Timber decay and structural repair
Decayed timber loses strength quietly until a floor bounces or a roof sags. We assess structural timber, diagnose the decay and design the repair, for owners and builders across the UK.
How do I know if decayed timber is a structural problem?
It depends on how much load-bearing section has been lost and where. Surface decay on a large member may be superficial, but rot at a joist or beam end, where the bearing reaction is highest, can remove most of its capacity. We assess the residual section, not just the appearance.
Diagnosing the decay
Structural timber fails for a small number of reasons, and telling them apart decides the repair. Wet rot, the most common, needs a persistent moisture source and stays local to it, softening and darkening the wood. Dry rot is more serious because it can spread through damp masonry well beyond the original wet timber, and it needs the source found and the full extent traced, not just the visibly rotten piece cut out. Beetle attack, from common furniture beetle to the more damaging deathwatch beetle in old oak, shows as flight holes and frass and matters where it has reduced the section that carries load.
We assess how much strength has actually been lost, which is the structural question behind the biological one. Surface decay on a large section may be superficial; a small joist with rot at its bearing may have lost most of its capacity where it matters most. We probe suspect timber, check moisture levels, look for the source of water, and judge the residual section, so the diagnosis distinguishes cosmetic surface damage from genuine structural loss.
Where timber decay bites structurally
The critical locations are the ones exposed to moisture and carrying load at the same time. Joist and beam ends built into external or damp walls are the classic failure point, because that is where water reaches the timber and where the bearing reaction is highest. Wall plates, lintels, bressummers over old openings, roof feet and purlin bearings are the others. In older buildings, embedded timber bonding and lintels hidden in masonry decay unseen until movement above reveals them.
The consequences follow the load path. A rotten joist end loses its bearing and the floor drops or springs; a decayed beam over an opening lets the masonry above crack and sag; a failed roof foot lets the rafter slip and the roof spread, pushing the wall out. Recognising which of these is happening tells us not just what to replace, but what else to check, because the visible symptom is often one step removed from the decayed timber causing it.
Designing the repair
The repair has to do two things: stop the decay recurring and restore the structural capacity. Stopping it means fixing the moisture source first, because new timber put into a wet pocket will simply rot again. Restoring capacity is a structural design, and the choice runs from splicing a new timber section onto sound wood, through resin and reinforcing-bar repairs that preserve historic fabric, to replacing the member in timber or steel where decay is extensive. We size the repair to the Eurocodes so the mended structure carries its loads with proper margin.
In older and listed buildings we lean towards conserving as much original timber as possible, cutting back to sound wood and splicing or resin-repairing rather than wholesale replacement, which suits both the building and the conservation requirements. We set the diagnosis, extent and repair detail out in a report a builder can price and build from. This work is provided across the UK, with remote assessment from photographs and moisture data plus a coordinated site visit for European clients.
What we check.
The points our calculations resolve for a project like this.
- Distinguishing wet rot, dry rot and beetle attack, and tracing the full extent
- Judging residual section and actual loss of load capacity, not just visible damage
- Critical locations: joist and beam ends, wall plates, roof feet and purlin bearings
- Finding and fixing the moisture source before any new timber goes in
- Repair choice: splice, resin repair, or replace in timber or steel
- Conserving original fabric in older and listed buildings
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
How do I know if decayed timber is a structural problem?
It depends on how much load-bearing section has been lost and where. Surface decay on a large member may be superficial, but rot at a joist or beam end, where the bearing reaction is highest, can remove most of its capacity. We assess the residual section, not just the appearance.
What is the difference between wet rot and dry rot structurally?
Wet rot stays local to a persistent moisture source. Dry rot is more serious because it can spread through damp masonry well beyond the original timber, so the repair must trace the full extent and address the source, not just replace the visibly rotten piece.
Can decayed timber be repaired without full replacement?
Often yes. We can splice new timber onto sound wood, or use resin and reinforcing-bar repairs, which is especially useful in older and listed buildings where conserving original fabric matters. We design whichever restores the capacity with proper margin.
My floor bounces. Could that be timber decay?
It can be. Bounce or a dropped floor often traces to a joist end that has lost its bearing to rot in a damp wall. We check the bearings and the residual section, find the moisture source, and design the repair.
What does a timber decay assessment cost?
Assessment and a repair report start agreed once we know the extent. Tracing dry rot through masonry or opening up hidden bearings can add to that and is agreed with you before we proceed.
Related in structural assessments & specialist services.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
Start a project