What is thatching reed?
Thatching reed is a grass that grows in water or on wet, fresh to brackish ground at the water’s edge, and which is used in dried form as a roof covering. The stems grow 1 to 3 metres tall, are delivered in bundles and are fixed to the roof in thick layers. Botanically this is common reed, Phragmites australis, often sold as water reed.
Reed is light, insulating, vapour-open and long-lasting if maintained. It gives a roof a character no other material matches, and it is a renewable material.
Constructions
There are three basic build-ups, with very different behaviour:
- Open roof — reed laid directly on horizontal battens, so the roof breathes fully. The centuries-old method, suitable only for unheated spaces such as a barn
- Closed roof with a ventilated cavity — an insulated, vapour-tight roof deck, then a ventilated air space, then the reed. The reed can dry from both sides, which helps its life
- Screwed roof — the reed is fixed with wires and screws directly onto a closed, insulated deck, with no cavity. This is the fire-safe solution, but the reed can only dry outwards
Life and roof pitch
Pitch is the single biggest factor in how long a thatch lasts. The Dutch trade federation for thatchers takes 45 degrees as the lower limit: below that, expect a shorter life. Rules of thumb from practice:
- Around 30 degrees: roughly 25 to 30 years
- From 45 degrees: broadly 20 to 40 years, averaging just under 30
- At 50 degrees and steeper: 30 years and more
North-facing slopes, shaded pitches and roofs under overhanging trees last less well, because the reed stays wet longer.
Points to watch
- Ventilate all round. Cut trees back; reed that cannot dry will rot
- Fire safety. Building regulations impose requirements on thatched roofs, including distances to boundaries. A screwed roof or a fire-resisting underlay is usually the answer. Involve your insurer early
- Cost. For a new thatched roof, expect something in the order of 100 to 200 euros per square metre, depending on build-up, roof shape and region
Related terms
- Thatching
- Roof pitch
- Mortar
- Facade
- Renovation
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