Repair plaster cracks wider than 3 mm or running vertically in summer by cleaning out loose material, wetting the edges and packing matching plaster in layers, earth plaster inside and lime plaster outside, and curing lime damp for several days. Fill hairline cracks in earth plaster with a thin slip coat, and when more than 20% of a wall face is damaged, re-plaster the whole face instead of patching.
Part of Building Maintenance Schedules
Plaster crack repair is the summer restoration of a wall’s plaster skin, which keeps rain out, protects straw and earth from decay and, on straw bale, acts as the fire barrier.
The Plaster Is The Wall’s Waterproofing And Its Fire Protection
The maintenance guide’s summer method has five steps: clean out cracks and remove loose material with a pointed tool; wet the crack edges; pack with fresh plaster mix, earth plaster for interiors and lime plaster for exteriors; for large cracks or missing sections build up in layers, a scratch coat first, left to cure, then a finish coat; and cure lime slowly by misting for several days. It adds that if more than 20% of a wall’s plaster is damaged, re-plastering the whole face is better than patching, since patchwork never looks or performs as well as a unified coat.
On straw bale walls the plaster also carries the fire rating, which depends on there being no exposed straw anywhere (Plaster as a Fire Barrier). A crack that exposes straw is therefore both a moisture path and a fire path.
Worked Example: Patch Or Re-Plaster
Inputs: a wall face 5 m by 3 m = 15 m²; the 20% rule; coats of 15-20 mm each, the thickness the earth plaster guide gives for interior coats (Earth Plaster, Interior); three coats, the system both plaster guides use. The lime guide does not state its coat thickness, so applying 15-20 mm to lime is an assumption.
- The patching limit: 15 × 0.20 = 3 m² of damage.
- Plaster for a full three-coat re-plaster: one coat is 15 × 0.015 = 0.225 m³ to 15 × 0.020 = 0.30 m³; three coats are 0.68 to 0.90 m³.
- A patch of 0.25 m² (a hand-wide strip 2.5 m long) in one coat takes 0.25 × 0.02 = 0.005 m³, 5 L.
Twelve such patches would reach the 3 m² limit, so the rule says to stop patching at about that point and do the whole face.
What You Need
- Matching plaster: earth plaster for interiors, lime plaster for exteriors, never cement on bale walls (Lime Plaster, Exterior, mixed 1 part lime putty to 2.5-3 parts sharp sand)
- A pointed tool, a brush, a trowel and a sprayer
- Water, for wetting the edges and curing
- Protection for eyes, hands and skin when working with lime
- Shade, cloth or damp sacking to protect fresh work from sun, wind and frost
Method: Repair In Summer
Step 1: Find the cause first. A crack that is moving, or that is wet at the foot, is a settlement or water problem; fix that before the plaster.
Step 2: Clean the crack to sound material with the pointed tool and brush out dust.
Step 3: Wet the edges so the old plaster does not draw the water out of the new.
Step 4: Pack the crack firmly with matching plaster, pressing it into the straw or earth; for a wide crack or a missing area build up in layers: scratch coat, cure, then finish coat.
Step 5: Cure lime damp, misting it for several days and shading it from sun, wind and frost.
Step 6: Check the finished repair for cracking in a week, and record the repair in the log.
Common Mistakes
- Cement patch on a lime or earth wall. It traps moisture and cracks at the edges.
- Dry edges. The patch debonds.
- Rapid drying. The lime dusts instead of hardening.
- Filling hairlines in lime plaster with heavy mix. Lime self-heals fine cracks by carbonation (Lime Mortar).
Safety
Wet lime burns, and ladders fall
Wet lime has a pH around 12-13 and burns skin and eyes, sometimes without pain at first. Wear eye protection and gloves, and if lime gets on skin or in an eye, flush with plain water for at least 30 minutes (Chemical Burns). Work from a footed ladder or a stable low platform, not from an unsecured pile.
What’s Next
- Learn next: Foundation Drainage Checks — stopping the water that cracks walls
- Combine with: Wall & Foundation Care — the whole wall routine