Introduction to Roofing with Slate
Whatever the angle of roof pitch working on any roof is dangerous. And the hazards or working on a steep roof pitch are difficult for novices to overcome. Coping safely on a shallow roof pitch is possible as long as sensible precautions are observed but working on a steep roof pitch requires careful detailed prior planning, the right equipment and if necessary advice from experts familiar with working on such a steep roof pitch. Ladders with special hooks to secure them to the peak are available for use on a steep roof pitch for inspection purposes, but when actually undertaking repairs a Scaffolding should always be used.
KCS recommends that anyone inexperienced in working on a steep roof pitch (or indeed any roof) is best advised to find a contractor to do the work instead. Check in advance that the contractor has public liability insurance. A claim on your own policy in the event of damage or injury may be difficult.
Roof Pitch and Slate Roofs
Roof pitch is an important consideration when contemplating a slate roof. The degree of roof pitch will determine which slates can be used or even whether a slate roof is advisable at all. A roof pitch of at least 22.5 is commonly regarded as the minimum although some slate products can cope with a shallower roof pitch. Redlands Roofing Systems artificial interlocking slate ‘Cambrian’ specifies a minimum roof pitch as low as 15 Deg in limited conditionsand there are one or two other which say they are suitable for a slate roof pitch as low as 17.5. But in practice the product specification is only the starting point. Slate, like any roofing material is only suitable on any given roof pitch dependant on other variables such as rainfall levels, snow loading and the roof’s actual elevation. A slate roof pitch which works on a three story house will not necessarily work on a very low pitch bungalow.
A steeper slate roof pitch copes better with adverse weather. Any shallow slate roof pitch is susceptible to the wind forcing snow and rain under the slates. Although some slates are treated to make them rain resistant, natural slate is a natural material and is therefore porous. So slate is suited to a steeper roof pitch. Reclaimed slates in particular may not have the necessary rain proofing treatment to make them usable on a shallow roof pitch. A steeper slate roof pitch will shake off rainwater and even snow but large amounts of rainwater lingering on a shallow slate roof pitch will soak into the slate. Snow in particular on a shallow slate roof pitch will stay in place for some considerable time and the underside of the snow might be liquid owing to heat rising from the house. Anyone contemplating roof slates at the lowest end of the manufacturer’s recommended roof pitch should therefore consider placing a membrane under the slates to ensure that the slating battens under any shallow slate roof pitch are treated with preservative to cope with water seeping under the slates.
Slates take longer to fit than tiles. Each one needs to be secured in place by two nails. Depending on the product, the roof pitch, the degree of exposure and the size of the slates, the supplier will recommend the minimum spacing ‘gauge’ for the batten and the batten size required. But if the distance between the rafters is higher or the slates are heavier than usual, larger battens may be necessary. ‘Tanalised’ treated battens are recommended to cope with the damp or wood boring pests. Roofers using reclaimed slates however need to work out the ‘gauge’ and other variables for themselves depending on the roof pitch and the degree of exposure. If the roof pitch varies, as it might do when there’s a window in the roof, or if different slopes are present for other reasons, roofers always rely on the lowest gauge present on the roof for the entire roof area otherwise the slates won’t align correctly. In some instances the manufacturers also specify the slate overlap. With slate the ‘headlap’ is typically at the lower end of a range of between 75mm and 100 mm. But the terms ‘overlap’ and ‘headlap’ can be confusing. ‘Headlap’, the term commonly used when referring to slate only refers to the part of the slate between its lower edge and the fixing holes half way down.
Starting at the lowest edge of the roof, the battens are placed to ensure that the slates provide the necessary overhang at the fascia. Roofers use a string line to ensure that the set up is consistent across the roof and nail the batten into each rafter to a minimum of 40mm. The process in then repeated right up the roof. Slates are then laid upon the fixed battens in a pattern known as ‘brick bond’, with the joints aligned with the centre point of the slate below and above. ‘Double lap’ pattern involves covering the top part of each slate with two slates. The steeper the slate roof pitch the smaller the overlap necessary. Aluminium, copper or stainless steel nails are regarded as preferable to galvanised ones to avoid corrosion and slates are nailed either in the middle or close to the top. Provided that non corroding nails are used roofers usually nail the slates at the centre to reduce the risk of them being lifted in the wind.
KCS will be publishing further articles on the details of laying each pattern and how to cope with verges (the gable end) and the ridges and hips. We will also be publishing guidance on the calculations relating to fixing of battens where no manufacturers advice at all is available. In the meantime anyone with any specific enquiries can contact us on our email address for a reply by return .