Roundwood Trusses and Roof Details

CONSTRUCTION DRAWINGS

Here are a few ideas for framing a ridged roof in roundwood. Usually the ridge is supported by the gable (end) walls, but
the design used in the Gatehouse is more like a hipped roof, with the ridge held aloft by the rafters themselves.
The framing for the compound curve of the limpet roof is quite complex and relies on steel cables for
transferring forces to the inside edge of the wallplate. However the ridge itself is simple in concept and execution, well suited for first time efforts.

In general roof construction is similar for all forms of building. In strawbale the box girder wallplate takes the place of
the simple timber bearer more usually found at the top of cob or stone walls. We built this roof frame at ground level
without fixings, then dismantled it, built the walls and re-assembled it on top. Ideal for straw, where a quick covering is
essential in a wet climate. Also saves on scaffolding and the need for volunteers with a head for heights! However
most roofs will probably be just as easily built in situ.


click image to enlarge

Rooting the base of the rafters firmly in place is essential to prevent the roof collapsing under its own weight, usually by spreading apart at the feet, which opens up the joints and even allows rafters to fall freely.

The technique most frequently used to prevent this is collaring, fixing a horizontal stretcher between the rafters, either at the base as here, or half-way up as with A-frames. Cruck frames are either anchored into a floor-level baseplate, the base of stone walls or the ground itself.


click image to enlarge

In the heavier construction typically found in house framing, I prefer to rely on joinery first, with fixings as a backup, and to prevent incidental movement, rather than resist the main forces. Applying a little thought can soon determine the direction of forces in a typical roof, and designing your joints to compress under those same forces ensures a sturdy, silent framing.

The size and strength of individual timbers is determined by the expected loading (including snow). These timbers are somewhat heavier than usual because they are carrying an extremely heavy load of wet soil, approx 8 tonnes in winter. The walls themselves can carry in the region of 400 tonnes as strawbale is remarkably strong in compression.


click image to enlarge

Here is a drawing of the main forces acting on the roof of the Gatehouse. Strawbale or Cob houses need a good pair of boots and wide hat. Because of the great weight of a wet living roof, I used secondary rafters to create the hat (overhang) deriving the main strength of the roof from the triangular configuration of the inner primary rafters. This also allowed rafters to bear on both inner and outer sides of the wallplate, evening the loading on the walls, an especially important consideration when using straw.

The secondary rafters could easily be greatly extended onto columns to provide extra storage/workshop space under the same roof.

This drawings are added to periodically. Please feel free to contribute any of your own.

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