Single and double lap
Natural slate, being thin, flat rectangles of stone, are only weather resistant when laid with a double lap giving the traditional half bond appearance. Fibre cement slates and some resin slates are also laid in the same way. The remainder of the roofing products called slates or single-lap tiles, made from either concrete, resin slate, clay or metal.

Single lap slates require far fewer units per square metre of roof than a double lap product. Between 70 to 50 percent of a roof slope covered with single lap slates has one layer, with between 50 to 30 percent with two layers at the head lap and side interlock. Natural slate has between 78 and 34 percent of the roof slope covered with two layers and between 66 and 22 percent with three layers at the head lap. This must not be confused with the weight which tends to be greater with concrete slates, due to their greater thickness.

In most instances the minimum rafter pitch for single lap slates can be as much as 5° lower than for a natural slate. The resulting reduction in rafter pitch, and material quantity, added to the cost of the base material, makes single lap slates a very cost effective roofing product.
 

On paper single lap slates outperform the equivalent double lap slate. But single lap slates have some limitations that should be considered before they are used in place of double lap slates.

All our slate and tile roofs are installed in accordance with the BS 5534 Slating & Tiling regulation