterioration of both the roof timbers and cupola structure.

Phase I works completed in 2005 dealt primarily with the north elevation only. Funding for Phase II works, a comprehensive package to address all outstanding issues, was allocated in 2011.

Left Ashdown House prior to Phase II

2Building design guide Phase II Re-roofing and Structural Repairs | Ashdown House

Project brief

This project aimed to encompass the following elements:

n New roof coverings Replacement of flat roof coverings with either lead or copper. Taking up and re-laying of stone slates to main roof slopes and dormers;

n Roof structural repairs Surveying the roof structure and implementing repairs;

n Roof access system Investigation and installation of safest means of temporary access for routine maintenance;

n Dormer window repairs Investigation and repair;

n Chimneys Investigation and repair;

n Lintel repair/replacement Investigation and undertaking of repairs/replacement to window lintels;

n Wall consolidation Investigating the condition of external walls and undertaking the most appropriate method of consolidation;

n Floor joists Investigation and repair/provision of additional lateral restraint;

n Cupola repairs Investigating and undertaking timber repairs to refurbish the cupola and associated supports;

n Roof balustrade Repair/replacement of existing balustrade and plus the most appropriate way of providing compliant edge protection for visitors to the roof/viewing platform;

n Staircase supports Investigation into condition of existing support timbers and repair if necessary;

n Internal partitions Investigation and repair of internal partitions and their associated plasterwork;

Key factors n Ashdown House is currently open to visitors on Wednesday and Saturday afternoons between the beginning of April and the end of October. If safe to do so, the intention was for the property to remain open to visitors throughout the duration of the works.

n The property is presently leased to a private Lessee. All access for surveys, investigations and the work itself had to be agreed in advance with the Lessee’s representatives. Allowance had to be built into the project programme to provide sufficient notice for access and for the Lessee to move contents and clear rooms, when internal access was required. In addition, items belonging to the Trust needed to be securely and safely stored.

n Ashdown House is a confirmed roost site for at least two and possibly three species of bat. The proposed works were likely to result in an offence and so it was considered necessary to apply to Natural England for a European Protected Species Licence before works could commence.

n Ashdown House is a Grade I listed building, however, despite its national importance it has not been the subject of systematic historic building analysis. As a consequence the interiors and exterior are not well understood. The role of the Architect and Lead Consultant was to include archaeological recording to enhance our understanding of the building, also to record historic fabric likely to be compromised or removed by the works.

Consultees n Architectural Panel n English Heritage n Leaseholder n Vale of White Horse District Council

Above Cupola roof braced and suspended from scaffold whilst frame below is replaced


Site issues

Above left Limited opening up of chalk facing to inspect condition of oak lintels

Above right Typical area of elevation containing eroded chalk block

FundingBudgetary constraints meant that external consultants had to be appointed, specification/drawings prepared, a contractor appointed, works commenced on site and budget spend maximised in 2011.

BatsThe property was known to contain bats, a fact subsequently confirmed by various bat surveys. Therefore, a European Protected Species Licence had to be obtained to allow the property to be re-roofed. To mitigate the impact of the works, all of the roof coverings had to be removed prior to hibernation in November. This single factor largely drove the whole project programme.

MaterialsThe project team wished to return the current copper roof covering to the cupola and crown flat roof back to its original lead covering. Comprehensive historical documentation supported this change and following consultations with the Architectural Panel, Local Authority Conservation Officer and English Heritage, consent was given.

Designationsn Grade I listed buildingn Site of Special Scientific Interest (SSSI)

Observation and the limited opening up undertaken during Phase I led the project team to conclude that the original oak window lintels had failed in the majority of instances. Owing to the exposed location of the property and porous nature of the chalk elevations, there were concerns at an early stage that like for like replacement in oak would rapidly lead to re-occurrence of the existing problem. The large sections of oak required would initially shrink and then deteriorate in the medium term. After detailed and in depth consideration, pre-cast concrete lintels were proposed. Much debate with the consultees ensued, following which consent was given for the change to pre-cast concrete lintels.


