Why your shower needs tanking, not just tiles

Tiles and grout are not waterproof. Here is what tanking is, where it goes, and why skipping it is the most expensive mistake in a bathroom.

Most people assume that once a shower is tiled, water can't get past it. In practice, cement-based grout is porous, and tiny gaps open up around corners, valves and the shower tray over time. Water that gets behind the tiles has nowhere to go. It soaks into plasterboard, timber joists and the ceiling below.

We regularly strip out Cork bathrooms that are less than ten years old where the tiles look fine but the board behind them has turned to mush. The cause is almost always the same: no waterproofing layer.

What tanking actually is

Tanking is a continuous waterproof layer applied to walls and floors before tiling. It usually takes one of two forms:

• Liquid membrane: a paint-on coating applied in two coats, with reinforcing tape bedded into every internal corner and around pipe penetrations.
• Sheet membrane: a thin fleece-backed sheet bonded to the wall or floor with adhesive, with overlapping joints and pre-formed corners.

Both work well when installed correctly. What matters is that the system is continuous, with no gaps at corners, around the shower valve, or where the wall meets the tray or floor.

Where it needs to go

• All walls inside the shower area, to at least full shower-head height.
• The whole floor in a wet room, turned up the walls by at least 100 mm.
• Around baths with an overhead shower, behind the bath and along the shower end.
• Around every pipe, valve and drain penetration, using the manufacturer's collars.

The board behind matters too

Standard plasterboard should never be used in a shower enclosure, even under a membrane. We use cement-based backer board, which won't swell or crumble if moisture ever reaches it. On timber floors, backer board is screwed down at close centres to reduce movement, which protects the grout as well as the tiles.

How to check your tiler does it

Ask what waterproofing system they plan to use and whether it will be in the written quote. A good tiler will happily show you the membrane on site before the tiles go on. If tanking isn't mentioned at all, ask why.

Porcelain or ceramic: which tile should you choose?

They can look identical in the showroom, but they behave differently once they're on your floor. A plain-English comparison.

Both porcelain and ceramic tiles are made from clay fired in a kiln. Porcelain uses finer, denser clay and is fired at a higher temperature. That makes it harder and much less absorbent. Porcelain absorbs less than 0.5% of its weight in water; most ceramic wall tiles absorb considerably more.

When porcelain is worth it

For any floor, especially kitchens and halls where people come in from wet Cork streets, porcelain is the safer choice. It also handles frost, so it's the only sensible option for outdoor patios. Through-body or "full-body" porcelain has colour running all the way through, so a chip barely shows.

When ceramic makes sense

For bathroom walls, kitchen splashbacks and feature walls, ceramic is perfectly suitable. It's lighter, which matters on older plastered walls, and it often comes in glazes and colours that porcelain doesn't.

Things to check on the box

• PEI rating (for glazed floor tiles): wear resistance from 1 to 5. Aim for 3 or higher on residential floors.
• Slip rating: look for R10 or above for kitchen and bathroom floors.
• Rectified edges: tiles ground to exact size, which allows tighter grout joints.

Both materials need the right adhesive. Porcelain's low absorption means it needs a polymer-modified adhesive rather than a basic one, because it won't grip by suction the way ceramic does.

Tiling over underfloor heating: what to get right

Tile is one of the best floor finishes over underfloor heating, as long as the screed is ready and the right materials are used.

Many new and extended homes around Cork have water-based underfloor heating set in a screed. Tile and stone conduct heat well, so they pair nicely with it. They also expand and contract with every heating cycle, and that's where problems start if the job is rushed.

1. Let the screed dry

A sand-cement screed is commonly said to dry at roughly 1 mm per day for the first 40 mm, and slower after that. A 65 mm screed can take two to three months to dry properly in an Irish winter. Tiling onto a screed that's still wet can cause adhesive failure and cracked grout.

We test moisture before tiling instead of guessing. Ask your builder what screed was used and when it was laid.

2. Commission the heating first

The heating should be run and brought up to temperature slowly, held, and then turned down again before tiling. This drives out moisture and shows up any shrinkage cracks in the screed while they can still be dealt with. Follow the screed and heating manufacturers' schedules.

3. Anhydrite screeds need special care

Many new houses have anhydrite (calcium sulphate) liquid screeds. These are smooth and flat, but:

• The surface layer (laitance) must be sanded off.
• The screed must be primed with a compatible primer, because cement-based adhesives can react with gypsum.
• Moisture limits are stricter than for sand-cement screeds.

4. Use flexible adhesive and grout

Over heated floors we use a flexible S1-rated adhesive and flexible grout, and we aim for full-bed adhesive coverage under each tile. Hollow spots under a tile concentrate stress and are where cracks start.

