A bond beam or top plate is a continuous timber or reinforced beam laid along the top of an earthbag wall so that roof loads spread evenly along the wall instead of landing on a few bags, and so the top courses are tied into one ring. Size it from the roof load, and test the tie between beam and wall before the roof goes on.
Part of Earthbag Building
The bond beam is the ring at the top of the wall where roof framing meets earthbag courses.
A Roof Is A Set Of Point Loads Until Something Spreads Them
Rafters or roof beams bear on the wall at intervals. Without a continuous member, each bears on a few bags, which can crush or shift, and the top courses are free to spread. A bond beam solves three things at once: it spreads the loads, it ties the top courses into one ring, and it gives the roof framing something solid to fasten to.
Two roof effects matter. Wind can lift a roof and pull on its connections, which is the subject of Wind Loads. Sloping roof rafters can push the tops of the walls outward unless the ring holds them. A continuous, well-lapped beam resists both better than separate pieces.
What You Need
- Continuous timber in lengths long enough to lap at corners, or a reinforced beam if cement or lime concrete is available. See Concrete Making.
- Fasteners to tie the beam into the top courses, such as wire from the barbed wire courses or spikes.
- The roof framing plan, so you know where loads land; see Roof Systems.
Method: Spread, Tie, Check
Step 1: Finish the top course level, tamped and wired like every other course.
Step 2: Lay the beam continuously around the top of the wall, lapping or joining pieces at corners and in the length so there are no breaks.
Step 3: Tie the beam to the wall so it cannot slide or lift. Tie wire from the upper courses to the beam is the common way. Make a test with a short run before committing.
Step 4: Fasten roof framing to the beam, not to the bags.
Step 5: Check the connection. Push and lever on the beam. If it moves relative to the wall, add ties.
Common Mistakes
- Roof bearing directly on the bags. Loads concentrate and crush the top courses.
- A beam in short unjoined pieces. It does not tie the ring together.
- Tying the roof to the beam but not the beam to the wall. In a high wind the whole assembly lifts.
Safety
Working at height on an unfinished wall
The top of an earthbag wall is not a safe platform until the beam is in place and tied. Do not load the wall with roof framing until the beam is fixed, and brace the framing against wind while it is being erected.
What’s Next
- Learn next: Finishing and Protection — plaster once the roof is on
- Combine with: Timber Framing — if the roof is a timber frame