Site issues

Above Copse and quarry before operation to extract chalk commences

Left CCTV security tower providing security surveillance

Contractor issuesDespite a favourable credit check before the contractor was appointed, within the first eight weeks of the contract, they suddenly ceased trading. To maintain continuity and a presence on site, the Trust directly employed the contractor’s Site Manager and subsequently insisted that he be employed by the contractor who was appointed to continue the works. A rapid and concerted effort by the project team saw an alternative contractor appointed and works re-commence on site within a month. This meant that the roof could be stripped in time to ensure compliance with the bat license.

SecurityIn the period immediately following the original contractor abandoning the site, the lightning protection system was removed by thieves. Additional security was put in place immediately. This, along with concerns about the large quantities of lead which were due on site, resulted in a review of security arrangements. Subsequently, remotely monitored CCTV cameras were installed for the remainder of the contract, a cost which had not originally been budgeted for.

ChalkInitial enquiries revealed that an appropriate match for the chalk block elevations was not commercially available. The previous phase of work had opened a disused quarry on an adjoining owners land and following negotiations and payment, permission was given to enter upon their land and extract the chalk required.

Unfortunately, despite initial observations to the contrary, the initial first batch of chalk gleaned from the quarry was of poor quality, containing many fissures and faults which resulted in very high rates of wastage. To obtain the quantity of chalk required, a second visit had to be made to the quarry to extract a further 40 tonnes of chalk. Fortunately, there was enough time in the programme to enable the additional chalk to be extracted and converted by the sawyers to keep the stonemason fixers supplied.


Design approach

Above Ashdown House with temporary roof and enclosed in scaffolding

Ashdown House is leased to a Lessee as a private dwelling with provision for limited opening two afternoons a week. At project inception, the terms of the lease were clearly summarised with regard to responsibility for the various elements of the structure to ensure liability for the cost of the works was clearly defined for both parties.

The timing of the works fortuitously coincided with a change in Lessee. Prior to the lease being sold, the General Manager was able to make the new Lessee aware of the extensive nature of the impending works. Having purchased the lease, the new Lessee embarked upon a programme of internal repairs and refurbishment and so chose not to occupy the building until both the internal work and the Trust’s external works were completed.

Whilst the Lessee undertook his internal refurbishment works, the property was completely empty. Therefore, before Phase II of the works commenced, it was possible to take advantage of the opportunity to inspect the floor voids, whilst many of the floorboards were raised. The Structural Engineer was able to inspect the connections between the floor joist ends where built into the external walls, which hitherto, had been largely inaccessible. Previously an area of concern, these connections were found to be in generally good condition and not in need of any remedial repairs or further intervention. In addition, the good access available allowed a detailed refurbishment asbestos survey to be carried out.

Repairs and conservation of the chalk elevations were kept to the minimum in order to retain as much of the historic fabric as possible.

Each of the timber lintels were individually inspected following the careful removal of small facing chalk blocks above each window. Some were found to be in good enough condition to be left. In the end, just over half required replacement.

Archaeology The archaeological brief included for recording to enhance our understanding of the building, also to record historic fabric which was likely to be compromised or removed by the works.

In addition, the debris cleared from the roof spaces was carefully bagged by the contractor and then sifted through by Volunteers. Unfortunately, there were no significant finds.

Engagement Following briefings and training by both the Trust’s Project Manager and the contractor’s Site Manager, Volunteer-led scaffold tours were undertaken to show visitors the conservation works being undertaken at close quarters. Tours were provided on a weekly first-come-first-served basis.

These proved very popular with both the visitors and Volunteers alike.

Environmental Whilst the parapet gutters were being re-lined in lead, the opportunity was taken to re-configure the bays and falls slightly to make the detailing more robust and better able to cope with the rainwater storm surges witnessed in recent years.

Roof insulation was increased using hemp-based insulation, whilst ensuring there was adequate ventilation provided.

