Cyril Potter, Chief Chemist at Ardex UK

[deanotile]

Sub-floor preparation for ceramic floor tiling

Cyril Potter, Chief Chemist at Ardex UK looks in detail at sub-floor preparation for ceramic floor tiling.

Any preparation work carried out on sub-floors is intended to provide a surface that is clean, dimensionally stable, rigid and sufficiently flat so that the ceramic floor tile can be well adhered and provide a hard wearing and durable floor finish.
Some of these requirements may not be in the control of the tile fixer, especially the requirement for dimensional stability of concrete and cement sand screeds.Rigidity of the sub-floor can be improved by stiffening the construction of the sub-floor and steps can be taken to achieve the required flatness and cleanliness.

This article deals with tile fixing with adhesives and the Codes of Practice state that where adhesives are used to fix floor tiles the sub-floor should be sufficiently flat so that when checked with a 2 metre straight edge any gap under the straight edge does not exceed 3mm between points of contact with the sub-floor, i.e surface regularity 1 (SR1).SOLID SUB-FLOORS.In the case of new concrete or screeded sub-floors the required drying out times may not have elapsed when the tiler is expected to commence fixing.

At site temperatures of 20C and a relative humidity of 65% a new concrete slab requires at least 6 weeks drying after curing and a new screed requires at least 3 weeks drying after curing.These times should be extended under poor drying conditions, e.g cooler and poorly ventilated conditions.In most cases other trades install concrete and cement sand screeded sub-floors but in some areas the tile fixer may install the screed.

In the latter case the tile fixer should ensure that the freshly laid screed meets the surface regularity requirement and is covered with an impervious sheet for about 7 days to prevent drying and then allowed to air dry for at least a further 2 weeks. If these drying time are not available then the use of a rapid drying cement and sand screed will allow the ceramic floor tiles to be installed after 24 hours.

When new concrete floor slabs or cement and sand screeds are installed the surface will have surface laitance and lime bloom on the surface.In many cases this may not be visually evident but if surface laitance and lime bloom is not removed then the ceramic floor tiles will not be adhered to the concrete or screed surface but to these cohesively weak materials.
Surface residues, contamination and other barriers to adhesion should be removed using appropriate mechanized equipment that is available from plant hire firms and select the quietest and least dust generating equipment if working in occupied premises.

Primers do not replace the correct preparation of the concrete or screed surface! Calcium sulfate based screeds require special treatment and for guidance on testing and preparation, when fixing floor tiles on this type of screed a guidance document from the Tile Association is available.
Note that these gypsum plaster screeds should not be used in locations where they could subsequently become wet.
Where heated screeds are to be tiled it is necessary to pre-heat the screed before the ceramic floor tiles are fixed and there are requirements for movement joints at specified spacing in the screed and possible use of metal mesh reinforcement in the screed.Concrete and cement and sand screeds that previously had other types of floorings adhered to them can sometimes present problem since the old adhesive residues may be very well adhered and difficult to remove.

If it is possible, the best procedure is to remove all surface residues and expose a clean surface.If this is not possible then the tiler should check with the adhesive manufacturer as to the best procedure.In some instances an appropriate primer will adhere to the old adhesive residues, or the adhesive residues can be covered with an appropriate smoothing compound before the tiles are adhered.In these cases the tile fixer should use his past experience for assessing that the adhesion of the old adhesive residues will provide adequate restraint for the floor tiles.

[deanotile]

FLOORING GRADE ASPHALT.These sub-floors can be difficult to assess for suitability as a base for ceramic floor tiles.Flooring grade asphalt will slowly deform under stress and is normally laid over a scrim or separating membrane so that it is not restrained.If there are signs of cracks in the asphalt then these are likely to cause problems.

Avoid the use of asphalt as a sub-floor for ceramic floor tiling where the tiling will be exposed to a large range of temperatures.If the asphalt is sound then the procedure will depend on the surface finish, prime smooth asphalt before fixing the tiles, direct fix on asphalt with a sanded finish providing the sand is clean and well adhered.

Cleaning the surface of the flooring grade asphalt with an alkaline cleaner and thoroughly rinsing off with clean water appears to be effective.There are risks associated with fixing tiles on external asphalt, as external asphalt is usually a roofing grade, designed to function as a waterproof membrane and remain deformable at high and low temperatures.Where it is trafficked it usually has an additional layer of paving grade on top, except for a 150mm perimeter of roofing grade asphalt, however fixing tiles to this type of sub-floor is still a risky operation.

[deanotile]

SUSPENDED TIMBER SUB- FLOORS.Where wood based sub-floors are to be tiled the tile fixer has to ensure that the sub-floor is rigid and at the correct moisture content.The first requirement can give problems if the sub-floor has to be strengthened to achieve a rigid base for the ceramic floor tiles.Fixing new plywood to stiffen the existing sub-floor will raise the floor level and this can cause problems where the new tiled floor abuts existing floors.

The recommendations given in the BS Code of Practice are to ensure that the plywood sheets and boards are conditioned to the correct moisture content and then the backs and edges should be sealed with a suitable sealer to prevent the ingress of atmospheric moisture.Note that an adhesive specifically recommended for use on wood based sub-floors should always be used and any priming recommended for use with the adhesive should be applied to the upper unsealed plywood surface.

