Tiling on Uneven Walls: How to Level and Tile Properly

You can tile an uneven wall, but only after you have measured how uneven it is and corrected it. Small dips can be absorbed by the adhesive bed. Bigger ones need filling, skim coating or new plasterboard. Skipping that step is what produces wavy tile lines, uneven joints and tiles that catch the light badly. In a bathroom it can also lead to leaks.

This guide shows you how to check a wall, which fix suits which gap, how to set out tiles so the wall’s flaws do not show, and where bathrooms need extra care. It is written for homeowners, landlords and property managers in Cardiff and the Vale of Glamorgan who want to understand the job before they hire or approve it.

TL;DR

• Hold a 2 m straightedge against the wall. Gaps over 3 mm need local correction, and the wall should not go beyond a 6 mm gap.
• Dips of about 3 to 6 mm are filled or skim coated. Anything over 6 mm usually calls for plasterboard over the old surface.
• Adhesive can fix tiny variations only. Building the bed up from 3 mm to 6 mm in isolated spots is a minor correction, not a levelling method.
• Set out from a level batten, not from the floor or ceiling, and work upward in small areas.
• Showers and wet rooms need a waterproof tanking system. Water-resistant adhesive and grout do not replace it.

Table of Contents

• How to tell if a wall is too uneven to tile
• Match the repair to the size of the gap
• Preparing the wall step by step
• Can tile adhesive make up the difference?
• Bathrooms and showers: waterproofing comes first
• Setting out tiles on a wall that is not perfectly true
• Tile size, mosaics and lippage
• Finishing: grout, sealant and trims
• Our perspective
• Need a tiler who prepares the wall properly?
• FAQ
• Sources

How to tell if a wall is too uneven to tile

Do not judge by eye. Walls that look fine in daylight often show their flaws once glossy tiles go up and a lamp rakes across the surface.

The standard test uses a long straightedge. Homebuilding quotes the British Standard for tile backgrounds: place a 2 m straightedge with 3 mm thick feet at each end against the wall. The straightedge should not be obstructed by the background, and no gap under it should be greater than 6 mm. Gaps over 3 mm need local correction.

The finished result is held to a similar standard. The Tile Association’s guidelines for tile fixing set a tolerance of plus or minus 3 mm under a 2 m straightedge for flatness and for variation from plumb.

A simple way to check your own wall

1. Clear the wall and remove old fixings, loose paint and flaking plaster.
2. Lay the straightedge horizontally at three heights: near the floor, mid wall and near the ceiling or top of the tiled area.
3. Repeat vertically and diagonally.
4. Slide a ruler or a stack of coins under the gap and note the largest one. Mark each dip in pencil.
5. Check plumb with a long spirit level. A wall that leans can be flat and still ruin a tiled corner.

Write the numbers on the wall itself. The biggest figure decides which repair you need.

Match the repair to the size of the gap

The right fix depends on how far the wall departs from flat. This table follows the Homebuilding guidance on the three ranges.

If the wall sits within the 6 mm limit but has a few isolated dips, you are in the middle row. The aim is a surface that passes the straightedge test everywhere, not just on average.

Preparing the wall step by step

The Tile Association says preparing the surface is vitally important and that tiling needs a flat surface, in its Tile It Right guide. In practice the sequence runs like this.

1. Make it sound

Tap the plaster. Hollow sounds mean it has lost its bond. Remove loose areas, cut back to a firm edge and repair. Tiles are heavy, and adhesive is only as strong as the surface it grips.

2. Fill the dips

Use a suitable filler or repair mortar for deeper hollows. Build up in layers rather than one thick pass if the product requires it, and let each layer cure. Check with the straightedge after each stage.

3. Skim or board when needed

Where variation is spread across the wall rather than concentrated in a few dips, a skim coat evens it out. Where the wall is badly out, new plasterboard fixed over the old surface gives a fresh, flat face. This is the job where a tiler and a plasterer, or one team doing both, saves time. On bathroom projects we prefer one team handling it so nobody blames the previous trade for a bad surface.

