Repairing
Collyweston slate roofs
- When to repair a roof
Collyweston stone slate roofs
can last hundreds of years. The slates themselves are extremely durable and
failure of a roof is usually due to the rotting of the oak pegs or battens on
which they are hung. Due to the weight of the slates, slippage can occur
quickly once is starts. Slippage of nailed slates can occur when the small area
of slate between the edge and the hole erodes. In such cases, the slates are
redressed to form a smaller slate and reused in the re-roofing. As with other
materials, failing roofs are also susceptible to wind damage. In the majority
of cases, it is the corrosion of the nails fixing the battens or laths that
cause the slippage.
Traditionally, slates were
pegged (now nailed) with a little lime mortar used at the top of the slates,
known as 'spot bedding'. Some slaters also point up the roofs, using lime
mortar to fill joints and gaps under the Front edges of the slates. Internally,
more lime mortar was applied directly to the underside of the slate, a
technique referred to as 'torching. When re-slating a roof, this practice has
generally been replaced by the use of roofing underfelt.
If a roof is relatively sound
and is not suffering from large areas of slate slippage or batten failure,
minor repairs will prolong the life of the roof by several years. Small areas
of failing woodwork should be repaired and the roof pointed (or repointed if
this has been done before) to improve its weatherproofing properties.
The occasional slipped slate can be spot-bedded back in place. This will also
allow any torching to be retained and repaired if necessary.
If the battens have decayed and
the fixings are rotten or corroded, there is little alternative but to strip
and re-slate the roof. This will unfortunately result in the loss of any torching,
as, whilst the roof could be re-torched, modern requirements are such that
underfelt tends to be used. A flow of air is needed around the roof
timbers and slates to keep the roof ventilated and prevent condensation. Where
felt is used, therefore, the slates should not be fully pointed.
2. What does it involve?
The work should comply with the Specification for Collyweston
Before work starts, the
contractor will need to erect a scaffolding to access the roof.
The existing stone slates and
ridges will be carefully stripped from the roof and set aside for parting up,
either on the ground or on the scaffolding, where space allows.
The old battens are removed,
roof timbers brushed down and repairs to the roof structure are carried out.
New timbers should always be inserted alongside the existing roof
timbers, to retain the historic roof structure. The removal of roof timbers
will normally require the agreement of the Conservation Officer, and Listed
Building Consent may be needed if the building being repaired is listed. The
roof may also be treated against decay and infestation at this point.
- The roof is then felted, if felt is to be used,
and the battens fixed, spaced to suit the diminishing courses of the stone
slate.
- The slates are parted up, with each stack having
enough slates to cover one course.
- Replacement slates may be needed to make up any
shortfall. These can be new or reclaimed slates. Where reclaims are used,
the source of them should always be checked. New slates are preferred as
this helps maintain demand and ensures the survival of the craft.
- The slates are fixed in diminishing courses,
starting with the largest at the eaves and working u p the roof to the
ridge. The slates should lap the heads of the two slates below (vertical
lap), and the vertical joints of the course below (horizontal lap), by a
minimum of three inches (75mm). Both laps are required to ensure the roof
is waterproof. There is sometimes a temptation to reduce the lap to save
money (as less slates are required to cover the roof). This is known as
'stretching' the slates, and is considered to be bad practice. If the
vertical lap is reduced, the slates, particularly at the lower courses,
appear unusually large, spoiling the appearance of the roof, and there is
a tendency for- the roof to leak. Details, such as swept and laced valleys
(see Details), should always be copied in the re-slating
work. Some contractors have traditionally added their own detailing, such
as bedding a bottle end in the gables of dormers, and is acceptable
providing it is discreet and does not detract from the overall character
of the roof.
- The leadwork around chimneys, at abutments, and
in valleys should be repaired. Lead should not normally be visible on a
Collyweston roof and is typically covered with a lime mortar fillet.
- The ridge tiles are then refixed, with any
shortfall being made up with matching new or reclaimed ridge tiles. They
are bedded in lime mortar, with the joints as narrow as possible. The ends
of each tile are filled solidly using a combination of broken tile, slate
and mortar.
- Debris should then be removed from the
guttering. It is a good idea to repair or renew any leaking or defective
gutters whilst the scaffolding is erected. Painting to the gutter, fascia
and bargeboards etc. is also far easier to carry out at the same time!
- Upon completion, the scaffolding will be
dismantled and the site should be left clear and tidy. You should notify
the contractor if you wish to inspect the work before the Scaffolding is
removed.
- When carrying out roofing work for a domestic
client, it is the responsibility of the contractor to ensure that all work
is carried out in accordance with the requirements of the Health and
Safety legislation.
This Note has been prepared by
Christine Leveson and Jon Burgess, in association with the Collyweston Stone
Slaters Trust
