Lino Ciriaco, ARDEX Group UK training and technical manager, explains how to avoid deflection when preparing the subfloor
Before tiling it’s crucial to ensure your subfloor is not only clean, dry and flat – but also free from deflection. Deflection, otherwise known as “bounce”, needs to be controlled and secured before tiling.
As a general guide, the floor should have minimal vertical deflection under load. Typically for ceramic tiles, the floor should not deflect any more than L/360 of the span of the structure, or L/720 of the span in the case of most natural stone.
However dependent on individual site requirements – including static and dynamic loading – floors may be required to be more rigid than L/360.
When preparing timber floors, extra noggings may be required between joists to ensure the floor can carry the additional static load (the weight of the tiling system) but also the anticipated dynamic loading in service.
Currently, British Standards recommends that: “Where it is necessary to further reduce or eliminate the risk of movement, an additional layer of plywood sheets of minimum thickness at least 12mm, or a proprietary tile-backer board resistant to moisture and thermal movement, should be screwed/bonded over the timber.”
So effectively here, according to British Standards, an overlay board must be 12mm minimum in thickness. Anything less than this, is not seen as sufficient, and will not effectively protect against deflection.
Note that current British Standards state that plywood overlays should be a minimum of 12mm when over boarding existing timber floors.
However please be aware that new British Standard guidance states that timber floors should not be tiled on directly.
This will mean that over boarded timber will need to be overlaid with a further tile backer board or uncoupling mat.
When over boarding timber, the overlay board (i.e. cement, XPS or magnesium oxide such as BAL Board), should be screwed over the timber at 300mm centres or as per the manufacturer’s recommendations – crucially ensuring that the joints in both layers do not coincide.
While best-practice is to install the board in brick-bond fashion, this isn’t always possible, but as long as the joints do not continue through the boards then you will be compliant with British Standards.
While overlay boards can be directly screwed into the timber floors, if there are voids or unevenness in the floors, we would recommend bedding the boards into a layer of flexible tile adhesive.
First, prime the timber (plywood, T&G floorboards or suitable P5 chipboard) with 2 coats of primer undiluted, with the second coat applied at 90° to the first coat.
Once the primer is dried, apply a thin bed of flexible tile adhesive to the floor with a 6mm notched trowel. Bed the tile backer board into the adhesive to achieve solid bed fixing and mechanically screw the board down following manufacturer’s guidance.
It is important to understand that uncoupling mats don’t provide protection against deflection (vertical movement), they are purely designed to protect against lateral (horizontal) movement of subfloors.
This lateral movement is caused by different rates of thermal expansion of the subfloor and the tile. Without such a buffer movement, this tension between subfloor and tile as they expand and contract due to temperature, heat, moisture, shrinkage etc, can cause tiles to crack or de-bond.
That said, installing an uncoupling mat on any floor is highly recommended to remove that risk of lateral movement, which can occur on any subfloor type – whether it’s a screed or timber floor. This is known as the “belt and braces” approach.
The use of S2 adhesives is also not an excuse to fail to prepare against deflection in the subfloor. The “S” classification refers to deformability – the capacity of a hardened adhesive to be deformed by stresses between the tile and the fixing surface – without the loss of adhesion.
It doesn’t necessarily refer to its flexibility – where an adhesive can be “bent” and return to its original form – great for where small movement might occur in the substrate.
In fact an adhesive might be highly flexible and highly-polymer modified, but not actually meet the deformability requirements to be an S1 or S2 adhesive.
While an S2 adhesive can actually be quite soft and thus not be suitable for a highly trafficked area for example.
www.ardex.co.uk
