Cambrian age Welsh slate

At one extreme, a straw or reed is very narrow and there are thousands of joints on a thatched roof. The covering needs to be comparatively thick and the slope quite steep to avoid water penetration. Water falling on thatch is carried along each stem gradually moving down the slope, but also moving into the depth of the thatch where it is caught by the next stem, but, because of the angle of the stems and the thickness of the thatch, it is also being transported outwards towards the roof surface again. This process repeats down to the eaves, but as the amount of water builds up, it soaks further into the depth of the thatch so the thickness of the thatch and the steepness of the roof is tailored to ensure it all exits at the eaves before soaking through to the interior.

Plain tiling and slating work in a similar fashion, but because the individual tiles or slates are very much wider there are fewer joints and fewer overlapping layers are required: typically, two layers over most of the roof and three at the head lap. Also, because they are comparatively wide (and the width is known), they can be arranged on the roof so that sideways spreading water does not pass over the edges and into the roof. The rate of sideways spread is proportional to the roof pitch – the lower the pitch, the greater the lateral spread – so narrower slates and tiles are always laid at steeper pitches than wider ones. This is the double-lap system where each slate or plain tile acts as a soaker for the joint in the course above and a cover flashing for the joint in the course below.

This is a very efficient system because it is two and three layers thick, but it does require a lot of material.


Many materials have been used for roofing and have given us diverse and beautiful roofscapes. But attractive as they are, their most important attribute is the ability to keep out water and, in this respect, size of the roofing unit – particularly width – is most significant. Size of the individual roofing materials and the way they overlap has most influenced how they have been used. The wider the material, the fewer joints are needed and, on the whole, the lower the pitch of the roof can be. 

If a roof could be covered with a single piece of material with no joints, then as long as the covering remained in good condition, there would be no routes for water to penetrate and only one layer of material would be needed. Mastic asphalt is an example of this sort of roof covering. Many roof coverings comprise smaller-sized components that rely on overlapping layers to catch any water that penetrates through the joints between them and direct it out onto the surface lower down the roof.

An effective single-lap system is the obvious way to reduce the amount of tiling or slating required. This can be achieved using tiles, such as pantiles, Roman tiles or various patent interlocking tiles, that overlap laterally. Water is kept away from the joints between such tiles by raising the overlap above the flow of water or by the interlocking detail that resists water penetration. As the joints are less vulnerable to leaks a lower pitch can be used.

For slates, single lapping can be achieved by using extremely large slates with a narrow soaker beneath the perpendicular joints. The soaker simply has to be wide enough to prevent sideways-spreading water from reaching the edge of the soaker and thus penetrating into the roof. Alternatively, a slate cover flashing can be used to substitute for the overlying course.

There are also the malleable materials: sheet metals and corrugated sheets, which either work by a raised seam system which places the joints above the water, or are designed to ensure water is kept out. How the raised system is applied mainly depends on the size that can be conveniently handled. Stiff and strong materials like sheet iron and steel have traditionally been several metres in length, but softer materials like lead and copper are laid much shorter. However, long strip is commonly used nowadays for copper and zinc due to new laying practices. These materials are laid in bays with the lengths and widths determined by the physical properties and thickness of the material, and the need to accommodate thermal movement. The great advantage with these systems is that they perform well at very low pitches.

If the ‘size’ of a roofing material is unconstrained (that is, when it is amorphous), as with asphalt or mass concrete, then joints can be completely eliminated and the roof covering can be formed in one piece with a pitch of just a few degrees to permit runoff.

This text is taken from the English Heritage book Practical Building Conservation: Roofing.  But it isn’t plagiarised because I wrote the original text.

Image by Peter Brugge

Master Thatchers