Repair rotten, sagging or damaged floor and roof timbers safely

Cutaway of a house roof and floor timbers with a rotten joist end spliced in teal, a new purlin and rafter foot, and a skip

For owners of homes with soft or sagging floors, rotten joist ends or damaged roof timbers. Describe what you have found and compare quotes from structural engineers who design timber repairs.

Timber floors and roofs usually fail for a reason. Damp can get into joist ends (the ends of the timbers a floor sits on) where they rest in a wall. A leak can rot a purlin (a main roof beam). Beetles can attack, a joist can be cut away for pipes, or the timber may carry loads it was never meant to. The result may be a bouncy floor, a sagging ridge, a bowed rafter or a floor that slopes. A structural engineer works out which timbers have lost strength, why, and the least disruptive way to put it right: splicing in new timber, fixing a plate alongside, replacing a section or forming new supports. They work with a timber or damp specialist on the cause, and give your builder a design to follow.

How a professional can help

Finding what has actually failed: They check joist ends, the timbers on top of walls that carry the roof (wall plates), rafters and purlins. They probe the timber and lift boards to find which timbers are sound, which are weakened and how far the problem extends.
Joist-end repairs: Rotten joist ends in a damp wall can often be cut back and supported with new timber, a steel shoe or a bonded splice, without taking up the whole floor. They design the detail and the new bearing.
Notched and drilled joists: Where pipework or cabling has cut deep notches or large holes, they work out whether the joist is still strong enough. Where it isn’t, they design a plate or an extra timber alongside it.
Sagging ridges and roof beams: They investigate sagging ridges, spreading rafters and rotten wall plates. They design repairs such as new ties, props, supports for the main roof beams or replacement timbers, and check what the roof rests on.
Woodworm and fungal decay: They explain when beetle holes are old and when an attack may still be active. They work with a timber and damp specialist, so the structural repair and the treatment of the damp or decay fit together.
Replacing and reinforcing: Where timbers are overloaded or can’t be saved, they design replacements or reinforcement. That might be steel plates bolted to the timber, deeper joists or doubled rafters, sized for the loads they now carry.

Is this right for you?

Your floor feels springy, slopes or has soft patches, and you suspect rot, beetle or overloading.
A damp or timber survey has found rotten joist ends or wall plates and recommends structural repair.
A plumber or electrician has cut away part of a joist, or an earlier alteration has weakened the roof.
You are buying a period property and the survey mentions sagging roof timbers.

How the project usually runs

Inspect the timbers: The engineer lifts boards, checks where timbers rest on walls and probes them, measuring sag and span, and looks at the floor or roof from above and below for signs of moisture.
Identify the cause: They work out whether the damage comes from leaks, damp, beetle, overloading or an old alteration and, where needed, bring in a damp or timber specialist to deal with the cause.
Repair details: Details are drawn for each repair, such as new supports in the wall, splices, plates, replacement timbers or props, with sizes, fixings and any temporary support needed while the work is done.
Timbers repaired in sequence: The builder carries out the work to the design, normally one section at a time, with the floor or roof propped while timbers are cut out. The engineer can inspect hidden stages.
Finishes put back: Finishes are replaced once the repairs have been checked, and you receive a short note of what was done and where, which is useful for a later sale.

What affects the cost

How many joists, rafters, roof beams or wall plates need repair, and how much of each.
Whether the underlying cause, such as a leak, damp wall or poor ventilation, also needs fixing first.
How much of the floor or ceiling must be lifted to reach the timbers, and the finishes to be put back.
The amount of temporary propping needed while damaged timber is cut out and replaced.
Specialist timber or damp treatment, which a timber specialist prices separately from the structural repair.
Whether the building is period or listed, where matching timber and traditional details can add to the work.

Approvals for timber repairs and strengthening

Like-for-like repair often doesn’t need approval, but replacing or strengthening floors and roofs can be controlled structural work, so ask building control or your engineer.
Repairs to timbers in a listed building may need listed building consent and usually call for matching materials, and a conservation area can add controls on external changes. Check with your council before starting.
Building Regulations, building warrants and heritage rules differ across England, Scotland, Wales and Northern Ireland. Structural engineers cover the whole UK and work to the rules where you are.
In England and Wales, cutting out and replacing joist ends built into a wall you share with a neighbour can fall under the Party Wall etc. Act 1996. Ask a party wall surveyor before you start.

Questions people ask

How do I know if a joist is rotten?

Signs include a bouncy or sloping floor, timber that is soft or crumbling when probed, a musty smell and damp walls. Joist ends hidden in walls often rot first, so the problem can be out of sight. A structural engineer can inspect, probe and sometimes lift boards to see how far the damage has gone. Until then, keep heavy loads off any floor that is soft or visibly sagging.

Can a rotten joist end be repaired, or does the whole joist need replacing?

Often only the end needs repair. The rotten section is cut back to sound timber and supported with new timber, a metal shoe or a bonded repair. Replacing the whole joist is more disruptive and is used where decay is widespread or the joist is also overloaded. The engineer decides which suits your floor.

Do I need to fix the damp before repairing the timber?

Yes. Repairing rotten timber without finding the source of moisture, such as a leak, poor drainage outside or blocked ventilation under the floor, means it is likely to rot again. A timber or damp specialist can deal with the cause while the engineer designs the structural repair, and the two need to be coordinated.

Is woodworm a structural problem?

Many beetle holes are old and the beetles are no longer active, and the timber is often still strong. Heavy active attack can reduce strength, particularly in thin or already stressed timbers. An engineer can say whether the structure is affected, and a timber specialist can advise on whether treatment is needed.

Can I cut a joist for pipes or cables myself?

It isn’t a good idea. Joists carry load along their length, and notching or drilling in the wrong place can weaken them. Ask an engineer before cutting. If cuts have already been made, an engineer can check the joist and design a repair if one is needed.

Available across the UK

Professionals cover the whole of the UK: England, Scotland, Wales and Northern Ireland. Planning and Building Regulations differ between the four nations, and your professional will work to the rules that apply where your property is.