5. Plan movement joints

Large floors and doorway thresholds need flexible movement joints so the floor can expand without cracking. These should line up with any joints in the screed. A tiler who skips these on a big open-plan floor is storing up trouble.

Large-format tiles: what you need to know first

600×1200 porcelain looks stunning with fewer grout lines. It also exposes every bump in the floor underneath.

Tiles where one side is 600 mm or more are usually called large-format. They're popular because they make rooms feel bigger and cleaner. They are also less forgiving than smaller tiles, and they cost more to lay properly.

Flatness is everything

A long tile laid on an uneven floor rocks, and its edges sit proud of the next tile. This step between tiles is called lippage, and it catches light and toes. Many tile manufacturers ask for a substrate tolerance of around 3 mm over a 2 m straightedge for large tiles. Most existing floors need levelling compound to reach that.

Tile bow

Long porcelain tiles are rarely perfectly flat. They often have a slight bow from firing. That's why manufacturers recommend offsetting rectangular tiles by no more than a third of their length, rather than the classic half-brick pattern. A 50% offset puts the high point of one tile next to the low point of the next.

How we lay them

• Level the substrate first, not with extra adhesive.
• Back-butter each tile as well as combing adhesive on the floor, for full contact.
• Use a tile levelling clip system to hold edges flush while the adhesive cures.
• Use a rapid-setting or large-format adhesive with good non-slump properties for walls.

Handling and access

A 600×1200 porcelain tile can weigh over 20 kg. Even bigger slabs need two people and suction lifters. Check that tiles can physically get up your stairs and around corners before you order them.

Is it worth it?

In kitchens, open-plan living areas and bathrooms with clean lines, yes. In a small, awkward bathroom full of corners and pipes, a 300×600 tile often looks just as good with far fewer difficult cuts.

Porcelain patios in Cork's wet weather

Outdoor porcelain is low-maintenance and holds its colour. In a climate this wet, how it's laid matters more than which tile you pick.

Porcelain paving has become a popular alternative to sandstone and concrete flags. It doesn't absorb water, so moss and algae have little to grip, and it won't fade like some natural stone. In Cork, where rain is a regular feature, those properties make a real difference.

Use proper outdoor porcelain

Outdoor paving porcelain is usually 20 mm thick, compared with 8–10 mm for indoor tiles. The extra thickness gives it strength over a mortar bed. Indoor floor tiles should never be used outside.

Check the slip rating too. For patios and paths, look for R11 or a high pendulum (PTV) value. Smooth indoor finishes become dangerous once wet.

Falls and drainage

Water needs to run off the surface, away from the house. We lay patios with a fall of around 1:60 to 1:80, and keep the finished level at least 150 mm below the house's damp-proof course. Where the garden slopes towards the house, a channel drain along the edge is usually needed.

The base underneath

• A compacted sub-base of crushed stone, typically 100–150 mm.
• A full mortar bed, not dot-and-dab. Voids collect water, and frost can then lift the slab.
• A slurry primer brushed onto the back of each slab, because porcelain won't bond to plain mortar.

Jointing

Joints between slabs are normally 3–5 mm and filled with a jointing compound. Brush-in resin jointing compounds are popular and resist weeds well, but they need to be applied in the right weather conditions.

Choosing grout: colour, width and type

Grout is a small share of the cost and a big share of how the finished job looks. Here's how to choose it.

Cement grout or epoxy?

Modern cement grouts with water- and mould-resistant additives are fine for most homes. Epoxy is worth the extra cost in wet rooms and on kitchen floors where staining is a concern.

Joint width

Rectified porcelain can take joints as narrow as 2–3 mm. Non-rectified tiles vary slightly in size, so they need wider joints, often 3–5 mm, to keep the lines straight. Floors over timber or heated screeds also benefit from slightly wider joints, which handle movement better. A "no-grout" look is not possible. Tiles always need joints.

Colour

• Matching grout makes the surface read as one and suits large-format tiles.
• Contrasting grout shows off the tile pattern, such as metro or herringbone, but also shows every imperfect line.
• Mid-grey is forgiving on floors; bright white floor grout rarely stays white.

Grout dries lighter or darker than it looks wet. Ask to see a cured sample against your actual tile before deciding.

Grout isn't silicone

Internal corners, the joint between tiles and the bath or tray, and perimeter edges should be finished with flexible silicone, not grout. These joints move, and rigid grout there will crack within months.

Regrout, repair or re-tile? How to tell

Not every tired bathroom needs ripping out. Here's how to judge whether a repair will last or just hide a bigger problem.