4. Prime and let it dry

New plaster and absorbent surfaces generally need priming before tiling, following the adhesive maker’s instructions. Rushing a damp skim is a common cause of tiles drifting after fixing.

Can tile adhesive make up the difference?

Partly, and only slightly. Homebuilding says small amounts of unevenness can be corrected with adhesive: the recommended 3 mm bed can be built up to 6 mm in isolated areas as a minor correction, not a substitute for proper wall preparation.

The trouble with using thick adhesive to chase a bad wall is that coverage suffers. The Tile Association guidelines say that in wet areas it is essential to achieve a solid bed with 100% adhesive coverage and no voids. Voids behind tiles in a shower are places for water to sit.

Two habits keep the bed reliable:

• Spread only as much adhesive as tiles can cover within its open time. The guidelines for tile fixing say this plainly.
• Press each tile with enough pressure, using a beating action or a slide and twist, to ensure good contact.

Bathrooms and showers: waterproofing comes first

Flat is only half the job in a bathroom. Walls around a bath or shower get repeated wetting, so the surface behind the tile matters as much as the tile.

NHBC technical guidance says walls surrounding bath and shower enclosures may be subject to repeated or persistent wetting, so substrates need careful selection. Additional protection can include an impervious waterproofing layer, known as tanking, behind the tiles. For a wet room, where the shower is open or behind a single wall and the floor is flush with the surrounding floor, the same document says walls and floor should be made fully watertight whatever the substrate.

One point catches many homeowners out. The Tile Association states that water-resistant adhesive and grout reduce the risk of water getting through the finish, but they are not a substitute for a tanking system, as BS 5385 Part 4 makes clear.

The highest-risk spots are the junction between tiling and the bath or tray, the joints between wall and floor tiling, and anywhere pipes or fittings pass through. Technical guidance for wet areas recommends trays and baths with integrated upstands built in behind the tiling, or proprietary upstand sealing kits for the same purpose.

If you are choosing a tray, our guide to tileable shower trays covers sizes, fitting and waterproofing. A shower wall with a window needs its own detailing, which we cover in window in shower waterproofing. And if you already have a damp patch or stained ceiling below a bathroom, read about a leaking shower enclosure before you add new tiles over the problem.

Setting out tiles on a wall that is not perfectly true

Even after correction, the floor, ceiling and corners rarely line up. The trick is to base the layout on a true reference, not on the room.

The Tile Association’s method goes like this: find the lowest point with a spirit level, place a tile there and draw a line along its top, then fix a level batten along that line. Draw a plumb line at the wall’s centre. Spread adhesive over about a square metre at a time, press and twist tiles into it, start at the bottom and work upward, and check horizontal and vertical lines every few rows.

TradeTraining’s setting-out guidance adds two useful details. A gauge rod, a batten marked in tile widths plus joints, shows you where courses land before you commit. Centring means marking the middle of the wall and testing which position leaves the bigger cuts at both ends, so you avoid thin slivers in corners.

What this does for an uneven wall

• The visible lines, the tops of courses, stay level even if the floor slopes.
• Any mismatch is pushed into the bottom row, which is cut to fit once the batten comes off. A cut row against the bath or floor is far less noticeable than a sloping row at eye level.
• Joints stay consistent. The Tile Association says joints must be at least 2 mm wide.

Tile size, mosaics and lippage

Large-format tiles show every wave in a wall. A long rectangular tile bridges dips and rises over humps, so any flaw appears as an edge standing proud of its neighbor. That edge difference is called lippage. The tile fixing guidelines state that there should be no appreciable difference in level across joints, with maximum deviations of 1 mm for joints under 6 mm and 2 mm for joints of 6 mm or more.

That is a tight limit, which is why flatness matters more as tiles get bigger. Smaller tiles forgive more. Mosaics follow the contours of a surface, which can suit a deliberately characterful wall in an older Cardiff terrace, though on a plain bathroom wall most people want crisp flat lines, so correction still comes first.

