How Does a Pile of Straw Keep Rain Out? Water Running Down Stems

Thatch sheds water by orientation rather than by any barrier. Stems are laid with their butt ends outward and their length running up the slope, so water landing on the surface runs down the outside of the stems and off the eaves, penetrating only a few centimetres into a covering that may be a third of a metre thick. That requires a steep pitch, since a shallow roof lets water travel further into the material before it leaves, and roofs below about forty five degrees fail. It requires the stems to be straight and parallel, since any that lie across the flow carry water inward. And it requires the thatch to be packed densely, since loose material admits water between the stems.

What is available locally has determined regional styles:

• •Water reed, durable and stiff, lasting longest and giving a hard-edged appearance
• •Wheat straw grown long and threshed carefully to keep the stems intact
• •Combed wheat reed, which is straw prepared so the butts face outward uniformly
• •Heather, sedge, rush and turf, used regionally where those were what grew
• •Sedge for ridges, since it bends without breaking
• •Modern crop varieties bred short and harvested by machine, which destroyed the traditional straw supply

The technique is specific and the details determine how long the roof lasts. Bundles are laid in courses from the eaves upward, each course overlapping the one below so that only the last few centimetres of each is exposed. They are fixed to the roof timbers or to the previous layer with hazel spars, rods and twisted ties rather than nails. Each course is dressed into position with a tool that beats the butts flush and aligned, which is what produces the even surface and what packs the material densely. The ridge is finished separately with material that can be bent over the apex and is the part that wears fastest. On some roofs, new material is laid over old, which is why very old thatch can be over a metre thick.

A thatched roof is a habitat as well as a covering, which is part of its charm and part of its maintenance problem. Insects colonise the material, and birds pull at it to reach them and to take material for nests, which is why netting is fitted over many roofs and is disliked for its appearance. Rodents nest in it where the eaves admit them. Several specialised insects live in old thatch and some are scarce enough to be of conservation interest, which occasionally conflicts with renewal. Moss and algae grow on shaded northern slopes and hold moisture against the surface, which shortens the life of the material beneath. A steep pitch, good airflow and keeping overhanging trees cut back address most of this.

Thatch persisted where the alternatives were expensive and disappeared where they were not. It is made from an agricultural by-product, requires no transport of heavy materials and needs no specialist supply chain, which made it the default roof for most buildings in most of Europe until slate and tile became cheap with industrial transport. It burns, which is why it was prohibited in cities after major fires and why insurance costs more now. It requires skilled renewal every few decades. The craft survives as a heritage speciality with a small number of practitioners, protected in many places by planning rules requiring like-for-like replacement, and the shortage of suitable long straw is now a real constraint on that.

Stems laid along the slope direct water down their outside rather than through the material, so penetration is a few centimetres into a covering a third of a metre thick, provided the pitch is steep and the packing dense. Water reed, long wheat straw, heather and sedge were used according to what grew locally. Modern short-strawed crop varieties destroyed the traditional supply.