How do I know if my roof timbers are failing?

Look for 'sagging' or dips in the roofline from the outside. Inside the loft, look for bowing rafters, damp patches (rot), or small holes with piles of 'frass' (dust), which indicate active woodworm infestation.

Do I always need a structural engineer?

If you are replacing or modifying any structural elements—like cutting rafters for a Velux or removing a load-bearing purlin—you **must** have a structural engineer calculate the loads to ensure the roof doesn't collapse. This is also required for Building Regulations approval.

What is 'Sistering' rafters?

Sistering is a cost-effective repair where a new, sound timber is bolted directly alongside a damaged or weak rafter. This avoids the labor-intensive process of removing the old rafter and stripping the entire roof.

The Structural Core

Roof Timber Repairs 2026

Your tiles and slates are only as good as the timber skeleton underneath. In 2026, addressing structural issues like sagging, rot, or woodworm is a critical part of a long-lasting roof replacement.

Structural Roof Timber Engineering

Repair Rates (2026)

Individual Rafter Replacement£250 – £550

Purlin Strengthening (per m)£35 – £50

Structural Engineer Fee£250 – £500

Full Timber Treatment (Spray)£600 – £1,200

*Timber prices have stabilized in 2026, but labor remains the primary driver. Extensive structural work often requires scaffolding.

Common Issues

Roof timbers generally fail due to moisture ingress or biological attack. Early detection can save thousands.

Wet/Dry Rot: Fungal attack that eats timber fibers.

Deflection (Sagging): Caused by undersized timbers or heavy tiles.

Woodworm: Beetle larvae that tunnel through the wood structure.

The "Overweight" Trap

One of the most dangerous mistakes is replacing old slate with heavy concrete tiles without strengthening the rafters. Slate is much lighter; adding concrete tiles to a slate roof structure can cause the roof to bow or even collapse over time. Always verify load capacities before switching materials.

Fast Structural Fixes

'Sistering' rafters can often be done from inside the loft, saving on scaffolding costs.

Long-Term Safety

Fixing timbers during a re-roofing project ensures your home remains structurally sound for 60+ years.

Resale Value

A survey that shows healthy, treated timbers is a massive plus for potential home buyers.

Roof Timber FAQ

Factor in Timber Repairs

Our 2026 calculator allows you to add allowance for structural timber work so you have a realistic budget for your project.

Use the Calculator