Amanda Searle, Head of Technical Services at Norcros Adhesives, offers advice on fixing decorative stone finishes. In this issue she discusses fixing natural stone to timber floors.

Timber substrates are not the most ideal surfaces to receive a rigid tiled finish but are, of course, commonplace in houses.

Before starting the tiling, ensure that the timber is strong enough and rigid enough to support the additional weight of the stone (which will exert a force of up to 0.8kN/m² or 81.5kg) without excessive deflection.

The easiest method of checking the rigidity of the timber is the ‘Meniscus test’. Just fill a glass to the rim with water and place it on the floor. Stand one metre from the glass and jump up and down. If the water spills out of the glass you can assume the floor needs strengthening.

British Standard BS 5385: Part 3 recommends that new timber bases are constructed with noggings between the joists. Water and boil proof (WBP) exterior grade plywood at least 15mm thick should be screwed to both joists and noggings at no more than 300mm intervals. Make sure all junctions between boards are supported by either noggings or joists. To reduce the risk of movement further, it is a good idea to screw an additional layer of plywood at least 10mm thick on to the boards at 300mm centres, making sure the joints of the two layers do not coincide.

On existing floors not constructed to the above standard, the boards should be removed and the floor stiffened with noggings and joist support sleeper walls. Alternatively, the floor may be strengthened by overlaying the existing boards with WBP exterior grade plywood at least 15mm thick screwed at no more than 300mm centres to both joists and existing boards.

To avoid shrinkage defects, the moisture content of the timber sheets and boards should be close to the equilibrium moisture content that prevails under service conditions. In heated domestic buildings where the temperature is above 21ºC, the moisture content of the timber sheets should be between 8% and 12%.

In conservatories, on heated timber sub-floors and areas with high temperatures or lower humidity levels the moisture content value of the timber sheets may need to be around 8% or lower.

It may be necessary to pre-condition the timber to the appropriate moisture content level before installation.

It is also essential to ensure that there is adequate ventilation beneath the boards and that an effective damp-proof course has been installed.

The design of the installation should take into account the initial drying shrinkage of the timber and subsequent movement due to seasonal moisture changes, with consideration being given to the type of heating used.

Prior to installation, the plywood sheets should be sealed with a styrene butadiene rubber (SBR) primer on the back, face and edges. This will prevent the ingress of moisture and subsequent distortion of the boards due to atmospheric humidity.

The safest option when fixing natural stone tiles to timber floors is to lay an anti-fracture membrane such as Norcros Permalayer on to the timber before tiling. Tiles may then be fixed using a suitable polymer modified (BS EN 12004 C2) cement-based adhesive such as Norcros Thick Bed Stone & Porcelain adhesive.

Joints between the tiles should be filled with a flexible (BS EN 13888 CG2) cement-based grout. Movement joints should be provided according to the recommendations in British Standard BS 5385:Part3, which in most domestic situations is going to involve an expansion joint around the perimeter of the floor and where floor tiles abut fixed machinery and structural fixtures. Intermediate movement joints should be incorporated into larger floor areas, as described in BS 5385:Part3, Clauses 19 and 23.6.