The main cause of problems on wood based sub-floors is the difficulty in conditioning any new wood based sheets or boards to the moisture content they will have once the floor is in use.The problem applies to new build construction and refurbishment work where the new sheets and boards may be stored under conditions of high humidity so that the boards take up moisture vapour and expand.If the tiles are fixed whilst the boards have too high a moisture content cracking of the tiles can occur.

Guidance on the suggested moisture contents of wooden sub-floors and boards to receive ceramic tiling under different heating conditions are given in The Tile Association guide 'Tiling to Timber Sheets and Board, Timber Substrates and Alternative Products'.The lowest moisture content of the boards is necessary where under tile heating is used on a wooden sub-floor.Wood based boards with piped integral heating should always be brought up to operating temperature to ensure that they are of appropriate dryness when the ceramic floor tiles are subsequently fixed.

When fixing plywood sheets and boards over existing floorboards it is also good practice to check that the old floorboards are well screwed down and rigid.It is always advisable to pre-smooth the existing floorboards with a suitable latex smoothing compound to ensure that the new plywood sheets fit flush so that no voids are present where localised movement under loads can occur.When fixing plywood sheets try to use one large sheet if possible or take steps to minimise the number of board joints.If 2 boards are necessary then try to set out the tiling so that the tile joints coincide with the joint between the boards.

If movement occurs at the plywood board joint, from subsequent drying of the plywood, replacing the grout along one joint is easier than replacing a cracked tile.The plywood boards should be laid broken bonded and running in the opposite direction to the joints between the existing floorboards.Where large areas of tiling are required on existing wooden sub-floors it is better to utilise some of the proprietary intermediate substrate materials specifically designed for use on wood based sub-floors.

[deanotile]

EXISTING FLOOR FINISHES.These can be divided up into those that are frequently suitable to receive ceramic floor tiles and those that should be avoided.The floorings that should be avoided include those containing linseed oil, e.g linoleum and some composition block floorings.Resilient floorings, wood block floorings (parquet), Magnesite (magnesium oxychloride on direct to ground sub-floors) and other floorings that are insufficiently rigid or will deteriorate once covered with an impervious flooring.

The frequently suitable floorings include existing ceramic floor tiles, terrazzo tiling, hard resin based floorings.These will only be suitable if they are solidly and strongly adhered to a hard rigid sub-floor and can be suitably prepared.When tiling on top of old ceramic floor tiling thorough cleaning, combined with a degree of abrasion, will be necessary to remove any surface deposits.Obviously any movement joints present in the old floor tiling will have to be brought through the new tile bed.

Terrazzo tiling is frequently installed using a semi-dry bedding technique with the terrazzo tiling laid in panels separated by pre-formed strips.The condition and soundness of the terrazzo tiling should be assessed and care taken to ensure that no differential movement occurs at terrazzo tile joints and at the joints with the pre-formed strips.Ceramic tiles can be adhered to some well-adhered, hard resin based floorings and coatings however advice should be sought from the adhesive manufacturer on any preparation work that will be necessary.Check that the resin based flooring or coating is not designed to repel contamination as this can cause difficulties in achieving adhesion.

A simple water droplet test should show if the surface is easily wetted by water.If the droplet sits on the surface as a globule and does not spread out then the surface might require specialised preparation.The basic requirements are to ensure that a clean, dimensionally stable, rigid and sufficiently flat sub-floor is provided as a base for the ceramic floor tiling.In some cases the use of mechanised equipment such as dust-free contained shot blasting, scabbling or surface grinding may be necessary.

If priming is recommended the primer used should be the one recommended by the adhesive manufacturer.Correctly installed ceramic floor tiling should provide in most domestic situations a floor finish that will last for 40 or more years.It is therefore important to carry out the correct surface preparation so that this durable flooring will meet the expected performance.

[TilingLogistics]

Great posts Dean

Kev

[atec]

excellent posts deanotile, if putting in undertile heating, after using a self leveling compound, is it best to put tile adhesive on with straight edge trowel or ribbed, reason i ask is recently i have been informed that a ribbed trowel forms hot airpockets that in turn will lift the tiles, to be honest i have never heard this before.

[deanotile]

excellent posts deanotile, if putting in undertile heating, after using a self leveling compound, is it best to put tile adhesive on with straight edge trowel or ribbed, reason i ask is recently i have been informed that a ribbed trowel forms hot airpockets that in turn will lift the tiles, to be honest i have never heard this before.

In my opinion the tiles should be fixed using a solid bed notched trowel

[atec]

yeah a notched trowel would be better than ribbed, must have been thinking of condoms or something LOL

[DEtileD]

You put tiles down with condoms? Wow...maybe I should start selling those instead of trowels.

[Branty]

Great post's Dean.
And all together. Nice one Cyril, nice one son, nice one Cyril........................







P.S you couldn't put the post in to clearer sentences, just to make it a bit easier to read?

[Dave]

Great info Dean....cheers mate....


p.s spaced text out for you..hope that looks better..

[deanotile]

Great info Dean....cheers mate....


p.s spaced text out for you..hope that looks better..

Thanks m8 look’s much better.

[nybor62]

great post

[TF Ed]

thats weird that post coming up. i have just been talking to phil hobson today and we were talking and cyril carter (tiling author) and cyril potter.