8.1 — The Fundamental Principle: Surface First, Product Second

One of the most common mistakes made by inexperienced operatives is selecting a biocidal product first and then applying it to whatever surface the mould is growing on. The correct approach is the reverse: identify the substrate, assess its condition and porosity, determine whether treatment in situ is viable, and only then select the appropriate product and method.

This matters because different surfaces respond very differently to moisture, biocidal chemicals, and mechanical agitation. A method that works well on ceramic tiles will damage plasterboard. A product safe for masonry may bleach timber. A treatment that kills surface mould on a painted wall may leave mycotoxins embedded in the substrate beneath.

8.2 — Plasterboard (Drywall)

Plasterboard is the most commonly affected substrate in UK residential and commercial properties, and it is also the most problematic to remediate. It is highly porous, with a paper facing that mould colonises rapidly, and a gypsum core that absorbs moisture and provides an ideal growth medium.

Why Plasterboard Is Difficult

• The paper facing is organic — mould digests it directly, causing structural weakening
• Mould hyphae penetrate the paper and into the gypsum core within days of germination
• Surface treatment kills surface mould but cannot reach hyphae embedded in the core
• Mycotoxins produced by embedded mould cannot be neutralised by biocidal treatment — they remain in the substrate
• Wet treatment adds moisture to an already moisture-affected material, potentially accelerating deterioration

Treat In Situ vs Remove and Replace

In Situ Treatment Method for Plasterboard

• HEPA vacuum the surface to remove loose spores before applying any liquid treatment
• Apply a quaternary ammonium or hydrogen peroxide-based biocide by low-pressure spray — do not saturate
• Allow full dwell time as per product SDS (typically 10–30 minutes)
• Do not scrub — agitation releases spores and damages the paper facing
• Allow to dry fully before applying a mould-resistant encapsulant primer
• Encapsulant seals residual spores and provides a barrier against future growth
• Redecorate only after full drying and encapsulation

8.3 — Timber

Timber is the second most commonly affected substrate in mould remediation, particularly in older UK properties with solid timber joists, rafters, window frames, and skirting boards. Timber presents different challenges to plasterboard: it is more structurally significant, more variable in porosity, and more susceptible to damage from certain biocidal chemicals.

Surface Mould vs Wood Rot

It is critical to distinguish between surface mould growth on timber and structural wood rot (decay caused by wood-rotting fungi such as Serpula lacrymans — dry rot — or Coniophora puteana — wet rot). These are different organisms requiring different interventions. Surface mould on timber can be remediated; structural wood rot requires specialist treatment and often timber replacement.

Treatment Method for Timber

• HEPA vacuum loose surface mould before applying any liquid treatment
• Use a fungicidal timber treatment product — boron-based or quaternary ammonium formulations are appropriate for most timber species
• Apply by brush or low-pressure spray — ensure penetration into the grain
• Allow full dwell time; do not rinse
• For painted or varnished timber: lightly abrade the surface first to allow product penetration, then treat
• Allow to dry fully before repainting or varnishing
• For exposed structural timber (joists, rafters): treat all accessible faces, not just the visible contaminated face

8.4 — Masonry (Brick, Block, Concrete, Render)

Masonry surfaces are generally more robust than plasterboard or timber and are better candidates for in situ treatment. However, porous masonry can harbour mould deep within the surface, and efflorescence (salt deposits from moisture movement) is often confused with mould growth.

