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Structural failure of timbers in thatched roof

We deal with a considerable number of thatched properties around the country. The first question we are always asked is'what condition is the thatch - and what is it?'.

There are three main types of thatch:

Water reed, or march reed - which has a life of up to 40 years in good conditions. Has a very smooth, precise appearance when laid.

Combed wheat straw - estimated life of 20 to 30 years.  This is cereal straw that has passed through a comber, but not a threshing drum.

Long straw - estimated life of 15 to 20 years. This is threshed wheat straw, moistened and drawn by hand. It has a shaggy appearance when laid.

Ridges are much higher wear areas - expect to re-ridge every ten years.

Thatch is often 'top dressed' using a spar coat - leaving the base coats intact. This means it can build up considerably in thickness over the years - this is often a clue as to the age of the building - we've seen some houses which still have the original thatch in place under nearly a metre of top coats.  One issue that arises is that chimneys, which were way above the top of the thatch are now much closer to the ridgeline, with potential for fire damage. 

The question of what condition the thatch is in, and how long it will last for is usually left to the experts. We will always put ballpark figures on re-thatching to help with budgets, but sometimes advise clients to obtain a detailed report and costings from a reputable thatcher. The National Society of Master Thatchers are a good place to start - they have a register of members, and publish good advice notes about fire risk management for example, together with a newsy website - the Thatch Advice Centre.  We've good contacts with one or two thatchers who work all over the country, so don't let a thatched roof be a problem - properly maintained, they are a joy to live with. As a guide, the last roof we looked at needed at least partial re-thatching, but in the end we decided to completely re-thatch, together with extensive repairs to the timber frame. Overall cost is just short of £30,000 - considering the size of the building, I think a very fair price.

Timbers within the roof of thatched properties can be quite delicate. Often they are split poles of hazel or birch - no more than 2 1/2" in diameter, with the lower layers of thatch tied to them with cord - made from oakum soaked in stockholm tar and twisted into a rope.  Sometimes the underside of the thatch is coated with daub, and painted with limewash - indicating that the roof space was occupied for living.  These timbers can be very delicate, and often beetle frassed - as they are so thin, beetle living under the bark when the wood was cut would have eaten much of the timber before it dried out. The fact is that the roof is still secure, and unless there is obvious movement, we leave well alone. A more common problem is for purlins to break - often the purlins which carry the rafters are poor quality timber, and become brittle over the years - especially if they are elm.  I've seen numerous examples of shattered purlins allowing the thatch to gradually subside into the roof space.  I'm not keen on the modern practice of ripping out these historic timbers and replacing with modern treated timber - unless absolutely necessary, we won't advise this. The old SPAB strapline of 'repair, not replace' is best here.

Collapsing timbers of a thatched roof

The main purlin here has cracked and is collapsing - above it, rafters have also cracked and are falling in. The battens have rotted and are falling out. Outwardly, the roof looks in good condition - a metre of thatch covers the roof and is hardly indented. Other suveys had missed the condition of timbers, as there was no access to the roof space.

Split hazel poles used as rafters of thatched roof

To the right, split hazel poles have bent out of shape and are collapsing - the battens have been almost completely eaten away by beetle over hundreds of years, and the whole pitch is about to collepse. There is almost no evidence of this from outside - a metre of straw thatch looks almost pristine.

Elm rafter split and broken in thatched roof

Another rafter of this roof is elm - eaten away by beetle and split. The roof is nearly ready to collapse, despite looking almost perfect from outside.

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