Whole life costing: Floor decking

Peter Mayer

How much does it cost to ensure your timber flooring complies
with regulations on preventing moisture damage.
Peter Mayer of Building LifePlans compares various
specification options

Introduction


The 2004 edition of Approved Document C Site preparation and resistance to contaminants and moisture for England and Wales introduced a requirement intended to minimise floor 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 a moisture-resistant grade of flooring is minor compared with the potential cost of repairs, which could run into thousands of pounds if the 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 as well as increasing susceptibility to fungal attack. Design should minimise the risk of moisture accumulation, for example by providing under floor 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• draft standard DD EVN 12872 for installation and use (BS 7916 is now withdrawn)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 2400mm x 600mm sheet is equivalent to at least five softwood planks. However there are a number of installation issues with wood–based panel boards which do 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 a far from perfect 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, are the two moisture-resistant grades. Grade P7 offers a higher load-bearing properties.

Particleboards manufactured from melamine-fortified urea formaldehyde (MUF) adhesives increase significantly less in swelling 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. For 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 Grade OSB–4 offers higher load bearing properties. OSB generally has mechanical properties in excess of particleboard, but has 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 floor coverings may attract higher costs due to addition of a hardboard layer and latex screed over the floorboards to achieve a smooth and level surface.

 

 

Specification options

 

(Note: An allowance for surface protection during installation is included in the capital costs.) Capital cost
£/m²
Net present value for 60 years£/m² Design life
Years
Particleboard      
Particleboard wood chipboard to BS EN 312 Type P5. 8.4 50 60
Particleboard wood chipboard to BS EN 312 Type P5, 22mm thick. 9.7 51 60
Particleboard wood chipboard to BS EN 312 Type P5, with protective plastics peel-off layer. 8.9 45 60
Particleboard wood chipboard to BS EN 312 Type P5, with thermosetting resin and kraft paper weather proof surface. 8.8 37 60
Plywood      
Standard grade marine plywood to BS 1088-1, square edged, suitable for use when exposed to regular wetting or permanent exposure to water: Hazard Classes 4 or 5 in BS EN 335-3. 18.5 50 60
Plywood to BS EN 636-3, suitable for use in unprotected external conditions: Hazard Class 3 to BS EN 335-3. 16.5 54 60
Plywood to BS EN 636–2 15.3 59 60

 

 

Table notes


• Costs relate to 18mm thick, tongued-and-grooved floor decking, for use in humid conditions: durability hazard class 2 to BS EN 335 only, unless otherwise stated. Cost issues relating to floor structure, thermal and sound insulation, fire, floor coverings and floor types (suspended or floating) have been excluded and assumed to be to good practice standards.

• Replacement of floors is based on a risk factor for each flooring option – not all floors are replaced.
• Nominal allowances have been allowed for protection (£0.5/mÏ) during installation where necessary and removing and replacing fittings (£2000 for each replacement event) during replacement which has been modelled at 0.5% probability.

• Best value should be determined from a whole life assessment of floor decking options which satisfy project specific criteria.

• A discount rate of 3.5% is used to calculate net present values.

First published in Building 2005

 

 

 

Further information


BLP provides latent defect warranties for buildings www.blpinsurance.com

Further information contact peter.mayer@blpinsurance.com or telephone: 020 7204 2450.

 

 

 

 

 

 

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