The roof of a straw bale building should project at least 45 cm beyond the wall face and preferably 60 cm or more, so rain is kept off the plaster, which eventually lets water through if it is rained on constantly. In wet weather, protect the bales during construction too, either by building the roof first on a post-and-beam frame or by covering the stacked courses every night.
Part of Straw Bale Construction
The overhang is the “good hat” that keeps rain off a bale wall, and the construction cover is the same idea during the build.
Plaster Sheds Rain, But Not Forever
Lime plaster is weather-resistant, but a wall that is rained on hard and often will eventually let water through, and water inside a bale wall is the hidden-rot problem. The overhang is the cheapest protection: it keeps most rain off the plaster, and it keeps the splash zone away from the wall base. The parent article’s numbers are a minimum of 45 cm, preferably 60 cm or more.
The overhang is measured horizontally from the finished plaster face to the edge of the roof, so a thick plaster coat on a thick wall shortens the effective projection if the roof was set out to the bale line. Allow for plaster when you set the rafters out. For roof construction see Roof Systems and Thatching.
What You Need
- Rafters or a roof frame that projects beyond the wall on all sides, including gable ends.
- A drip edge, gutter or open ground that carries roof water away from the stem wall.
- Tarps or sheeting for covering stacked bales.
- A load-bearing wall or a frame ready to carry the roof; see Wall Systems.
Method: Hat First, Where You Can
Step 1: Choose the system. Post-and-beam infill lets the roof go on first, with the bales protected under it.
Step 2: Set out the overhang at least 45 cm beyond the plaster face, and 60 cm or more where the design allows.
Step 3: Carry roof water clear of the stem wall and the splash zone.
Step 4: In load-bearing construction, cover the courses with tarps at night and in rain; avoid stacking bales in rain.
Step 5: Check after the first storm for water running down the wall.
Common Mistakes
- Short eaves on the windward side.
- Measuring the overhang from the bale line and forgetting the plaster.
- Roof water landing on the stem wall.
- Leaving bales uncovered overnight.
Safety
Roof work and wet bales
Working at eaves height is a fall risk; use a stable scaffold or platform. Wet bales are heavy and slippery to stack, and a wet stack can lean; brace the wall until the roof is on.
What’s Next
- Learn next: Bale Moisture Monitoring
- Combine with: Waterproofing and Fire Safety