CUPA's Official Guide to Fixing Slates
Method Statement for CUPA Natural Roofing Slates
It must be stressed that the overall appearance of a finished slate roof is dependent on the preparatory work undertaken before any slates are even fixed to the roof. This process starts on the drawing board of the architect and continues through the erection of the building to roof level. The accuracy of the timberwork to support the roof covering, the felting and battening (where applicable) of the roof and the initial work done before any slating starts.
Sorting/ Grading Slates
It is extremely important that slates are sorted prior to holing and fixing to ensure that the slates on any one course are of the same thickness to prevent “kicking” slates or unsightly gaps between subsequent courses (open mouths) on large roofs where several thousand slates – supplied in several crates – are to be used. The quantity required for one slope should be sorted at any one time before the slating starts on that slope.
We recommend that the slates are sorted into at least three grades of thickness even though the slate supplied are graded in accordance with current working procedures at the Quarries in Spain and produced to BS EN 12326, being a natural material there are still variations in thicknesses of roofing slates.
When the slates are then fixed on the roof, the thickest grade should be used for the course nearest the eaves and as work progresses up toward the ridge, the thinner grades can be used, finishing at the ridge with the thinnest grade of slates.
Holing Slates
Holing should always be from the bed (underside) of the slate using a drilling or boring method and positioned so that the thinner end is at the top when fixed. This will give a small countersunk depression on the face of the slate which allows for the head of the fixing nail. In general, we recommend that the holing be done by a suitable slate holing machine during sorting. If a machine with a punching method is used, it is vital that it is maintained adequately to prevent excessive breakages.
Holing with a spiked hammer, or pick can cause excessive spalling and should, therefore, only be used for the small amount of holing that has to be done on the roof whilst fixing.
Cutting Slates
Roofing slates should not be cut too narrow – a minimum of 150mm (6”) is recommend, as this will normally reduce the side lap and therefore the integrity of the roof. If possible, we advise that slate and ½ be used in cases where cutting normal slates would result in the width being narrower than 150mm (6”).
Any machine used to cut slates for features such as hips and valleys should be maintained and adjusted regularly. Similarly, hand tools such as a slate knife and trimming blade should be kept in good condition so that any cuts made to the slates are uniform and true.
Fixing Slates
Any slating work should be in accordance with the latest editions of BS5534:2003 and BS8000:part 6 or with any subsequent parts, editions or standards that may be published appertaining to the design and installation of roofing. The roof should also be properly marked out prior to any slates being fixed in place.
Natural slate is individually split and individually dressed and should be treated as an individual unit when being fixed on the roof. One slate may suit particular location on the roof better than another, perhaps for reasons of thickness, natural curvature, surface texture of appearance, etc, etc, therefore proper sorting/grading and selection of the slates is extremely important to achieve a neat and attractive finish.
General Safety
Throughout any roofing work all relevant site safety guidelines must be strictly adhered to. In addition, the following points are made for the handling of roofing slates.
Although natural slate inert, when cutting or drilling work is undertaken any excessive inhalation of dust should be avoided. Where adequate ventilation is not provided, dust masks should be worn. It is also advisable to wear eye protection. Slates can also have sharp edges and may splinter; therefore, the use of a sturdy pair of gloves is advised when handling roofing slates.