TN 23.23 Tiling to Timber Floors

INTRODUCTION
Before laying tiles on any timber base there are several factors which have to be taken into consideration, and these design considerations are detailed in Section 6 of the current British Standard Code of Practice BS 5385: Part 3: 2014, clause 6.3.4. The appropriate mechanical properties required for floor finishes are discussed in section 6.3.4.1 of this document.
There are several timber boards on the market, which are not suitable to receive ceramic tiling. Always check the suitability of the board to receive a rigid ceramic tile finish.
The design of the floor should take into account the initial drying shrinkage of the timber and subsequent movements due to seasonal moisture changes in mind the type of heating.
Timber floors of faulty construction have often behaved unsatisfactorily for a period of years as a result of surface evaporation of moisture.
The laying of floor tiles on a timber floor may also provide a more impervious covering, and in these circumstances the moisture content of timber may rise sufficiently to create conditions suitable for fungal attack (e.g. dry rot) to occur.
Before fixing commences it should be established that the ventilation is adequate and that effective damp proof courses are correctly located.
New Timber Floors
New timber bases should be designed not merely to carry the additional dead load (weight of the tiling system) but also the anticipated static/dynamic loading in service i.e. in order provide a stiff floor. Noggins may be required to fix between joists to ensure rigidity.
In particular:-
1. An appropriate sheet or board of a minimum of 15mm is recommended in BS 5385: Part 3: 2014.
2. The boards should be conditioned to adjust to the site conditions i.e. a moisture content appropriate for the site conditions
3. The lower face and edges are sealed e.g. with BAL BOND SBR to prevent moisture atmospheric moisture ingress and thus reduce risk of distortion/warping.
BS 5385 : Part 3 : 2014 recommends in clause 6.3.4.2 “If it is considered necessary to further reduce or eliminate the risk of movement, an additional layer of sheets of minimum thickness 10mm, resistant to moisture and thermal movement, should be screwed over the plywood at 300mm centres ensuring that the joints in both layers of sheets do not coincide”.
Note: The BS 5385 standard does not specifically recommend 10mm thick plywood overlays onto new timber bases

Existing Timber Floors
BS 5385: Part 3: 2014 recommends “Existing timber bases to be covered by tiles should be checked to ensure that they are sufficiently strong and rigid. They should be examined to determine whether they can carry the additional dead load of up to 0.8kN/m², and the probable dynamic loading, without excessive deflection. Where possible, existing
boards should be removed and the floor stiffened with noggings and joist support sleeper walls before following the recommendations for new timber bases”
PREPARATION
Method 1 – Timber Overlay
Some existing floors may require strengthening, e.g. with an overlay of exterior grade WBP plywood, to BS EN 313-2 or marine grade plywood to BS 1088.
BS 5385: Part 3: 2014 recommends “The required rigidity might be achieved by fixing WBP exterior grade plywood of 15mm minimum thickness over existing boards” and “screwed to joists and existing boards at 300mm intervals”.
Prior to fixing, seal the underside and all edges of the plywood with BAL BOND SBR. Lay sheets with cross-joints staggered and a 0.5 to 1mm gap between boards. Fix with countersunk screws at 300mm centres with screw heads set flush with the surface.
Method 2 – Direct Fixing - Plywood
Sheets or boards should be of a suitable, moisture resistant or exterior grade (as detailed above) and be screw fixed to seasoned timber joists at 300mm centres maximum, including cross noggings, if required, to ensure a stable deflection free floor. It is important to ensure that the edges/junctions between the boards are fully supported and that the reverse side and edges of the board are sealed using BAL BOND SBR to prevent distortion from atmospheric changes. Prime the surface to be tiled with undiluted BAL PRIME APD. Always check the suitability of the board for the intended application with the manufacturer.
Method 3 – Direct Fixing- T&G Floorboards
Where the floor is considered rigid enough to allow direct fixing, all traces of previous finishes (e.g. stains, varnishes) should be thoroughly removed by sanding prior to tiling.
T & G floorboards should then be primed with 2 coats of undiluted BAL PRIME APD and allowed to dry prior to the laying of tiles.
ADHESIVE
Fixing of Tiles to a stable Timber Floor (For Methods 2 & 3)
When fixing directly to a stable single layer of timber tiles may be laid in a solid bed of BAL-SINGLE PART FASTFLEX adhesive keeping the final bed thickness to the minimum that the tile and floor will allow, ideally a 3-4mm final bed thickness beneath the tiles.
Fixing of Tiles to an Overlaid Timber Floor (Method 1)
Alternatively, for timber overlay floors (Method1), BAL FLEX ONE or BAL RAPID FLEX ONE adhesive can be used. Tiles may be laid in a 3-6mm solid bed of adhesive, ensuring no voids are left beneath the tiles. The use of an uncoupling mat (e.g. BAL RAPID MAT) should also be considered.

GROUTING
Once the adhesive selected has set and dried thoroughly, joints, minimum 3mm wide, between the tiles may be filled using:
Method 1
BAL GROUT FLEX WIDE JOINT or BAL MICROMAX3 ECO (Method1)
Method 2
BAL MICROMAX3 ECO with BAL ADMIX GT1 at a dilution of 2 parts water with 1 part BAL ADMIX GT1 by volume (Methods 2 & 3).
Check for any potential risk of staining of tiles. Consideration should be given to the use of BAL PROTECTIVE SEALER prior to grouting especially with porous type tiles.
MOVEMENT JOINTS
Movement joints will be required in these installations. These joints should be a minimum of 6mm in width and to the full depth of tile and adhesive bed, and joints may be filled around perimeters and in light traffic areas using BAL MICROMAX SEALANT.
Movement joints should be incorporated as outlined in British Standard BS 5385-3, Clauses 6.8 and 7.1.6. Briefly, this document requires that joints be located: -
a) Over existing and/or structural movement joints.
b) Around the perimeter of the floor and where tiling abuts columns, curbs, steps and plant fixed to the base.
c) In large floor areas tiles should be divided into bays of size not greater than 10m x 10m intervals.
d) On suspended floors the bay size should be reduced and additional joints provided over supporting walls or beams.
e) For floors subjected to significant thermal changes tiles should be divided into bays of size not greater than 40m² with an edge length not greater than 8m.
Note: Rubber-based materials may cause staining of silicone sealant. It will therefore be necessary to insert a bond breaker, e.g. PTFE tape to isolate the two products.
In shower areas/wet rooms the floor should be tanked. This is to ensure no passage of water to the underlying substrates. This may be achieved using BAL WATERPROOF 1C or BAL TANK IT.

TECHNICAL ADVISORY SERVICE
For free expert guidance on the use of BAL products, or any aspect of ceramic tiling with BAL products, contact the BAL TECHNICAL ADVISORY SERVICE on 01782 591120.
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NOTE
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