SPECIFYING FOR THE REPAIR OF COLLYWESTON ROOFS
These notes have been prepared primarily for the benefit of architects, surveyors and other building professionals who need to draw up Specification Documents and supervise roofing contracts and who may be unfamiliar with the special requirements of Collyweston stone roofing.
They may also be useful for building owners, especially those who are experienced in undertaking repair and restoration projects, and those involved in Local Authority Planning and Conservation.
Most Collyweston roofs are on historic buildings, many of which are as much as 300 years old. As such they vary widely and are rarely "standard" in the way that a modern roof usually is. Many Collyweston roofs now needing attention, in the form of stripping and re-laying have been in place for at least 100 years. Some roofs may have remained undisturbed since the 17th century. These roofs therefore represent the work of expert craftsmen and examples of a tradition of roof laying which has survived to the present day.
Drawing up a suitable Specification for localised repairs, or for a complete re-laying, will need tailoring to suit each individual project. The purpose of these notes is to assist in the process of compiling the most suitable specification by including all the generally applicable items and indicating others, which may need consideration under particular circumstances.
A specification is a detailed set of instructions to the craftsman. The clearer these are, the less room there is for misunderstanding and the easier it is to obtain an accurate Quotation. It also enables the firms that may tender for the contract, to compete fairly.
WHEN TO REPAIR A ROOF
Stone slates are re-usable and can last for hundreds of years.
The failure of a roof is usually due to the deterioration of the nails or pegs used to fix the slates to the battens or to the decay of the battens themselves.
Some weakening can occur in the small area at the head of the slate. These slates can be re-dressed and made into smaller sizes for subsequent use higher up on the roof.
Occasionally the main structural timbers of the roof can need repair or replacement due to insect or fungus attack, ingress of water or overloading. Areas of slates will need to be stripped in order to allow for repairs to take place.
If a roof is relatively sound and is suffering from minor slippage of slates, localised repairs will prolong the life of the roof. Examination of the underside of the roof should reveal whether the repairs need to be more comprehensive. Small areas of failed battens can be replaced as necessary and local areas of slate re-laid in the same style as the rest of the roof (spot bedded or fully bedded). Internally, the underside can be re-torched, if torching was in place before. Torching is the weather proofing of the underside of the slates by applying a coat of lime plaster.
Due to the weight of the slates areas of slippage can increase quickly, once they start, so regular and timely maintenance is essential.
For minor patching repairs it is usually most practical to contact a specialist Collyweston Slating Firm (see list) and ask for advice and/or an estimate.
Under no circumstances should local repairs be carried out by simply wedging slipped slates back into place with mortar (especially mortar with any cement in it).
The use of bituminous products to "stick" the slates down should also be avoided at all costs.
Re-pointing whole pitches or large areas of slates by forcing mortar into all the vertical and horizontal joints as a form of repair or to try to prolong the life of a roof is not recommended. In the past this has been a widespread practice, as a stop-gap repair.
The use of any sort of spray- on sealing foam, which is applied to the underside of the roof, is highly detrimental to the roof.
Not only are all these very unsightly options but they can prevent the roof from being properly ventilated. They can inhibit the roof from heating and cooling as individual slates by effectively joining them together into one mass. This can lead to cracking of the slates and greatly accelerated deterioration of the roof in general and to its timber supporting structure. In addition any wholesale pointing up of the roof with a mortar which contains any cement will effectively end the life of the stones as it is virtually impossible to re-use Collyweston slates which have hard cementitious mortar adhering to them.
Such "repairs" are a false economy.
CHOOSING A SLATER
Stone slating is a specialist craft and not all roofing contractors are either familiar with the particular methods involved or skilled in the Collyweston tradition.
In the area where Collyweston stone roofs occur the relative scarcity of the slates and their steep decline in use for new roofs in the latter part of the 20th century, has resulted to some extent in the loss of traditional skills and care should be exercised when selecting a suitable contractor.
The Collyweston Stone Slaters Trust maintains a list of specialist contractors.
Planning and Historic Building Legislation
Many buildings with Collyweston roofs are "Listed".