Distinguishing Mould from Efflorescence

• Mould: Fuzzy or powdery growth, various colours (black, green, grey), biological — will test positive with mould test kits, has musty odour
• Efflorescence: White crystalline deposits, no odour, not biological — caused by soluble salts migrating to the surface as moisture evaporates. Requires different treatment (salt neutraliser, not biocide)

Treatment Method for Masonry

• Brush off loose surface growth with a stiff brush — collect debris and bag as contaminated waste
• Apply a masonry biocide (quaternary ammonium or sodium hypochlorite-based products formulated for masonry) by brush or spray
• Allow full dwell time — typically 15–30 minutes for masonry products
• Rinse with clean water where specified by the product SDS
• Allow to dry fully — masonry retains moisture; do not redecorate until moisture readings confirm dryness
• Apply a masonry biocidal paint or anti-mould coating as a preventive measure

8.5 — Grout, Silicone, and Tile Surfaces

Bathroom and kitchen surfaces — ceramic tiles, grout lines, and silicone sealant — are among the most common locations for mould growth in UK properties due to persistent high humidity. These surfaces are non-porous (tiles) or semi-porous (grout) and are generally the most straightforward to treat, with one important exception: silicone.

Ceramic Tiles

• Non-porous — mould grows on the surface only, not within the material
• Apply a bathroom biocide or quaternary ammonium spray, allow dwell time, agitate with a brush, rinse
• Tiles are robust — stronger mechanical agitation is acceptable compared to plasterboard or timber

Grout

• Porous — mould can penetrate the grout surface
• Apply biocide, allow extended dwell time (15–20 minutes), agitate with a grout brush
• If mould is deeply embedded and grout is discoloured throughout its depth: regrout rather than treat
• Apply a grout sealer after treatment to reduce future porosity

Silicone Sealant

Silicone is the most problematic bathroom surface for mould remediation. Mould grows within the silicone matrix — not just on the surface — and cannot be killed by surface biocidal treatment. Once mould has penetrated silicone, the only effective solution is removal and replacement.

8.6 — Soft Furnishings and Carpets

Soft furnishings — carpets, upholstery, curtains, mattresses — present some of the most challenging remediation scenarios. They are highly porous, difficult to dry thoroughly, and often harbour mould deep within the fibres and backing materials where biocidal treatment cannot reach effectively.

Decision Framework for Soft Furnishings

8.7 — HVAC Ducting and Ventilation Systems

Mould within HVAC (heating, ventilation, and air conditioning) systems is a specialist area that most general remediation operatives should not attempt without specific training and equipment. However, understanding the risks and knowing when to refer is essential.

Why HVAC Mould Is High Risk

• A contaminated HVAC system distributes spores to every room it serves — it is the most effective mould dispersal mechanism in a building
• Mould within ductwork is often invisible and can only be confirmed by internal inspection with a duct camera
• Treatment requires specialist duct cleaning equipment (rotary brush systems, HEPA-filtered extraction) and biocidal fogging
• Incorrect treatment can dislodge large quantities of spores and spread contamination throughout the building

What You Can Do

• Inspect and clean accessible ventilation grilles and covers — these can be removed, cleaned with biocide, and replaced
• Replace HVAC filters — contaminated filters are a significant spore source and should be replaced as part of any remediation job in a property with HVAC
• Advise the client that internal ductwork inspection and cleaning is required and refer to a specialist duct cleaning contractor

8.8 — The Bleach Myth

Household bleach (sodium hypochlorite) is the most commonly used DIY mould treatment in the UK — and one of the least effective for professional remediation. Understanding why bleach fails is important both for your own practice and for explaining to clients why professional treatment is different.

Key Terms — Module 8

Module 8 Summary

• Always identify the substrate before selecting a remediation method — surface type determines approach, not the other way around.
• Plasterboard is the most problematic substrate: treat in situ only for minor surface contamination; remove and replace where mould has penetrated the core or where Stachybotrys is suspected.
• Timber requires fungicidal penetrating treatment — not bleach. Distinguish surface mould from structural wood rot and refer dry rot to specialists.
• Masonry is generally robust but porous — treat with masonry-specific biocides and address moisture ingress behind render or paint.
• Silicone sealant cannot be effectively treated once mould has penetrated — remove and replace.
• Soft furnishings with mould in backing, foam, or wadding must be disposed of — mattresses always.
• HVAC duct mould is a specialist area — refer internal duct cleaning to a specialist contractor.
• Bleach is not a professional remediation product — use BPA-registered biocides with defined dwell times.

Version 1.0 — August 2026