TN 07.24 Deflection of Sub-Floors and Rigid Finish

INTRODUCTION

All rigid finishing materials will be affected by any movements arising from the supporting substrates. Rigid coverings such as tiling are unforgiving and require a substrate with minimal deflection as possible to guarantee success. Although a floor may feel solid underfoot, deflection may still be present, even if no bounce is felt. The basic rule is that if the finished tile surface span deflects more than the calculated maximum deflection amount, the tiled surface will fail.
Note: Deflection is more of an issue with floors that with walls, however walls can still be subject to loads. In these situations, if the wall background is not prepared correctly, a tiling finish will fail.

CALCULATING DEFLECTION

Allowable deflection is expressed as a fraction of the span. We take a nylon string and anchor in position across the longest span of the area. Ensure that the string is tight. In the centre of the room, carefully measure the distance between the string and the floor. Next, place an estimated load* in the centre of the room & re-measure the distance between the string and the floor. See table below for allowable deflection calculations for different areas. If the distance exceeds the maximum requirements, the floor will need to be made stiffer.

*Estimated load = the ultimate weight the floor might have to support, including the floor installation.

The accepted minimum requirement before ceramic tiles are installed is L/360 (L/720 for natural stone finishes) e.g. If the span of the joists is 3.05m between supports, when a load of 135kg is applied, the deflection between the centre and the end should not exceed 7.6mm.

Guidance may be found from the Tile Council of North America (TCNA) & in BS 5385-3 on the acceptable deflection requirement for a rigid ceramic tile and natural stone finish.

However, in the draft of the National Annex to BS EN 1990 (Eurocode – Basic of Structural Design) the table for vertical deflections advises a value of ≤ L/500 for rigid flooring e.g ceramic tiles.

The TTA technical document of Internal Ceramic Tiling to Sheet and Board Substrates advises that in practice “Failures at L/600 have been observed. It is for this reason that recommendations for floor rigidity are not solely based on deflection measurements but on empirically established methods found to work over normal construction”.

SUBFLOOR PREPARATIONS

The aim of subfloor preparations is to stabilise and flatten the floor to stop it from moving to reduce stresses affecting the tile installation. This is best achieved using a combination of the following:
• Reinforcing floor supports / joists – ensure the joists are strong enough to take the additional weight of a tiled floor. Ensure joists are fully secured, tightly packed into the wall junctions. Add noggins between the joists and at strategic points (e.g. beneath where 2 sheets of plywood meet) to add strength.
• Secure the subfloor – ensure that all the floorboards are securely screwed down to the joists, ready for over-boarding.
• Over-boarding – over-boarding the floor with a product to stabilise the area “as a whole” and provide a rigid base for tiling.
• Uncoupling - Adding an uncoupling layer that acts as a buffer, absorbing lateral movement in the floor so that it is not transferred into the tiles.

OVER BOARDING OPTIONS
Over boarding the floor or wall stabilises the area and provides a strong and smooth surface to install tiling. 

Over boarding with a Tile Backer Board – Floor
Providing the existing timber floor is rigid and stable, the theory to over-boarding with tile backer boards is to provide a thermal and moisture stable background, suitable for tiling. In this case, this can be achieved with the use of a thinner board (e.g. BAL BOARD 6mm) meaning reducing any threshold steps.

BS 5385-3:2024 advises for new timber floors in clause 9.4.4.2 that; “Where it is necessary to further reduce or eliminate the risk of movement…or a proprietary tile backer board resistant to moisture and thermal movement should be screwed/bonded over the plywood at 300mm centres so that the joints in both layers do not coincide”
When installing BAL BOARD ensure that:
•Prime all required faces
•Using a 6mm notched trowel, apply a thin bed of flexible tile adhesive to the floor
•Install the board in a broken bond or brick pattern
•Screw at 300mm centres
•Board joints must be reinforced using BAL SCRIM TAPE before tiling

For other proprietary tile backing boards, always follow the manufacturers installation guidelines.
Further advice can also be found in BAL Technical Note TN 23.23.

ANTI-FRACTURE UNCOUPLING

An uncoupling system cannot compensate for deflection of the floor (vertical movement) but can accommodate some limited lateral movement (horizontal movement) from being transmitted from the subfloor through to the tiling layer above. This help to reduce the risk of reflective cracking, lifting/debonding of tiles.

It is recommended that an anti-facture uncoupling layer be installed to a floor that has underfloor heating in the assembly, areas of direct sunlight, areas of large temperature variations to address the affects of thermal expansion and contraction.
• Apply a flexible, BAL cement-based adhesive using a 4 mm notched trowel.
• Place the matting into the freshly applied adhesive, observing open times. (Do not test adhesion by pulling the matting back out).
• All joints must be butt jointed but not overlapped or folded.
• Tiling onto the matting can begin immediately using the appropriate BAL adhesive for the tile and background.
• Adhesive bed thickness above uncoupling mat must not exceed 6 mm.
• Uncoupling mat must be separated where movement joints are located in the background so that the joint can be continued through to the tiled surface.

UNDERFLOOR HEATING

High levels of stress are applied to hard materials when they experience expansion and contraction. Therefore, particular care needs to be taken when using rigid floor coverings with underfloor heating systems. The thermal expansion of the rigid floor and the heated sub-floor should not be too dissimilar otherwise excess stresses will affect the tiling during the heating and cooling cycles.

As well as consideration of an anti-fracture uncoupling system, the correct provision of movement joints needs to be made. Use adequately robust pre-formed movement joints.

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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