This means that consent may be needed from the Local Authority (list of Local Planning Authorities to be inserted) for any alterations which will affect the character or appearance of the building. For all but the most minor repairs, it is prudent to speak to the Local Authority to ascertain whether or not Listed Building Consent will be needed for the work. Buildings which lie in the curtilage of a Listed Building are also covered by the legislation. Curtilage buildings are those which although not fixed to a listed building form part of the land and have done so since before 1st July 1948. For example, farm buildings surrounding a listed farmhouse are curtilage structures. It is a criminal offence to undertake, or cause, unauthorised work to a listed building.
Buildings with Collyweston roofs are very frequently in parts of towns and villages that have been designated as Conservation Areas. These roofs, which are making a positive contribution to the character of the area, are frequently protected and may require permission to remove or alter them.
If Listed Building or Conservation Area Consent is required, at least 8 weeks should be allowed for this to be obtained and time programmed into the work schedule accordingly.
Once authorised, permission is valid for 5 years.
In exceptional circumstances, for example on an important roof on a Grade I or II* building and/or one where the roof is particularly ancient and undisturbed, it may be made a condition of consent for detailed recording of the roof to take place. This is in order to make a permanent record of the methods and materials used in this traditional craft. If this is the case, there may be a cost and time implications for the project to pay a professional building archaeologist and allow access during work at key stages.
Before the roof covering is disturbed the roof needs to be inspected internally for evidence of bats. If there is any possibility that bats are present English Nature must be consulted before work begins. All British bats and their roosts are protected and it is illegal for anyone without a licence intentionally to disturb, injure or kill a wild bat or obstruct access to any place of roost. Some timber preservative chemicals are toxic to bats and must not be used.
It is not normally possible to avoid VAT costs on roofing repairs, even to a listed building.
Although Listed Building Consent may be required for works because Planning Guidance deems the work to be partial demolition and reinstatement or an alteration, the VAT Office does not take this view and regards almost all work as a "repair" which currently attracts 17.5% VAT.
However, if there is new work, such as a completely new dormer or a new Collyweston roof over a new extension, then VAT may not be payable on these items, depending on the proportion of new work.
If the roof is over a building that has not been in domestic use since 1960 and is now being prepared for such use, it may not attract VAT.
It is always advisable to check the VAT situation with your local office (inc Tel and address), as the position is currently (Jan 2001) under review and the subject to a recommendation for change. Local offices can also vary in their interpretation of the complex legislation and regulations.
It is always worth checking the current situation with your local Conservation Officer (see list) to see if your repair project will be eligible for grant aid. In general the provision of grant aid by Local Authorities and English Heritage is becoming increasingly rare and where it is available tends to be tightly focused on particular priority areas. Apart from the two sources mentioned above there are no other potential sources of funding currently known which would contribute to the costs of re-roofing a domestic dwelling. For churches and Grade II* and Grade I buildings enquiries should initially be directed towards English Heritage (see list).
Recording the roof prior to start on site
The stone roof covering should always be re-laid and reinstated to its previous form and detailing, except where inappropriate or unsympathetic changes have occurred, or where there are known to be technical defects in the existing details.
It is therefore essential to have a record of the details of the roof covering and supporting structure, before it is disturbed. Additional records may also need to be made during the stripping down process.
The most effective way to record the roofing details is by a comprehensive set of photographs, including a scale where necessary. There is a checklist for points to note in the Appendix of "Stone Slate Roofs" Technical Advice Note by English Heritage (June 1998) which is helpful. As a minimum, information will be needed on: -
There are a number of reasons for recording the roof prior to start of work
It would normally be the responsibility of the specialist professional advisor to make this record, as the initial part of a well run contract, especially as some of the recording requires the scaffolding to be in place and few owners/clients relish heights.
If the roofing contractor is to be required to make this record, it will need to be carefully stipulated in the specification.
As mentioned above, in rare cases the recording may be a condition of consent for works. It is possible that the formal record required under these circumstances could be used as a working document to inform decisions on site.
All roofing work shall comply with BS 5534: Part I 1997 "Code of Practice for Slating and Tiling" unless subject to any qualifications stated below.
All works shall take account of the Planning (Listed Building and Conservation Areas) Act 1990 and Planning Policy Guidance Note "Planning and the Historic Environment" (PPG 15) 1994 and any conditions which apply as part of any consents.
