A practical guide to what should go under tiles before installation, including substrate prep, primer, waterproofing, levelling, and adhesive selection.
One of the most important parts of tiling is what goes underneath the tiles. The tile is only as strong as the surface, primer, waterproofing, and adhesive supporting it.
If the preparation is wrong, the result can be hollow tiles, cracked grout, loose tiles, water damage, and expensive repairs.
This guide explains what should go under tiles on concrete, screed, plaster, cement board, timber, and existing tiles.
If you are planning floor tiling or bathroom tiling, this is the part that determines whether the job lasts.
In most tiling jobs, the correct order is:
The exact system depends on the room, surface type, tile size, tile material, and whether the area is wet or dry.
Tiles are rigid. They do not handle movement well. If the surface beneath them is weak, uneven, dusty, damp, or moving, the tiles may sound hollow, crack, lift, or cause grout failure.
This is why surface preparation is not an optional extra. It is part of the tiling job.
A dusty slab can stop adhesive bonding properly. A flexible timber floor can crack grout and tiles. A poorly waterproofed shower can damage walls and floors behind the tile. A floor that is not flat can cause lippage, poor alignment, and hollow spots.
Before you start, check these surfaces you cannot tile on.
Concrete is one of the most common and suitable substrates for floor tiles in South African homes, but it still needs checking before tiling.
New concrete must be fully cured. A common minimum is 28 days before tiling, but product instructions and site conditions matter.
The surface must be flat enough for the tile size.
There must be no active cracks.
The slab must be clean and free from dust, grease, paint, curing compounds, loose material, and old adhesive.
Any moisture concerns should be checked before tiling.
A suitable primer or bonding agent where required by the adhesive manufacturer.
Self - levelling compound if the floor is not flat enough.
Waterproofing membrane in wet areas such as bathrooms, showers, laundries, and water - exposed kitchen areas.
The correct tile adhesive for the tile type, especially for porcelain or large - format tiles.
Screed is often used to level floors or create falls toward drains in bathrooms and showers.
The screed must be fully cured.
It must be strong, firm, and not crumbly.
It must not sound hollow or break away when scratched.
Cracks should be checked before tiling.
Shower and bathroom screeds must have correct falls toward the drain.
Primer or bonding agent if required.
Waterproofing membrane in wet areas.
Self - levelling compound only where appropriate and compatible.
Correct tile adhesive after the surface has been prepared.
Cement board or tile backer board is used where a stable tile substrate is needed, especially for walls and wet areas.
The board must be securely fixed with no movement.
Joints must be supported and correctly treated.
The correct board must be used for the room and moisture exposure.
Standard plasterboard or chipboard should not be treated as the same as cement backer board.
Joint tape and suitable adhesive or waterproofing treatment at board joints.
Waterproofing membrane in wet areas, especially showers.
Primer if required by the tile adhesive system.
Correct tile adhesive for the tile and board type.
Plastered walls can be tiled if the plaster is sound, clean, and properly prepared.
The plaster must be firm and not hollow or loose.
The surface must be flat enough for the chosen tile.
Paint, dust, grease, and loose material must be removed or correctly prepared.
Cracked or weak plaster must be repaired before tiling.
Bonding primer on smooth, dusty, porous, or painted surfaces if compatible.
Waterproofing membrane in wet areas.
The correct wall tile adhesive.
Extra preparation where large - format wall tiles are being installed.
Timber is one of the most difficult surfaces to tile over because it expands, contracts, and can flex.
The floor must be structurally strong and not flex underfoot.
Boards must be securely fixed.
Moisture issues must be ruled out.
Direct tiling onto timber is generally not recommended without a proper tile system.
A decoupling membrane designed to absorb movement.
Cement board only where the floor is stiff enough and the product system allows it.
Additional structural support if the floor moves.
Flexible tile adhesive suited to the chosen system.
Waterproofing where the timber floor is in a wet area, only as part of a compatible system.
Tiling over existing tiles can work, but only when the old tiles are firmly bonded and the surface is suitable.
Existing tiles must not be loose, cracked, hollow, or lifting.
The surface must be clean and free from grease, soap residue, wax, and sealers.
The height build - up must not create problems at doors, drains, skirtings, or transitions.
Glossy tiles may need abrasion or a specialist primer.
Wet areas must be checked carefully before over - tiling.
