Joist Strengthening Methods and Costs

Springy floors, sagging ceilings, and noticeable deflection under load all point to undersized or weakened joists. Several reinforcement options exist; the right one depends on access, headroom, and how much disturbance is acceptable.

Causes of joist inadequacy

• Original sizing for lighter use (e.g. ceiling joists pressed into use as floor joists in loft conversions)
• Excessive span without intermediate support
• Notching for plumbing or wiring (common DIY mistake)
• Wet rot or beetle damage
• Change of use bringing higher loads (e.g. installing a bath where there wasn't one)

Strengthening options

Sister joists

New timber joists bolted alongside existing. Cheap, effective when access allows.

• Pros: Simple, low cost
• Cons: Doubles ceiling depth, awkward in occupied rooms

Steel flitch plates

Steel plate sandwiched between two timber joists, bolted together. Excellent stiffness, modest depth increase.

• Pros: Strong, dimensionally efficient
• Cons: Heavy to install, requires drilling timber accurately

Steel beam underneath

RSJ installed under existing joists, mid-span, creating an intermediate support. Reduces effective span.

• Pros: Doesn't disturb floor above
• Cons: Reduces ceiling height below

FRP (fibre reinforced polymer) strips

Carbon fibre strips bonded to underside of joist. Lightweight, no disruption above.

• Pros: No structural depth added, fast install
• Cons: Specialist installer, more expensive per metre

Replacement

For severely undersized or damaged joists, full replacement is sometimes cheaper than reinforcement.

Cost benchmarks

• Sister joist (per joist): R1,600–R3,600
• Flitch plate (per joist): R4,000–R10,000
• Steel beam underneath (mid-room): R30,000–R90,000
• FRP strip per linear metre: R8,000–R14,000
• Full joist replacement (room): R70,000–R160,000+

Local Authority Building Control

Structural alterations to floor joists need calculations and Local Authority Building Control approval.

MCFAR specifies joist strengthening for renovations and change-of-use projects.

Frequently Asked Questions