All works involving insulation shall comply with BS 5250:1989 "Control of Condensation in Buildings" unless specifically agreed otherwise by the Client or his/her professional advisor, for sound technical reasons related to the nature of stone roofs.
All works involving insulation shall take full account of BRE Report "Thermal insulation- avoiding risks" 1994
All work involving Lead shall comply with Code of Practice CP 143 Part II and the Guidance issued by The Lead Development Agency.
All Collyweston slating work to be carried out on pitches of 45-47° on new or replacement roofs.
Any loss/theft of slates from site shall be made good at the contractors expense.
No slates shall leave the site without the express agreement of the owner/agent.
All work shall be finished to a sound, weather tight condition and guaranteed for*** years.(This may entail additional costs for the provision of a Bond!)
All work shall be carried out with full reference to current regulation and guidance including
"Control of Substances Hazardous to Health" 1988
"Control of Pesticides" 1986
Health and Safety at Work Act 1974
Check that all necessary consents have been obtained
Check that any recording prior to start of work on site has been done to the satisfaction of the local authority and/or the professional advisor to the works and/or the owner/client. OR Make a full photographic record of the roof to include all items of detailing.
Check source of fresh water supply for mixing mortar on site and cleaning down
Check access and agree working arrangements and times with client/agent
Check site security to ensure especially that slates can be securely stored on site.
Provide all necessary protection, tarpaulins, dustsheets etc, for the duration of the work.
The contractor shall be responsible for maintaining the building(s) in a weathertight condition at all times.
The contractor shall protect new work from extreme temperature conditions to prevent adverse effects from frost or rapid drying. No new work involving mortar shall take place in temperatures below 5°C on a falling thermometer.
Special care shall be taken in the erection of scaffolding to avoid any damage to the fabric of the building.
Any damage to property or the occupier's goods as a result of contract works shall be made good at contractor’s expense.
Defects in the roof structure or ancillary brickwork and/or stonework, shall be brought to the attention of the agent/client for a decision on remedying them, as soon as they become apparent.
Any historically interesting nails, fixings, features, artefacts or dated elements to be brought to the attention of the agent/client and/or the Recording Archaeologist.
All existing sound Collyweston slates from the roof shall be re-used for the new roof
All shortfalls to be made up with sound, good quality, natural Collyweston stone slates of the same general size range, thickness, texture and edge finish as those of the existing roof.
All shortfalls shall be made up with new slates wherever possible, rather than those reclaimed from another site, unless satisfactory evidence can be given of their source.
All undercloaks shall be of natural Collyweston, unless otherwise agreed.
All fillets, shales and galletting to be natural Collyweston slate. Materials Note 1
All existing sound ridge tiles shall be re-used on the new roof, with any shortfall made good with tiles to match the existing, in material, size, colour and profile.
No modern ridge ventilation tiles to be introduced.
All mortar used for repairs, re-laying the roof and torching shall be lime based without the inclusion of any Portland cement.
General mix for bedding shall be 1:3 mature lime putty: local sand, with some sharps. For torching (underside of roof) mortar to be 1:3 mature lime putty: local soft sand plus 50-100gms goat hair per bucket of mortar.
Mix for re-bedding coping stones, relaying or re-pointing stonework and for flaunching at abutments – 1:3 mature lime putty:local sharp sand
Milled sheet lead shall be to BS 1778
For all soakers, valley linings, and weatherings – Code 4
For dormer cheeks, box gutter linings, flat roof sections and other features requiring durability – Code 6
Solder to be to BS 219 Grade D or J
Nails for fixing battens to be of adequate length and gauge to provide secure fixing, without splitting the batten, galvanised steel to BS 1202 Part 1.
Nails for lead work to be copper to BS 1202 Part 2; large round (clout or flat) headed.
Nails for fixing Collyweston slates to be stout, non-ferrous, (ideally copper) clout headed.
For pegged roofs the pegs shall be oak to match those previously in position.
New or replacement battens to be pressure treated (vac-vac or tanalised) softwood and be free from splits, shakes, wane or large knot holes. Maximum moisture content at time of fixing 22%.