A specialist bonding primer or high - bond system made for over - tiling.
A suitable high - bond tile adhesive.
Waterproofing only if the system and substrate allow it correctly.
Never over - tile a shower or bathroom area if there are signs of leaks, hollow tiles, or failed waterproofing.
If an existing tiled area is failing, tile repairs and regrouting may be a better first step than covering defects.
Waterproofing is required before tiling in wet areas. Tiles and grout help protect the surface, but they are not the full waterproofing system.
Waterproofing is especially important in shower floors and walls, around baths, bathroom floors, laundry areas, kitchen areas exposed to frequent water, and floor - wall junctions and corners.
For full wet - area context, read bathroom waterproofing before tiling.
Self - levelling compound is used when a floor is not flat enough for tiling. It does not fix every structural problem. It should only be used on a suitable, stable substrate and according to the product datasheet.
It must be compatible with the substrate, the floor must be primed first where required, it must be applied at the correct thickness, and it must cure fully before tiling.
It should not be used to hide major structural movement or active cracking.
Use adhesive matched to the surface and tile type:
Standard ceramic floor tiles on concrete: Standard cement - based tile adhesive.
Porcelain tiles: Adhesive formulated for porcelain.
Large - format tiles: Heavy - duty or large - format tile adhesive.
Wet areas: Flexible, water - resistant adhesive suited to the waterproofing system.
Tiling over existing tiles: High - bond over - tile adhesive.
Timber with decoupling membrane: Flexible adhesive compatible with the membrane system.
Wall tiles: Wall tile adhesive suitable for tile weight and surface.
Natural stone: Adhesive suitable for natural stone, depending on stone type.
Always follow adhesive manufacturer datasheets. The wrong adhesive can cause poor bonding, staining, tile movement, or failure.
Avoid tiling over loose or hollow tiles, crumbling screed, damp or contaminated concrete, peeling paint, flexible timber without a proper tile system, standard chipboard in wet areas, dusty surfaces with no primer, active cracks not investigated, failed waterproofing, and old adhesive ridges that were not removed or levelled.
If the surface is not sound, clean, flat, and stable, the new tiles are unlikely to last.
Use these checks before buying tiles or booking installation:
What goes under tiles is just as important as the tile itself. In most jobs, the process starts with a clean, flat, stable substrate, then the right primer, waterproofing in wet areas, levelling where needed, and the correct adhesive.
Skipping these layers can lead to cracked tiles, hollow sounds, loose tiles, leaks, and expensive rework. A strong tile installation starts below the surface.
Planning tiling work in Cape Town? Send Excellence Tilers your suburb, a few photos of the surface, and the type of tiles you want installed. We can check what preparation may be needed before tiling, including primer, levelling, waterproofing, and the correct adhesive system.
When you are ready, use Request a Quote, WhatsApp Us, message us through Contact, or explore Floor Tiling Services, Bathroom Tiling Services, Kitchen Tiling Services, and Tiling Services.
Clear answers to common project questions.
What goes under tiles depends on the surface and room. In most cases, you need a clean, flat, stable substrate, primer where required, waterproofing in wet areas, levelling compound if the floor is uneven, and the correct tile adhesive.
Primer is often needed on dusty, porous, smooth, or difficult surfaces to improve bonding. The exact primer depends on the substrate and adhesive system, so follow the product manufacturer instructions.
Yes. In wet areas such as showers, bathroom floors, around baths, and laundries, waterproofing should be applied before tiling. Tiles and grout are not the full waterproofing system.
You can tile onto concrete if it is fully cured, clean, flat, dry, stable, and free from contaminants. Primer, levelling compound, waterproofing, or crack treatment may still be needed depending on slab condition.
Tiling over existing tiles can work only if the old tiles are firmly bonded, clean, flat, and not cracked, hollow, or lifting. A specialist bonding primer and high - bond adhesive are usually required.
Direct tiling over timber is generally risky because timber moves. A proper system such as a decoupling membrane, structural strengthening, and flexible adhesive may be needed.
A shower floor needs a stable substrate or screed with proper falls to the drain, waterproofing, compatible adhesive, suitable floor tiles, grout, and silicone joints. The tile alone is not the waterproofing.
Yes. Excellence Tilers helps Cape Town homeowners and property owners with surface preparation, waterproofing, floor tiling, bathroom tiling, and tile installation.
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