Nature conservation Early liaison with the Wildlife and Countryside Advisor enabled all of the relevant bat surveys to be undertaken and mitigation fully planned into the project. Following discussions on the type of roof felt to be used when recovering the slopes, modern non ‘bat friendly’ breathable membranes were discounted and bitumen felt used.


Above The newly installed chimney restraint system

The project team comprised people and companies providing internal and external expertise including:

n Project sponsor and curatorn Clientn Project Managern Conservatorn Environmental Advisorn Nature Conservation Advisorn Main contractors: Gary Jones, R W Armstrong & Sons Ltd.n Architect: Andrew Brookes, Rodney Melville & Partnersn CDM co-ordinatorn Quantity Surveyorn Structural Engineers

Project team Construction

Ashdown House sits in 500 acres of park downland and woodland between the Ridgeway path and the village of Lambourn. Part of the surrounding land is classified as a Site of Special Scientific Interest (SSSI) where sarsen stone strewn about the fields from periglacial periods support rare lichen.

Ashdown House is a Grade I listed seventeenth century house. Built around 1662 probably by Captain William Winde for William 1st Earl of Craven. The house is a unique architectural gem, constructed of chalk ashlar with Bath stone dressings and stacks with a crown flat roof and stone slate roof coverings. The house is of Carolean design, with Dutch and French influences. It was in the forefront of a new style of English country house building but is extraordinarily tall and narrow reflecting the style of Dutch town houses of the period.

The house is forever associated with the romantic tale of William, 1st Lord of Craven, who built the house, and Elizabeth, Queen of Bohemia, ‘the Winter Queen’. William was devoted to Elizabeth – the sister of the ill fated Charles I – and ‘consecrated’ Ashdown to her however; sadly she died before it was completed.

The building had been subject to considerable structural movement notably to the chimneys and south elevation.

Sub-contractors

Stone roof tiling and lead workAttleys Roofing Ltd.

Quarrying of chalk blockM J Church

Cutting and processing of chalk blockWolff Stone

Masonry repairs to facades and chimneysAlbion Stonemasons Ltd.

Scaffolding, temporary roof and structural supportBenchmark Scaffolding Ltd.

New cupola timber frameThe Oak Frame Carpentry Company

Timber balustradeClapham Carpentry & Joinery

DecorationsOxford Decorating Ltd.

New leaded lights to cupolaBrockweir Glass

Gilding to the ball finialHumphries & Jones

Lightning protection systemRodell Ltd.


Construction

External works n Chalk was quarried from an adjoining owners land;

n Cotswold stone slates for the roof were made up from a stock held at Coleshill;

n Sand cast lead was used for the cupola roof and crown flat roof.

Conservation building workThe racking of the cupola structure, originally thought to have been caused by the deterioration of the support beams below following prolonged water ingress, was actually found to have been caused by the failure of all the major structural timber joints from extensive rot and beetle attack. Whilst the integrity of the cupola structure had been lost and was beyond repair, the primary roof structure was found to be in generally good condition, with small repairs and additional bracing required. So as not to disturb the roof structure when the cupola frame below was replaced, the roof was fully supported and lifted intact, allowing the structure below to be removed and replaced. Once the cupola had been rebuilt, the roof structure was carefully lowered back to its original position.

Top left Extracted chalk at the quarry with bed marked in blue

Top right Typical panel to elevation with replaced chalk blocks

As a result of the structural alterations undertaken in the twentieth century, the property had suffered from progressive movement. Regular monitoring of the movement utilising electronic distance measurement (EDM) surveys revealed movement on the south elevation of up to 120mm. The condition of the structural ties restraining the north and south elevations had to be physically checked to ascertain their condition and an assessment of their remaining life made. Fortunately, they were found to be in good condition with another 25 year life expectancy. Whilst the roof covering was off the opportunity was taken to discretely introduce an addition restraint to the chimney stacks to arrest further outward movement.