All under felt to be of a breathable type to conform to BS 747 Part 4 1974
The following are acceptable;
All sound and serviceable historic rainwater goods shall be refurbished and re-used.
Any new rainwater goods shall be cast iron or cast aluminium to match any sound existing goods in size, profile and gauge and in the method of fixing.
Structural Roof timbers
No structural timbers shall be removed without the express approval of the owner/agent and any necessary planning consents.
New timber shall be dried to a moisture content of between 15 and 20%.
These notes are not intended to form part of any specification document but are designed to help inform the specifier of the issues, and options.
It is the policy of English Heritage and all local authorities in the Collyweston area that:
The Collyweston Slaters Trust fully supports this policy.
The objective of such policies is to discourage the loss of Collyweston roofs in the area and to encourage the overall increase in the production of new slates. This is in order to ensure that the craft has a long-term future and the character of the area is sustained.
Slates which have been stripped from buildings which were formerly animal shelters or barns can be unsuitable for re-use in domestic buildings. There is some evidence that salts and acids have been absorbed and this may shorten the life of the slate on the roof.
Where a mixture of colours and profiles are present, i.e. "white" and terra- cotta colours and half round and "hogs back"profiles, the sourcing for extra tiles would need to be agreed. It is quite common for the ridge to be a mix of the older pale "hogs back" shape and other newer replacements. Occasionally blue hard tiles with an angular profile have been substituted in the past. These are not suitable or traditional and the opportunity can be taken to remove them.
Traditionally a Collyweston roof is ventilated naturally by the flow of air between the slates and at the eaves. Today there can be a conflict between the modern desire for high levels of insulation and the functioning and appearance of a traditionally laid stone roof. In fact stone slate roofs are thermally efficient, but with increasing pressure to convert former roof voids to living accommodation, the traditional ventilation is not possible. Every effort should be made to ensure that modern insulation and ventilation materials and methods do not have a detrimental effect on either the life of the slates themselves or on the overall appearance of the finished roof. This is particularly important on buildings which are listed.
Not all contractors and building professionals are completely confident in their use of natural lime. Up until WW2 it was in common usage and all craftsmen would have been knowledgable about its characteristics and properties. All the buildings which now have Collyweston roofs were constructed and had their roofs laid without the use of portland cement
English Heritage, all the local authorities in the area and the Collyweston Slaters Trust wish to encourage the re-introduction of cement free mortar for the repair and laying of Collyweston roofs.
There are frequent courses and training days held locally on the use of lime.
Collyweston roofs where the slates are pegged over the battens are becoming an increasingly rare survival. In the recent past they have all been routinely replaced with nails as they have come up for repair.
Where a pegged roof is being repaired or re-laid on a listed building this method of fixing needs to be replaced like for like unless specifically agreed otherwise by the local authority. Oak pegs are available.
A pegged roof will need torching on the underside.
Where some of the battens have failed, it is recommended to replace the whole area.
Traditionally the battens are made from riven chestnut or oak. It is not normally feasible to salvage these as the nail fixing has usually weakened the ends too much. In special circumstances, for example, where an historically important pegged roof is being replaced, consideration can be given to using oak or chestnut battens. As they are naturally more waney than modern sawn softwood they add to the character of the finished roof, as the lines of the tails of the slates are not as even.
Riven battens are available.
As with the note on ventilation, modern standards are being applied to traditional roofing materials and methods. It has become standard practice in recent years for roofing felt to be laid under the Collyweston slates when a roof is stripped and re-laid. This was never done in the past and there has not been any research into to the long-term effects of this practice on the life of the slates.
For most roofs it is advisable to use only the best quality breathable felt on the market, to reduce any risk of harming the slates to a minimum. However where the roof is over an open barn or where the underside of the roof can be readily seen and appreciated roofing felt should not be introduced, and indeed is an unnecessary expense. For buildings which where formerly stables, coach houses, lytch gates and domestic barns keeping the traditional appearance of the underside of a Collyweston roof is
a worthwhile contribution to the character of the building.
The eaves of many Collyweston roofs are extended, by means of sprockets, so as to form a deep overhang. Before the general introduction of guttering this served to ensure that rainwater fell well clear of the building. Today guttering is mounted on long rise and fall brackets. Many examples are elegantly shaped and almost certainly made by a local blacksmith. Such examples should always be retained and any replacements made to match. There are blacksmiths who can do this work
These brackets are usually spiked straight into a mortar joint in the wall as a method of fixing.
