Applying Waterproofing Tapes to Corners

Why not just paint the corner with membrane? A corner is where two surfaces meet, and each one behaves a little differently. Walls and floors move slightly with changes in temperature and humidity. A new building settles, and a screed keeps shrinking for some time. A liquid membrane is only flexible up to a point. If the corner keeps moving, it can crack, and water starts to seep quietly under the tiles.

A tape takes up this movement without tearing. If water cannot reach the substrate, you do not get a damp corner hidden behind the tiles. The plaster does not come away, the grout does not turn black and you avoid mould that no amount of cleaning will remove. In a block of flats it also means no leak into the flat below. This small step has a big effect on how long the bathroom lasts without repairs.

Tapes are not a waterproofing layer on their own. They are part of a waterproofing system, so the work follows the same order as the membrane:

• Preparing the substrate: the surface must be sound, dry and free of dust, grease and old adhesive. Larger dips and holes in the corner are filled first.
• Priming: absorbent surfaces such as plaster, plasterboard or screed get the primer that belongs to the system and are left to dry.
• First coat of membrane: a band of membrane slightly wider than the tape goes into the corner.
• Bedding the tape: the tape is pressed into the wet membrane with its centre line exactly in the corner, then smoothed with a trowel or brush so no air stays underneath.
• Covering: once dry according to the manufacturer, the tape and the whole area get another coat, usually two, so the waterproofing reaches the required thickness.

Tiling can start only when the membrane has fully cured. The manufacturer states the drying time, and it gets longer in a cold or damp room.

Straight tape covers an ordinary corner. More awkward spots get preformed pieces made of the same material:

• Internal and external corners: ready-shaped pieces for places where three surfaces meet. Cutting and folding straight tape in a corner is an unnecessary risk.
• Pipe collars: small round pieces with a hole for the water outlets at the mixer or shower.
• Floor collar: a larger square around a floor drain or linear channel, joined to the floor waterproofing.

You will usually find tapes with a fleece edge that is worked into the membrane, and self-adhesive tapes. The membrane decides which one to use. The safest choice is tape and membrane from one manufacturer as a single system, because nobody guarantees a mix of brands. For a level-access wet room or a balcony, choose materials the manufacturer clearly states are suitable for that kind of exposure.

The simplest case is a flat, dry, new substrate with a few straight corners. The job takes more time mainly in these situations:

• Many short runs, recesses, niches and external corners
• Curved walls or sloping ceilings under the roof
• Lots of pipe penetrations that each need a collar
• An old substrate after tiles have been removed, which has to be cleaned and repaired first
• Fresh plaster or screed that has to dry before work can start
• A level-access shower with a sloped floor and a linear drain

In practice, problems rarely start in the middle of a wall. They start in the details:

• Tape pulled tight: there is no slack in the corner, so it tears away from the substrate when things move.
• Air bubbles and creases: water can run underneath the tape through them.
• No overlap: neighbouring pieces should overlap, usually by at least 5 cm or as the manufacturer specifies.
• Tape bedded on dust or a damp surface: it only looks stuck.
• Second coat left out: the tape stays exposed and the waterproofing is too thin.
• Waterproofing stops too low: in a shower the walls need protecting as high as the water reaches, not just at floor level.

A careful, handy homeowner can manage this job if they work steadily and follow the manufacturer's instructions. For a level-access shower or a connection to a linear drain, a tiler is the better choice.

1. Get tapes, corner pieces and collars from the same system as the membrane and primer.
2. Clean and vacuum the corners and check that the substrate is dry and sound.
3. Fill dips and holes in the corners with a suitable mortar and let it harden.
4. Prime the substrate and let it dry as the manufacturer specifies.
5. Cut the tapes to length in advance, leaving enough for the overlaps.
6. Fit the corner pieces and collars first, then the straight tapes.
7. Apply a band of membrane into the corner, wider than the tape.
8. Press the tape into the corner along its centre line, without stretching it, and smooth it from the centre outwards.
9. Overlap neighbouring pieces and press the joint firmly into the membrane.
10. Once dry, coat the tapes and the whole area with a second coat, and a third if the manufacturer requires it.
11. Let everything cure completely before you start tiling.

• Tapes are in every corner of the shower, around the bath and at the drain
• Corners have preformed pieces, not folded straight tape
• Collars are fitted around the water outlets and the drain
• The tape is bonded along its full length with no bubbles, creases or lifting edges
• Tape joints overlap
• The tapes are covered with membrane and no dry patches show through
• The waterproofing in the shower goes high enough up the wall
• The membrane has fully cured before tiling starts