Owing to the structural issues and fragility of the external fabric, the Structural Engineer instructed that works to the elevations, including lintel replacements could only be undertaken from every other lift, to avoid too much disturbance in any one area.

Following the long debate and concerns about installing pre-cast concrete lintels instead of oak to match the original, it was interesting to note that when the head of several of the windows were opened up, pre-cast concrete lintels had already been installed towards the inner face of the walls, in an apparent previous, unrecorded phase of works.

Associated repairsWhilst the Trust’s collection and chattels had been removed from the main staircase to safe storage, the opportunity was taken to redecorate.

Whilst the entire building was accessible from temporary scaffold, all minor repairs, pre-paint repairs and external redecorations were undertaken.

Nei

l Nig

htin

gale


Above Extensive conservation repair to cupola roof structure

Bath stoneSyreford Quarries & Masonry Ltd.Brockhill Quarry, Naunton, Cheltenham GL54 3BA01451 850677www.syreford-cotswold-stone.com

Sandcast leadCEL Leadwork Contractors Ltd.Progress House, 256 Station Road, Whittlesey PE7 2HA01733 206633

Lead roof vent – Airtrak-LV9/LV15Nicholson Ltd.Regent House, Queens Road, Barnet, Hertfordshire EN5 4DN08450 098980

Hemp insulationBlack Mountain Insulation Ltd.Unit B, Tir Llwyd Industrial Estate, Rhyl, Denbighshire LL18 5JA01745 361911www.blackmountaininsulation.com

Thermowood deckingVincent Timber Ltd.8 Montgomery Street, Birmingham B11 1DU0121 772 5511www.vincenttimber.co.uk

Productsand services

Funding

Internal n The project was funded by the National Trust.

Costn £1.3 million with a cost per m2 of £1,156.

ProcurementnThe architect was selected by competitive fee tender. nSpecialist suppliers/craftsmen were involved. n The contract was a JCT Standard Form of Contract 2011.

Project durationn Completion of tender documents: 13 June 2011n Tenders invited: 14 June 2011 n Tenders received: 8 July 2011 n Contractors selected: 18 July 2011 n Commenced on site: 22 August 2011 n Project completion: 31 August 2012


http://www.syreford-cotswold-stone.com

http://www.blackmountaininsulation.com

http://www.vincenttimber.co.uk

Post project review

The project is considered a success having met all of the original objectives. The property is now weathertight and structural issues highlighted in the Project Brief have been addressed.

The external fabric has benefited from extensive conservation. In addition, the re-configured parapet gutters are better equipped to cope with rainwater surcharges associated with changing weather patterns and increased insulation to the roof slopes and voids will reduce heat loss and ultimately save energy.

Due to the original contractor ceasing trading within eight weeks of starting on site, the approved programme was exceeded by one month. Through a concerted team effort, a new contractor was appointed and works recommenced on site promptly.

Best practicesThe project team was well structured. Everyone on the team was well informed and nothing was allowed to drift. The core team would bring in relevant internal consultants at appropriate stages, as and when required. This ensured good use of everyone’s time without the need to sit through endless meetings at which contributions were not required.

Above Replacement concrete lintel and consolidated rubble fill core

During the construction phase, separate Client Policy Meetings and Contractor Site Meetings ensured that the internal project team and external consultants could reach consensus on matters to be raised with the contractor.

The quality of workmanship achieved on site was considered to be extremely high. This was achieved through the selection of good contractors. For crucial sub-contract packages (eg. stonework) the Architect and Trust’s Project Manager interviewed the preferred sub-contractor prior to their appointment, to satisfy themselves that they had the relevant experience, management and conservation approach. This process was adopted as a direct result of the Trust’s Project Manager’s early consultation with the Senior Project Manager at Tyntesfield, where the same approach had been successfully implemented.

The volunteer-led scaffold tours proved to be very successful. Although they took much time and effort to organise, the feedback from visitors, staff and the volunteers meant that they were well worth doing. The contractor was excellent at both helping to organise the tours and train the Volunteers. Their site managers were good at engaging with the visitors and volunteers alike. The tours ensured that the volunteers were fully engaged with the project and were fully aware of progress on the works.