Soffitt and facia boards should never be introduced.
Replacement timbers need to be restricted to those whose condition threatens the structural soundness of the roof as most Collyweston roofs are supported by structures of high historic importance.
Any necessary replacement structural timbers to be on an exact like for like basis unless there are sound conservation reasons otherwise.
In general weak or failing timbers to be reinforced by "doubling up" or splicing in sound wood.
The introduction of steel should only be considered as a last resort
Carefully erect suitable scaffolding. Workmanship Note 1
Carefully strip off slates, working one pitch at a time. The roof shall be protected in a weather tight condition at all times when work is not in active progress.
Remove slates to a safe store for sorting and re-dressing as required. Damaged slates shall be put to one side for re-dressing, with sound slates stacked in rows and on end, sorted by length. They shall not be stored flat. This should not be done on the scaffolding. No slates shall leave the site, without the express permission of the agent/client.
Chutes shall not be used to get slates to the ground.
Allow time for any recording of constructional details by others
Preparation and Repairs
Clean down and inspect roof structure for defects, rot and insect attack. No timbers shall be removed without any required permissions.
Allow time and access for any timber treatment and timber repairs as agreed by agent/client/conservation officer.
Defective timbers shall be replaced on a strict like for like basis, matching both size and species. Workmanship Note 2
Retain any historic pegs and nails (or keep a representative sample) for use as templates for copies and/or for recording purposes.
Any defective stone or brickwork shall be repaired and allowed to go off before any adjacent slates are re-laid. All work to be carried out using matching materials and mortar (as specified above) brought to a brushed finish.
New barge boards and fascia boards shall not be introduced, where non existed previously.
Install roofing felt /underlay as follows
Lay parallel to eaves, commencing at eaves and with a slight drape between rafters.
Lap 150mm at sides and ends, with end laps centred over rafters.
Tack/nail sufficiently to make secure until battens are fixed
Lay over any sprockets or tilting fillets and extend into gutters. Prevent any sagging which could trap water
Do not extend over any firebreaks or party walls
Lay a strip over the ridge to over lay general felting by 150mm minimum
Lay 600mm minimum strip at hips over general felt and extend fully to eaves line.
Extend felt at abutments to ensure that it is tidily incorporated under the flashing.
Replace any defective battens as follows:
Battens shall be fixed securely to every rafter with nails to above specification
Battens shall be set out so that the slate courses will diminish in a regular order from eaves to the ridge.
Nail penetration into rafters shall be minimum 50mm.
Batten ends shall be cut square and centred over rafters and nailed without splitting. No more than 1in 3 batten joints should be on any one rafter.
Battens to be packed where necessary to maintain level
Battens to be not less than 1.2metres and laid over a minimum of 3 rafters.
Provide additional battens, sprockets/tilting fillets at eaves, ridge, and other perimeter features
Provide additional battens if necessary to prevent wind-lift of horizontal laps in felt
The roof shall be re-laid in diminishing courses using only sound slates reclaimed from the previous roof, the shortfall to be made up with slates from a source agreed by the owner/agent/conservation officer
Re-used slates shall be consolidated on one or more pitches and any shortfall made up with new (reclaimed) slates used together on a different pitch.
Re-used slates shall not be "turned", that is, slates shall be fixed to their original orientation (top face up, top edge up)
Eaves course shall be laid using the same size slates as the previous roof.
The undercloak shall be of Collyweston Slate (or large true slate, if this was previously in place). The use of continuous sheet material is not acceptable.
Eaves course shall be spot bedded on wall top. Workmanship Note 4
Slates shall be bedded using lime mortar only. Workmanship Note 5
Verges; Valleys; Hips; Dormers and Ridges to be laid as previous roof, unless otherwise agreed on site. Workmanship Note 6
Slates shall be laid with the 3" head and side laps.
"Stretching" of slates to reduce the number of courses or the overall number of slates required is not be acceptable. Workmanship Note 7
Wide slates shall be reserved to close the bond at verges, hips and abutments.
