Prepare a straw bale wall for plaster by trimming the bale faces flat and even with a chainsaw, hedge trimmer or hand shears, confirming the bales are dry, and misting the wall so it does not draw water out of the plaster too fast. Add mesh only where plaster crosses a joint between different materials or has nothing to grip, since the scratch coat pressed into the straw is the main key.
Part of Straw Bale Construction
Surface preparation is the trimming, dampening and reinforcing of a bale wall before the first plaster coat.
The Straw Surface Is The Key, And Mesh Covers Where It Is Missing
Straw bale faces have a natural tooth that grips a scratch coat pressed into the straw. A flat, even face means even plaster thickness, so trimming comes first. Mesh is a different case. The parent article does not call for mesh across ordinary bale faces. Mesh helps where plaster bridges two materials that move differently, such as bale and timber post, or at a corner where the plaster has no straw to grip, because cracks start at those joints. Salvaged chicken wire or wire mesh is the usual material; see Iron Wire if you have to make wire. Earthbag walls differ, since the bag courses give a rough key and mesh is usually unnecessary; see Plaster Systems.
What You Need
- A cutter for trimming: chainsaw, hedge trimmer or hand shears.
- A moisture check on the bales; see Bale Moisture Monitoring.
- A sprayer and water.
- Mesh, fixings and cutters, if the junctions call for it.
- A way to protect wiring, pipes and frames from plaster.
Method: Trim, Check, Mist, Reinforce
Step 1: Check moisture. The wall must be below 20% moisture before plastering. Sealed-in moisture causes rot.
Step 2: Trim the bale faces flat and even, removing loose and protruding straw.
Step 3: Pack gaps between bales and around frames tightly, so plaster does not have to bridge a void.
Step 4: Fix mesh across junctions between bale and frame or timber, if used. Overlap mesh sheets and lash them so there is no loose edge, and keep it tight against the surface.
Step 5: Mist the wall with water just before the first coat.
Common Mistakes
- Plastering over uneven faces. Plaster thickness varies, and thick patches crack.
- Plastering wet bales.
- Mesh left loose. Plaster sags off it.
- Leaving straw exposed around openings; see Plaster as Fire Barrier.
Safety
Cutters, dust and wire ends
A chainsaw or hedge trimmer can kick back and throw straw dust: wear eye and ear protection and a dust mask and keep both hands on the tool. Cut mesh ends are sharp; wear gloves and fold or embed the ends so they cannot catch.
What’s Next
- Learn next: Application Technique
- Combine with: Bale Preparation and Stacking