What Should You Put Under Tiles? (Complete Subfloor Guide)

One of the most overlooked aspects of tiling is what goes underneath the tiles. The tile itself is only as good as what supports it. Getting the subfloor or substrate wrong leads to cracked tiles, hollow spots, water damage, and expensive repairs.

This guide explains what you need to put before tiling - for different surfaces and different situations - so your tiles bond correctly and last for decades.

Tiles are rigid. They cannot flex or absorb movement. If the surface beneath them is uneven, unstable, damp, or incompatible with the adhesive, tiles will fail. The most common tile failures - hollow tiles, cracked grout, lifting tiles, and cracked tiles - all trace back to inadequate substrate preparation.

No amount of skill in laying tiles compensates for a bad surface beneath them.

1. Concrete Slab (Most Common in South African Homes)

Concrete is one of the most common and most suitable substrates for tiling. When properly prepared, it provides an excellent base.

What to check:

What to apply before tiling:

2. Sand - Cement Screed

Screed is commonly used to level floors or to create falls (slopes) toward drains. When properly mixed and cured, screed provides a good base for tiling.

What to check:

What to apply:

3. Cement Board (Tile Backer Board)

Cement board - products like Durock, HardieBacker, or Wedi - is installed specifically as a tile substrate. It is:

• The concrete must be cured (new concrete should cure for at least 28 days before tiling)
• The surface must be flat - check with a spirit level or straight edge; no more than 3mm variation over 1. 8m
• No active cracks (see below)
• Free of dust, dirt, grease, paint, and curing compounds
• Primer / bonding agent - apply a suitable concrete primer (such as a diluted PVA or a dedicated tile bonding primer) to seal the surface and improve adhesive bonding. This is particularly important on dusty or porous concrete.
• Self-levelling compound - if the floor is not flat enough, apply a self - levelling compound to flatten the surface before tiling. This must be fully cured before tiles go down.
• Waterproofing membrane - in wet areas (bathrooms, kitchens, laundry rooms), apply a proper waterproofing membrane before tiling. This is non - negotiable.
• The screed must be fully cured (at least 7 days for standard screed, often longer)
• Screed that sounds hollow or crumbles when scratched is not strong enough to tile on
• Check for cracks - hairline cracks from drying shrinkage are common; active cracks require investigation
• Primer/bonding agent before tiling
• Waterproofing in wet areas
• Dimensionally stable (does not expand or contract significantly with moisture)
• Resistant to moisture damage (unlike standard plasterboard or chipboard)
• Suitable for wet area use when joints and edges are properly sealed
• Seal the joints between boards with fibreglass tape and tile adhesive or waterproofing membrane
• Apply waterproofing in wet areas over the entire board surface before tiling
• Plaster must be firm - tap it; hollow or loose plaster must be repaired or replaced
• Plaster must be clean and free of paint (paint prevents adhesive from bonding to the plaster itself)
• Must be flat enough (within 3mm)
• If paint is present, it must be sanded off or a bonding primer applied
• Bonding primer on smooth plaster
• Waterproofing membrane in wet areas
• On a painted surface, either remove the paint or apply a high - bond primer specifically designed for painting over painted surfaces
• Decoupling membrane - a specialised mat (such as Schluter Ditra or similar) that is bonded to the timber and provides a stable surface for the tiles to bond to. The membrane absorbs movement from the timber below and prevents it transferring to the tiles above. This is the most important product for tiling over timber.
• Additional structural support may be needed if the floor flexes significantly underfoot
• Cement board can also be screwed over timber as an alternative, but only if the floor is very stiff and solid
• A high - bond, specialised adhesive designed for over - tiling
• A bonding agent or primer on the existing tile surface to improve grip
• Sand or abrade the existing tiles if they are highly polished to improve adhesion

What to apply:

4. Existing Plaster on Walls

In South Africa, most internal walls are plastered. Plaster can be a suitable substrate for wall tiles if properly prepared.

What to check:

What to apply:

5. Timber Subfloor

Timber subfloors are the most challenging surface to tile over. Timber naturally expands and contracts with changes in humidity and temperature - this movement cracks tiles and opens grout lines.

Direct tiling onto timber is generally not recommended without the correct preparation.

What to apply before tiling on timber:

6. Existing Tiles (Over - Tiling)

Tiling over existing tiles can work when conditions are right (see a separate guide for full details). When doing so:

What to apply:

Regardless of the substrate type, waterproofing membrane must be applied in all wet areas before tiling:

• Shower floors and walls
• Around baths
• Bathroom floors generally
• Kitchen areas with water exposure
• Laundry rooms

1. Application:
2. Prepare and prime the substrate
3. Apply fibreglass tape at all corners and junctions
4. Apply waterproofing membrane by brush or roller (usually two coats)
5. Allow to fully cure before tiling

If a floor is not flat enough for tiling - typically when the variation is more than 3mm over 1. 8m - a self - levelling compound must be used before tiling.

Self - levelling compound is poured onto the floor, spreads itself flat, and cures to a hard, smooth surface. It must:

• Be appropriate for the specific substrate (some products work only on concrete; others work on various surfaces)
• Be applied in the correct thickness (consult the product data sheet)
• Cure fully before tiling begins (usually 24 hours minimum, but check the specific product)

Once the substrate is prepared and any waterproofing has cured, the adhesive is the last thing applied before tiles. Choosing the correct adhesive matters:

• Situation
• Adhesive Type
• Standard floor tiles on concrete
• Standard cement - based tile adhesive
• Large format tiles
• Heavy - duty or large - format tile adhesive
• Wet areas
• Flexible, water - resistant adhesive
• Tiling over tiles
• High - bond over - tile adhesive
• Porcelain tiles (low porosity)
• Adhesive formulated for porcelain
• Timber (with decoupling membrane)
• Flexible adhesive only

• Do not skip the primer - it significantly improves adhesive bond strength on dusty or porous surfaces
• Do not rush waterproofing cure times - premature tiling damages the membrane
• Always use the correct adhesive for the specific tile type and surface - ask at the tile shop if unsure
• Keep the adhesive manufacturer's data sheets - they specify cure times, coverage rates, and application methods

What you put before tiling is just as important as the tiles themselves. For most situations, the sequence is: prepare and repair the substrate, apply primer or bonding agent, apply waterproofing in wet areas, allow to cure, then tile with the correct adhesive.

Skipping any of these layers leads to tile failures that are expensive and disruptive to fix.

Want tiling done properly from the substrate up? Contact Excellence Tilers in Cape Town. We prepare surfaces correctly, waterproof where required, and use the right materials at every step - because the work under the tiles is what makes everything above last.