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.
Bats
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.
VAT
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.
Grants
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: -
- Number of courses from eaves to ridge, on each pitch.
- Size of the largest slate (at eaves course) measured from nail/peg
hole to the tail.
- Eaves detail- type of undercloak, depth of oversail, sprocketting
details.
- Valley details, laying pattern, valley boards, lead work
- Hip, ridge,verge dormer and abutment details
- Bedding and fixing type
- Leadwork and rainwater goods
There are a number of reasons for recording
the roof prior to start of work
- To help inform decisions about specifying the re-laying of the
roof, as it is not possible to stipulate "like for like" without
having done this analysis.
- In order to retain the character of the roof and not erode it by
incremental changes
- To help sustain the traditional craft skills and methods which have
evolved over centuries in response to the special conditions imposed by
the use of Collyweston slate on local limestone buildings.
- To provide evidence in any disagreements as to the previous
configuration of details or the former state of the roof.
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.
SPECIFICATION
General Requirements
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
Preliminaries
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.
Workmanship
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.
Materials
Collyweston Slates
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
Ridge Tiles
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.
Mortar
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
Leadwork
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
Fixings
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.
Battens
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%.
Roofing Felt
All under felt to be of a breathable type to
conform to BS 747 Part 4 1974
The following are acceptable;
Tyvec
Rainwater Goods
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%.
Materials Notes
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:
- Where ever possible, new slates should be used
rather than second-hand
- Reclaimed slates should be used only on the
building or group of buildings from which they were removed.
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.
Note 6
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
Workmanship
Scaffolding
Carefully erect suitable scaffolding. Workmanship Note 1
Stripping
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.
Felting
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.
Battens
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
Slating
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.
Leadwork
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.
Workmanship Notes
Scaffolding
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
Collyweston roof.
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
Adam Farnsworth
James Kaye, Peterborough City Council