Abutments shall be lead soakers painted with 2 coats of bitumastic paint with lime mortar fillets.
All lead work shall match existing, unless otherwise agreed. Workmanship Note 8
Cleaning: Upon completion: -
All internal roof spaces shall be cleared of fallen debris and left in a clean state.
All gutters and hopper heads, hidden valleys and flat areas shall be cleared of debris and washed down and left in good free-running order.
All repaired chimneys shall be checked to ensure that fallen mortar and debris has not impaired the operation of any flue
All loose mortar, dust and debris shall be cleaned from the wall faces and cills of all elevations affected by the contract and be left in the same state as found or better.
All ground beneath the scaffold and any other site works shall be cleared and left in a clean state, within 48 hours of the dismantling and taking from site of the scaffolding, unless otherwise agreed by the client/agent.
The finished roof shall match the former roof as recorded prior to start of work.
As most buildings with Collyweston roofs are historic or listed buildings particular care needs to be taken in scaffolding. The contractor will normally need to provide a fully independent working scaffold to give safe access to eaves level, complete with ladders, hoists, safety netting and close boarding sufficient to support loads from stacked slates. Pole ends to be capped.
Do not attempt to correct apparent defects, which may be part of the original subtleties of construction and contribute to the character and technical performance of the roof.
For example, the rafters may have been installed with a slight concavity or with a slightly steeper pitch than the coping. It is also common for slates to have a slight upturn at abutments caused by raising the end rafter.
It is important to keep the original weather face uppermost because it is the more durable side.
Where new and older slates have to be combined on the same pitch they should be distributed evenly throughout the slope.
Where the eaves are easily visible from the ground the undercloak should always be Collyweston slate.
The treatment of the eaves may be dependent upon whether roofing felt (and/or certain sorts of insulation materials) have been introduced.
On a traditional roof the eaves are normally spot bedded to allow the free flow of moisture and air and assist in the wet/dry and heating and cooling cycles. Any interference with these needs to be carefully considered.
Slates may be spot bedded or fully bedded and pointed according to:
the tradition of the craftsman
the presence or absence of under felt
the intended use of torching for finishing the underside
If felt is being introduced, the slates should not be fully bedded and pointed as this may hinder breathability and cause sweating on the underside of slates.
One accepted method is to bed the slates fully at the perpends and spot bed at the centre, so that the tails are left open to assist in ventilation. Where there is no under felt, the tail can be pointed and the mortar raked back far enough to allow water to fall clear. This also ensures that the finished roof retains the characteristic shadow lines which define the diminishing courses.
Gallets or shales may be introduced for bedding to help avoid rocking or uneven surfaces and ensure weathertight finish. These should always be of Collyweston limestone material.
New extensions and features, such as dormers, to buildings with Collyweston roofs, and where there is to be a new Collyweston roof, it will be best practice to replicate all the traditional details from the older roof, including slate sizes and style of laying.
If there is any doubt that the existing roof or others nearby are not good examples of traditional good
practice, there is a photograph library attached (to be developed) which can help to establish a suitable pattern for copying in new work.
For head laps (the amount that the third course overlaps the first) the lap should be no less than 20% of the first course slate (measured from the fixing hole to the tail). In most cases this will be 75mm for all slates down to 300mm long. For slates less than 300mm long the head lap can be reduced to 50mm
For side laps the laps should be no less than 40% of the width of the slate.
In most circumstances it is not traditional for there to be visible lead work at the abutments of a
Comments by slaters about good and bad practice for the above Notes, would be very helpful to establish an agreed specification for general use with Collyweston. It would also be useful if any pictures or photographs of good practice and good examples of historic details could be made available to the Trust. These will be returned after scanning for use on the web site, but this information will act as a pattern and help to set a range of standards for Collyweston slating.
Text prepared for the Collyweston Stone Slaters Trust by
Ann Bond, Principal Conservation Officer, Northamptonshire County Council
Andrew Brook Managing Director, Historic Building Conservation Ltd
Gaydon (Ted) Edwards, Clerk of Works to the Fitzwilliam Estates
James Woolmer, Architect
With advice from
Peter Loft, E.Bowman and Son
Jon Burgess Cambridgeshire City Council
James Kaye, Peterborough City Council