Brief Overview of Collyweston Slating
A Heritage Skill
Collyweston roofs are either 'spot bedded' or 'fully bedded/pointed'
The slates are laid in diminishing courses starting with very large slates called 'eave slates' which are laid over a first course of 'under eaves'. These are usually blue slate but always used to be Collyweston slate. The next course is made up of slates called 'fullers' and, from these, the slates diminish right up to the out rules at the ridge.
The lath or battens are nailed on to the rafters on top of the membrane and gauged to the rod. The rod is a long rod or batten which has all the different courses of slate marked on it depending on what is available to the slater in the 'parting'.
The parting is all the slates which have been hand-dressed with a dressing hammer in order to get a flat edge to the tail of the slate with two square edges. The slate is then drilled with a nail hole near the head of the slate. The slate is measured and parted into different sizes using a parting stick.
The slate is measured from the square edged bottom of the slate to the nail hole and then put in vertical stacks, depending on the size, ready to be quantified at the end of the dressing so the rod can be worked out.
When laying 'spot bedded' roofs generally a slate is laid on two near vertical mortar beds which taper in towards the head of the slate approximately 1 inch wide. These are laid from the trowel like a “sausage” of mortar each side of the slate which is squashed down as the slate is nestled into position with the head of the slate resting on the lath above and the pliability in the mortar bed taking up any twist or unevenness in the slate. This leaves the slate firm so that no rocking takes place when the slates in the next course are laid on top.
The slate is then nailed through the nail hole at the head into the lath. If the shoulders of the slates taper then shale is placed over this gap and is held in position with the spot bed. Shale is a thin laminate/layer of Collyweston slate or blue slate which sometimes splits off the face of the slates when they are first being dressed.
These laminates are put to one side as a vital part of the material used to make up a traditional Collyweston slate roof. They are used to provide a secondary line of defence against water ingress and are placed across the gap which is formed between two slates which taper up towards the 'head' of the slate and the nail hole.