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.
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.