NHBC standards for ceramic wall tiling ask for courses that are straight and even, forming a plane and regular surface, especially around fittings and fixtures. They also call for proprietary water-resistant grouting used to manufacturer recommendations, and sealing up to sanitary fittings that allows for movement.

Finishing: grout, sealant and trims

Grout once the adhesive has set, following the maker’s timings. Use a water-resistant grout in wet areas. Seal where tiles meet a bath, tray or sanitaryware with a flexible sealant, not rigid grout, because those joints move.

Once grouting is done, a cloudy film can remain on the surface. Guidance on grout haze removal after tiling explains how that residue is dealt with, and it is worth wiping carefully as you go to keep it minimal.

Exposed tile edges at corners and ends of runs need a neat finish too. Our overview of tile trim options for bathrooms covers types, materials and sizes.

Our perspective

Most tiling problems we are called in to fix did not start with the tiles. They started with a wall someone decided was “good enough”.

When we run a bathroom job, one team handles the plumbing, the wall preparation and the tiling. That matters on uneven walls, because the person who finds a 10 mm dip behind old plaster is the same person who decides how to correct it, and the same person who has to live with the result. There is no handing the problem to a subcontractor or a showroom.

Our approach is to measure first, agree the fix with you in plain terms, and fix the price for the job so a wall that needs extra boarding is discussed before work goes ahead, not buried in a bill later. We would rather spend an extra half day on preparation than have you looking at wobbly lines for ten years.

In bathrooms and wet rooms we treat waterproofing as part of the wall build, not an afterthought, because we are plumbers as well as fitters.

James

Need a tiler who prepares the wall properly?

We are a local team covering Cardiff and the Vale of Glamorgan, and tiling, wall preparation, plumbing and full bathroom installation all sit under one roof. See our tiling services for wall and floor tiling, or look at a full bathroom renovation in Cardiff, where bathroom renovation is published from £2,500 to £4,000 as a one-off project price.

If you are planning a larger change, such as a wet room, an en-suite or a new layout, start from our home page and tell us what you have in mind. Fixed pricing means you know the cost of the job before we start.

FAQ

How uneven is too uneven for tiling?

Use a 2 m straightedge. Gaps over 3 mm need local correction, and the background should not exceed a 6 mm gap according to the standard quoted by Homebuilding. Beyond 6 mm, plan on plasterboard or replastering rather than adhesive.

Can I just use more adhesive to level the wall?

Only for tiny, isolated dips. A 3 mm bed can be built up to 6 mm in places as a minor correction. Heavier build-up risks voids and poor coverage, which matters most in wet areas.

Do I start tiling at the top or the bottom?

Work from the bottom up, but start from a level batten fixed along a line, not from the floor. The Tile Association describes fixing the batten at the lowest point’s line, tiling upward, then cutting the bottom row to fit after the batten is removed.

Will small tiles or mosaics hide an uneven wall?

They follow the surface more closely than large tiles, so they show less lippage but will still reveal real waves. For a flat, modern look, correct the wall first. Mosaics suit walls where you want the contours to show.

Is waterproof adhesive enough in a shower?

No. Water-resistant adhesive and grout help, but the Tile Association says they are not a substitute for tanking. Showers and wet rooms need a proper waterproofing system behind the tiles.

Sources

• Tiling an uneven wall to get a smooth professional finish, Homebuilding
• Guidelines for Tile Fixing, The Tile Association
• Tile It Right, The Tile Association
• Tile Association guidance on solid bed adhesive coverage
• Frequently Asked Questions, The Tile Association
• NHBC substrates for tiling bath and shower enclosures and wet rooms
• Tiling bath and shower enclosures, wet rooms and bathroom pods, 2024 guidance
• NHBC Standards 9.2.5 Ceramic wall tiling
• How to Set Out a Tiled Wall: Datum Lines and Centring, TradeTraining
• Tile grout haze removal

Recommended

• Tileable Shower Trays: Sizes, Fitting and Waterproofing