Costs: Moisture damage in timber floorings

The 2004 edition of Approved Document C for England and Wales, Site Preparation and Resistance to Contaminants and Moisture, introduced a requirement intended to minimise flooring failures due to moisture damage.

Any area at risk from water damage, moisture or high humidity, particularly kitchens and bathrooms, should have floor decking of a moisture-resistant grade. The extra cost of specifying moisture-resistant flooring is minor compared with the potential cost of repairs, which could run into thousands of pounds if a floor has to be replaced. But “moisture resistant” does not mean “waterproof”.

The critical moisture content level is 20%. Above this level, moisture-resistant flooring can at best only delay decay or delamination where a floor is in long-term contact with moisture. Ideally, having a moisture-resistant floor surface will ensure there is time to rectify the cause of the defect before damage and repair costs escalate.

The most common floor decking solutions are moisture-resistant particleboard (chipboard) and plywood. Alternatives include moisture-resistant oriented strand board (OSB), cement-bonded particleboard (generally used for non–domestic floors), preservative-treated softwood planking, or naturally durable wood planking.

Long-term exposure to high levels of moisture causes reduction in strength and stiffness and increases susceptibility to fungal attack. Design should minimise the risk of moisture accumulation, for example by providing underfloor ventilation. Moisture-resistant decking should be specified in combination with waterproof floorcoverings to provide a “belt and braces” approach. Alternatively, specifying a concrete floor avoids the potential problems associated with timber floor solutions.

Wood–based panel floor decking

The key standards for guidance on wood–based panels are:

• EN 12871 for specification by performance
• draft standard DD EVN 12872 for installation and use (BS 7916 is now withdrawn).

Wood–based panel flooring offers lower capital costs compared with traditional softwood floorboards. The basic material prices are lower and reduced installation times are possible where one 2400 mm × 600 mm sheet is equivalent to at least five softwood planks. However, there are a number of installation issues with wood–based panel boards, which have long-term cost consequences.

A recurring problem with wood–based panel flooring is squeaking. Saving money during the installation by not gluing tongued and grooved edges or skimping on fixings may incur future costs.

Installation is a high-risk period for floor deckings. On the one hand it makes sense to lay the floor to give following trades a safe and firm surface to work from, on the other hand exposing the floor to the elements, wet trades and impacts from tools, equipment or materials can leave the floor decking in poor condition for the final user. Options include laying a temporary floor or providing a protective layer such as hardboard or plastic sheeting.

Particleboard or chipboard
Particleboards should be to EN 312 grades P5 and P7, the two moisture-resistant grades. Grade P7 offers higher load-bearing properties.

Particleboards manufactured from melamine-fortified urea formaldehyde (MUF) adhesives swell up less when in contact with moisture than phenol-formaldehyde (PF) or isocyanate (PMDI) adhesives.

Plywood
Plywood should be to EN 636–2 for use in humid conditions and EN 636–3 for use in exterior conditions. Use enhanced moisture resistance standard marine plywood to BS1088 for very damp areas, such as shower rooms.

Plywoods are generally stronger than particleboards.

Oriented strand board (OSB)
Oriented strand boards should be to BS EN 300. Grades OSB–3 and OSB–4 are moisture resistant, and OSB–4 offers higher load-bearing properties. OSB generally has mechanical properties in excess of particleboard, but higher thickness swell and poorer surface smoothness.

Natural timbers
Softwood floorboards should be BS 1297. Timbers should be naturally durable or preservative-treated as a minimum for EN 335–1 hazard class 2, which describes conditions where the wood is protected from weather but humidity levels are high and there may be occasional but not persistent wetting. BRE Digest 429 offers durability details.

A long-term benefit of floorboards is the ease with which they can be lifted and replaced. However, laying floorcoverings may attract higher costs due to the addition of a hardboard layer and latex screed over the floorboards to achieve a smooth and level surface.