Roof slating and tiling

How does Roof slating and tiling work, step by step?

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   Step 1: Confirm the covering, the pitch and the exposure

   The designer selects the covering and confirms it is within its pitch range for the actual roof, then sets the headlap from the product, the pitch, the rafter length and the exposure of the site. Weight is checked against the structure, because concrete interlocking tiles, clay plain tiles and natural slate differ substantially and a re-roof in a heavier covering can overload existing rafters. Wind uplift is assessed and the fixing specification follows from it - which units are nailed, which are clipped, which are both, and how the perimeter zones are treated. All of that arrives on site as a written specification, and where site conditions differ from what was assumed, the question goes back to the designer rather than being settled on the scaffold.

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   Step 2: Prepare the deck, lay the underlay and set the ventilation

   The roof structure is checked for line and level, because a covering laid on a wavy roof shows every dip. The underlay is then laid horizontally from eaves upwards, each sheet lapped over the one below so water shed onto it runs out at the eaves rather than into the roof, and dressed into an eaves carrier or over a tray so it discharges into the gutter. The vapour permeability of the underlay and the ventilation strategy for the roof void go together and are set by the designer - an impermeable underlay on a warm roof without adequate ventilation is one of the more common ways to build condensation into a new roof. Ventilation provision at the eaves, at the ridge and at any abutment is installed as the covering proceeds, not retro-fitted afterwards.

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   Step 3: Set out the gauge and fix the battens

   Setting out is the skilled part. The roofer measures the true rafter length from eaves to ridge, works out how many courses fit at the specified headlap, and adjusts the gauge slightly so the courses divide evenly and the roof finishes correctly at both eaves and ridge - the eaves course and the top course are always a special case. The gauge is marked on the rafters and the battens are fixed to it, each batten fixed at every rafter, with batten joints staggered and always landing over a rafter. Battens are typically in the order of 25-50 mm in section, with the size for a given roof set by the rafter spacing and the specification. A batten line that wanders shows in the finished roof as a wandering course, so the roofer works from marks and a line, not by eye.

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   Step 4: Detail the eaves, verge and abutments as the work rises

   The perimeter details are built into the covering rather than added to it. At the eaves a shorter under-eaves course or a double course closes the bottom of the roof so the first full course has something to lap onto, and the covering projects far enough to discharge into the gutter. At the verge the covering is either bedded on mortar over an undercloak or finished with a dry verge system, and the courses are laid so that a cut unit at the verge is wide enough to hold its fixings - the roofer works this out at setting out, not when the course arrives at the edge. At an abutment the covering is closed with a soaker and flashing arrangement, or with a secret gutter where the design calls for one, all detailed by the designer.

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   Step 5: Lay and fix the courses up the slope

   Units are laid course by course working up the roof, maintaining the bond so that vertical joints in one course are covered by the units above. Slates and plain tiles are nailed, either head-nailed or centre-nailed depending on the type and the exposure, and interlocking tiles are hung on their nibs and then nailed or clipped according to the fixing specification. Every unit is checked for lie and for hairline cracks as it goes on. The general principle is that fixing is increased in the more exposed zones of the roof - the perimeter, the ridge, the verge and the eaves - and the fixing specification prepared for the project sets exactly what goes where. The roofer follows it as written; substituting a lighter fixing because the roof looks sheltered from the scaffold is how coverings come off in the first serious gale.

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   Step 6: Form the hips, valleys and any penetrations

   Hips and valleys are where two planes meet and where the water concentrates, so they carry the most careful work. A valley may be lined in metal, formed in a preformed valley trough, or swept or laced in plain tiles by a skilled tiler, and the cut units either side are supported so they cannot rock. Hips are finished with hip tiles, either bedded and mechanically fixed or dry-fixed, with a hip iron at the foot to stop the run slipping. Penetrations - flues, vents, roof windows and pipes - are weathered with proprietary units or lead work, always arranged so that water is led over the covering below and never behind it. Cut units around any of these details are still fixed, and a cut piece too small to hold a fixing is a defect, not a detail.

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   Step 7: Ridge, dry-fix or mortar, and final inspection

   The ridge closes the roof and is finished either in mortar-bedded ridge tiles or with a dry-fix ridge system. Dry-fix uses a mechanically fixed ridge batten with a ventilated roll, and it has largely taken over on new work because it does not rely on mortar staying sound and it builds ridge ventilation in. Mortar bedding remains the right answer on many conservation and repair projects, and where it is used the mortar mix, the bedding and the mechanical fixing behind it all matter. The roof is then inspected: courses parallel, gauge consistent, laps correct, all units fixed as specified, perimeter and junction details complete, underlay dressed correctly into the gutter, and ventilation paths clear and unobstructed.