Bowing Walls: Causes and Restraint Solutions

A wall bowing outwards is one of the more dramatic-looking structural defects. The cause is usually mundane — restraint failure between wall and floor — and the fix is well-understood. But ignored, a bowed wall is a wall on its way to falling.

Why walls bow

• Lateral restraint failure: Joists pulling away from wall, no longer pinning it back
• Wall tie failure: Outer leaf no longer tied to inner
• Roof spread: Inadequate ridge or collar restraint pushes wall plates outward
• Foundation settlement: Differential settlement pulls walls out of plumb
• Lintel failure: Localised buckling around windows

How to assess

Hold a long spirit level vertically against the wall:

• Up to 20mm out of plumb over 2.5m: monitor
• 20–50mm: action required, structural assessment
• Over 50mm or visibly leaning: urgent

Better: use a plumb bob from a stable point and measure the deflection at intervals over time.

Restraint options

Lateral restraint straps

Galvanised steel straps that tie the wall to floor joists or roof structure. Cheapest and most common retrofit.

Tie bars (pattress plates)

Decorative cast iron plates either side of the wall, connected by tension rods through the floor structure. Common on Victorian terraces.

Helical bars in bed joints

Stainless steel helical bars bedded in cut mortar joints provide horizontal reinforcement.

External buttress

Brick or concrete buttress against the wall. Permanent, visually obvious. Used where internal restraint isn't feasible.

Rebuilding

For severe bowing, the wall section may need to be taken down and rebuilt. Last resort.

Cost

• Lateral restraint straps: R1,600–R3,600 per wall location
• Tie bar pair: R10,000–R24,000
• Helical bar reinforcement: R4,000–R8,000 per linear metre
• External buttress: R40,000–R160,000+
• Rebuilding wall section: R200,000+

MCFAR assesses wall movement and designs restraint.

Frequently Asked Questions