The full inclusion of the Lessee’s property manager and his local on-site representative in the project team was crucial to the success of the project. They were both fully engaged in the project and integral to the project team, right from the start. Having intimate knowledge of the project and being authorised to make decisions on the Lessee’s behalf, meant that day to day decisions could be arrived at quickly. In addition, the prompt actions of the on-site representative assisted the smooth running of the project in many ways.


Post project review

Best practices continuedThe National Trust employed the original contractor’s Site Manager directly, once the company had ceased trading, ensuring that there was a presence on site during a time of great uncertainty. In addition by continuing to employ him and then insisting that the new contractor take him on as their site manager, there was good continuity which facilitated a smooth re-start. Crucial records and information were also retained which may have otherwise been lost.

Lessons learntHaving to plan, appoint consultants, specify, consult and implement a complex £1.3 million project in one financial year was extremely demanding.

Crucial records and information from the previous phases of work could not be located internally. Indeed, the best source of information and records were the external consultants who had amassed much historic information from their long association with the property.

The quality of the quarried chalk was initially quite poor. There were high rates of wastage (well in excess of that anticipated) when the chalk was converted into ashlars. This resulted in there being insufficient chalk available and the quarry had to be re-opened for a second time to glean additional block. Fortunately, the chalk extracted the second time was of much higher quality. This was a risk that had to be borne by the Trust in order to source chalk of an exact match and would not have arisen had the chalk been available from a commercial quarry.

Above left Completed safety barrier and viewing deck with new timber balustrade

Above right Completed cupola

End-user feedbackVolunteer“It’s been a long and complicated project but it has also been a fascinating one, revealing so much about the house and its history. We are very excited that the fully restored Ashdown House is about to be revealed!”August 2012

“Today I had the thrilling experience of taking the scaffolding tour to see the conservation work that is taking place on Ashdown House. I urge everyone (assuming that you are not on another continent or afraid of heights!) to come to Ashdown and take this tour while you can because it is an amazing experience! The tour first ascends three flights to look at the replacement of the chalk stone blocks on the walls. Visitors are then taken up to the viewing platform above the roof. At this stage you are four storeys up and on a level with Ashdown’s flat roof and cupola, the top of which is currently suspended to allow work on it to take place. This part of the tour has a lot of “wow” factor! From here you can see the replacement of the Cotswold slate roof, the work that is taking place on the flat roof and the cupola, and the massive leaning chimneys of Ashdown. You can also look out through specially appointed “windows” to view the surrounding countryside.”April 2012


Further information

ContactAshdown House, Lambourn, Newbury, Berkshire RG17 8RE www.nationaltrust.org.uk/ashdown-house www.facebook.com/Ashdown.HouseNT www.twitter.com/AshdownHouseNT www.ashdownhouse.blogspot.co.uk

If you require this information in alternative formats, please telephone01793 817791or [email protected]

Case study informationThis case study was researched and compiled by Andrew Harris, Building Surveyor. Acknowledgements to Ingrid Chesher.

Design by Inkcap Design.

Products and services: Use of products and services is not necessarily an endorsement by the National Trust.Copyright: National Trust retains copyright for this document.Please do not reproduce/photocopy without prior permission.

Building Design Guide concept devised by Rory Cullen and developed by Jonathan Howard, with acknowledgements to Jacky Ferneyhough, Ingrid Chesher and Angela Collins.

© 2013 National Trust. Registered charity no. 205846.

Above Ashdown House with all works completed


http://www.nationaltrust.org.uk/ashdown-house

http://www.facebook.com/Ashdown.HouseNT

http://www.twitter.com/AshdownHouseNT

http://www.ashdownhouse.blogspot.co.uk

http://www.inkcapdesign.co.uk

re-roofing and structural repairs - national trust · 2020-03-13 · background re-roofing and...

Documents