When regrouting is enough

• Grout is discoloured, mouldy or crumbling, but the tiles are firmly stuck.
• Tapping the tiles gives a solid sound, not a hollow one.
• There are no signs of water damage on the ceiling below or the walls next door.

We rake out the old grout to a consistent depth, regrout, and replace all the silicone. It transforms a shower for a fraction of the cost of new tiles.

When a single tile can be replaced

A cracked or chipped tile can usually be cut out and replaced, if you have a spare. If you don't, it's worth checking whether the range is still sold. For older bathrooms, a contrasting "feature" replacement is sometimes the honest option. Always keep a few spare tiles after any new job.

When it needs a full re-tile

• Several tiles sound hollow or move when pressed.
• The wall or floor behind feels soft, or the tiles have bulged.
• There's staining, a musty smell, or a water mark on the ceiling below.
• Cracks run across several tiles in a line, which points to movement in the floor.

These are signs that water has got behind the tiles or that the substrate is failing. Regrouting over that only hides the damage while it gets worse.

Tile slip ratings explained

R10, R11, PTV 36: what the numbers on the tile box mean, and which rating you need for each room.

A polished tile that looks great in the showroom can be lethal on a wet kitchen floor. Two systems are commonly used to describe how slippery a tile is.

The R rating (ramp test)

Under the German DIN 51130 test, a person in safety boots walks on an oiled tile while the ramp is tilted until they slip. The angle gives the rating:

The pendulum test (PTV)

The pendulum test swings a rubber slider across the tile surface, wet or dry, and gives a Pendulum Test Value. A wet PTV of 36 or above is generally considered low slip potential. This test is often used in Ireland and the UK because it reflects real wet-floor conditions in homes.

What we recommend

• Living areas: R9 is fine.
• Kitchens, bathrooms and halls: R10 at least.
• Walk-in showers and wet rooms: R11, or small mosaics where the extra grout lines add grip.
• Outdoor patios and steps: R11 or higher.

Grippier tiles have more texture, so they take a little more cleaning. A soft brush and a pH-neutral cleaner usually handle it.

Getting ready for your tiler: a homeowner checklist

A few decisions made early will save days on site and avoid last-minute compromises.

Weeks before

• Choose and order tiles early. Popular ranges can take two to four weeks to arrive. Order around 10% extra for cuts and breakages, and 15% for diagonal or herringbone layouts.
• Check batch numbers. Tiles from different production batches can vary in shade. Make sure all boxes share the same batch code.
• Decide on fixtures. Your tiler needs to know the exact position of the shower valve, niche, towel rail, vanity unit and toilet before tiling starts.
• Book the plumber and electrician. First-fix pipes and cables must be in place before boarding and tanking.

The week before

• Clear the room completely, including the bathroom cabinet and anything on windowsills.
• Decide on trim colour and finish: chrome, brushed steel, white or mitred edges.
• Choose grout and silicone colour.
• Agree where waste will go and where tiles can be cut, ideally outside or in a garage.

While the job is on

• The bathroom will be out of use for several days. Tanking, adhesive and grout all need curing time.
• Keep pets and children out of the work area.
• Don't use a new shower until the grout and silicone have fully cured. Your tiler will tell you when.

What affects the price of a tiling job in Cork?

Two bathrooms the same size can have very different quotes. These are the factors that move the price.

People often ask for a price per square metre. It's a fair question, but the tiles going on the wall are only part of the job. The biggest cost differences come from what has to happen before the first tile goes up.

1. Removing what's there

Stripping old tiles is quick on plasterboard and slow on solid walls, where tiles may be bedded on thick sand and cement. Removing old adhesive and patching the wall adds time.

2. Preparation and levelling

Walls that are out of plumb and floors that are out of level need boarding, render or levelling compound. Old Cork houses with timber floors often need overboarding to stop movement cracking the grout.

3. Waterproofing

Showers and wet rooms need a tanking system. It adds material and labour, but it's the part that protects the rest of your house.

4. Tile size and layout

• Mosaics and very small tiles take longer per square metre.
• Large-format tiles need a flatter substrate, levelling systems and two people to handle.
• Herringbone and diagonal layouts mean more cuts and more waste.

5. Detailing

Shower niches, mitred external corners, boxing around pipes and tiled bath panels all add time. They also make a big difference to the finished look.

6. Access and the room itself

A small ensuite with a sloping ceiling, a window in the shower and lots of pipework takes longer than a simple square room of the same area.

The most reliable way to get an accurate price is a site visit. Photos sent by text help us give a rough idea first.