How to Price Dry Rot Treatment: Labour, Materials and Margin Guide

Quick Answer: Dry rot treatment in the UK prices at £1,500-£4,000 for a contained single-area outbreak and £4,000-£15,000+ for a widespread one involving wall plates, joists, and masonry sterilisation. The chemical treatment is a minor cost; the money is in finding and stopping the moisture source, cutting out all affected timber (plus a 300-450mm margin of sound timber), and replacing structural members. A specialist survey (£200-£450) is essential — misdiagnosing dry rot as wet rot, or vice versa, leads to either over-treatment or recurrence.

Summary

Dry rot (Serpula lacrymans) is the most feared timber decay in UK buildings, and with reason: it can spread through masonry, travel metres from its food source, and re-establish from spores or surviving mycelium if not fully eradicated. Pricing it is genuinely difficult because the visible damage is almost always smaller than the actual spread — strands (rhizomorphs) and mycelium travel behind plaster and through walls where you cannot see them until you open up.

The defining feature of dry rot, and the reason it is more serious and more expensive than wet rot, is that it does not need a continuing water source once established. It generates its own moisture and can move through and over inert materials (brick, plaster, even steel) in search of fresh timber. This is why the standard treatment is so destructive: you cut back well beyond the visible decay, sterilise the surrounding masonry, and isolate any remaining timber with physical or chemical barriers.

This guide separates the survey, the moisture remediation (the actual cure), the strip-out of decayed timber, masonry sterilisation, and the rebuild. It draws the critical distinction between dry rot and wet rot — which are treated completely differently — and explains why a competent diagnosis is worth more than any chemical. For related work see woodworm treatment pricing guide, damp proofing pricing guide and damp survey pricing guide.

Key Facts

Survey and treatment costs

Repair and rebuild costs

Regulatory and standards

Quick Reference Table

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Scope What's Involved Typical Cost Risk Level
Diagnosis only Survey + opening up £350-£800 —
Contained outbreak Fix moisture, cut back, sterilise, minor timber £1,500-£4,000 Moderate
Floor structure affected Joists + wall plate replacement £4,000-£8,000 High
Widespread / multi-room Major strip-out, masonry isolation, rebuild £8,000-£15,000+ Very high
Listed / period property Conservation methods, breathable repair £6,000-£20,000+ Very high

The chemical is typically <10% of the total. The cut-out, replacement, and rebuild dominate the cost.

Detailed Guidance

Dry rot vs wet rot — get this right first

The single most important diagnostic decision. They are different organisms requiring different responses, and confusing them is the most common and most expensive mistake.

TIMBER DECAY DIAGNOSIS
│
├─ Cuboidal cracking (cubes), dry crumbly timber,
│   white/grey cotton-wool mycelium,
│   rust-red spore dust, mushroom smell,
│   spreads OVER masonry ──────────────> DRY ROT (Serpula lacrymans)
│                                          Destructive cut-back + sterilise
│
└─ Decay confined to wet timber, darker,
    stringy or cracked along grain,
    NO spread beyond the damp area,
    stops when dried out ───────────────> WET ROT (Coniophora etc.)
                                           Fix moisture, replace local timber

Wet rot is confined to timber that is actually wet — fix the water, replace the rotted timber, and it stops. It does not justify masonry sterilisation or wide cut-backs. Dry rot spreads beyond the wet zone, generates its own moisture, and demands the full destructive protocol. Pricing wet rot as dry rot massively over-charges the customer; pricing dry rot as wet rot guarantees recurrence and a claim against you.

Step one: find and stop the moisture

Dry rot cannot establish without an initial moisture source pushing timber above ~20% moisture content. The cure starts by eliminating it — a leaking gutter or downpipe, penetrating damp through defective render, a plumbing leak, a failed DPC, or blocked sub-floor ventilation. If the moisture source is not stopped, no amount of chemical will prevent recurrence, and the guarantee is worthless. This is the part inexperienced firms skip and specialists obsess over. Budget for DPC renewal (£60-£120/lin m), gutter/render repair, or sub-floor ventilation (£40-£90 per airbrick) as appropriate.

Step two: remove the decayed timber

All decayed timber is cut out and removed, plus a margin of apparently sound timber beyond the visible decay — BRE guidance is typically 300-450mm past the last sign of fungal growth, because mycelium extends ahead of visible damage. Removed material is treated as contaminated and skipped, not reused. This is £30-£70/lin m for cutting and removal, before replacement.

Step three: sterilise the masonry

Where dry rot has spread into or over masonry, the masonry within the affected zone (and a margin around it) is sterilised with a fungicidal treatment — either spray application or, for thick walls, irrigation via drilled holes. £20-£45/m². The modern view (and the conservation-led view) is that thorough drying is the primary control and chemicals are a supplement, not a substitute — so over-irrigating sound masonry "to be safe" is poor practice and inflates cost. Treat to the extent of the spread, no more.

Step four: replace and isolate

Replacement timber is pre-treated (boron or organic-solvent preservative) and where new timber contacts potentially damp masonry it is isolated — DPC membrane between joist ends and walls, joist hangers instead of built-in bearings, or treated bearer details. New wall plates sit on a fresh DPC. This isolation is what prevents the next outbreak. Replacement runs £40-£90/lin m for joists and £45-£100/lin m for wall plates installed.

Step five: rebuild and guarantee

Re-plastering after sterilisation (£25-£50/m²), reinstating floors, and redecoration follow. For mortgage and resale purposes, an insurance-backed guarantee (20-30 years) from a PCA-member contractor is usually required — price it as a separate line. Be clear the guarantee covers re-treatment of the fungus, not future moisture defects the owner allows to develop.

Period and listed buildings

Traditional solid-wall buildings rely on breathability. The aggressive chemical-and-replace approach can do more harm than good in a listed building — trapping moisture, replacing sound historic timber, or introducing impermeable materials. The conservation approach prioritises drying, breathable repairs, and minimal intervention. These jobs need a conservation-accredited specialist and often a heritage-aware structural engineer; costs are higher and timelines longer.

Frequently Asked Questions

How do I know it's dry rot and not wet rot?

Dry rot produces cuboidal cracking (the timber breaks into brick-like cubes), white cotton-wool mycelium, rust-red spore dust, a distinctive mushroomy smell, and — crucially — spreads over and through masonry away from the damp source. Wet rot stays in the wet timber and stops when it dries. If in doubt, get a specialist survey; the treatment cost difference is so large that a £300 survey is trivial insurance.

Can dry rot come back after treatment?

Yes, if the moisture source isn't permanently fixed or if mycelium/spores survive in untreated masonry. This is exactly why proper dry rot treatment is destructive and thorough: stop the water, cut well past the visible decay, sterilise the masonry, and isolate new timber. A reputable insurance-backed guarantee exists precisely because recurrence is the risk being underwritten.

Why is dry rot so much more expensive than woodworm?

Woodworm is usually a surface spray of accessible timber. Dry rot requires finding and stopping a hidden moisture source, cutting out and replacing structural timber, sterilising masonry, and rebuilding — often opening up walls and floors to chase the spread. The chemical is a small part; the structural repair and the destructive investigation are what cost.

Do I need a structural engineer?

If load-bearing timber — joists, beams, wall plates, roof members — is affected, yes. An engineer confirms what can be repaired versus replaced, specifies replacement sections, and signs off the rebuild. Budget £350-£600 for an engineer's visit and calculations. For listed buildings, add a conservation specialist.

Regulations & Standards