Thatching

How does Thatching work, step by step?

1. 1

   Step 1: Survey the existing roof and agree the material

   Almost all thatching in the UK is re-covering rather than new build, so the project starts with a survey of what is already there. The thatcher opens up sample areas to establish the tradition used, the depth of the existing coat, the condition of the base coat where one survives, the state of the timbers and the pitch actually achieved. Many old roofs carry several coats laid over one another and a base coat that may be very old indeed and worth keeping. The designer, the thatcher and, on a listed building, the conservation officer then agree whether the roof is re-ridged, spar-coated over what is there, or stripped to the timbers. That decision drives the cost, the programme and the weight the roof structure has to carry.

2. 2

   Step 2: Expose, repair and prepare the roof structure

   Thatch is heavy when wet, and adding a coat adds load, so the timbers are inspected once the covering is opened up. Rafters, purlins, wall plates and the fixing timbers or battens are checked for rot and beetle, particularly at the eaves and around the chimney where water has had the most chances. Repairs are made in matching timber before any new material goes on. On a spar coat the existing surface is dressed back and combed clean so the new work has a sound bed. On a full strip the roof comes back to the structure and the thatcher can see exactly what is being worked with, which is usually the point at which the real scope of the project becomes clear.

3. 3

   Step 3: Set the eaves course and establish the line

   The eaves course is the datum for the whole roof. It is laid first, packed and dressed to the projection the design calls for, and it fixes the line, the overhang and the thickness that every course above will follow. Gable and barge courses are set at the same time. Thatchers work to a generous overhang because the eaves throw water clear of the wall below, and a mean eaves detail puts water down the face of the building for the rest of the roof life. Everything laid afterwards is judged against this first course, so time spent getting it straight and true is time saved over the whole roof.

4. 4

   Step 4: Lay the coatwork in courses up the slope

   Bundles are laid in courses working up the slope, each course overlapping the one below so that only a short length of stem is exposed. In water reed and combed wheat reed the butts face outwards and the thatcher dresses each course up tight with a leggett, driving the butts into line to form a dense, even surface. In long straw the material is laid with the stems running down the roof and is drawn into place rather than driven. Coat thickness is commonly in the order of 300 mm and above, but the depth for a particular roof is the thatcher's and the designer's call, taken from the tradition, the material and the exposure. The thatcher works from a ladder or a light scaffold and moves steadily across and up, keeping the courses parallel and the surface plane true, because a hollow or a bulge in the coat will collect water for the rest of its life.

5. 5

   Step 5: Fix each course back to the structure

   Every course is secured back to the roof, historically with hazel or willow sways held by hooked iron pins or spars driven into the coat below, and on modern work commonly with steel rods and screw fixings into the rafters. The fixings sit within the thickness of the coat and are covered by the courses above, so nothing that holds the roof down is left exposed to weather. Fixing pattern and centres are set by the specialist contractor to suit the material, the pitch and the exposure of the site, and they are not something to lift from another roof. Around the perimeter, at the gables and at the eaves, the fixing is closed up because that is where wind gets under the surface and where a roof most often starts to lift.

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   Step 6: Form the ridge and the details

   The ridge closes the top of both slopes and takes the hardest wear on the roof. It is formed in a more flexible material than the coat, usually straw, either as a flush ridge that follows the line of the coat or as a block ridge standing proud of it, and it is held with liggers and cross rods sparred down into the coatwork. Patterned block ridges are a regional and personal signature and can be a planning matter on a listed building. Chimneys, dormers, valleys and the junction with any adjoining tiled or slated roof are detailed and dressed at the same time, with lead or other flashing worked into the coat where the design requires it. These junctions are where thatched roofs leak, so they get the most careful work on the project.

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   Step 7: Trim, dress and hand over with a maintenance regime

   The finished surface is trimmed and dressed to an even plane, the eaves and gables are cut to line, and wire netting is fitted over the roof where birds and vermin are a problem, though it is not universal and some thatchers argue it holds moisture in. The roof is left to settle. Handover should include a written maintenance regime, because a thatched roof is a maintained roof: gutters and valleys kept clear, overhanging branches cut back, moss and shading dealt with, the ridge inspected regularly and replaced well before it fails, and the coat inspected after any severe weather. The owner also needs to know what the fire precautions on their building are and why they are there.