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How to install tile underlay

Caulking gun being using to apply silicone sealant in between tiles.

Overview

Before laying floor tiles you need to install tile underlay. Easy to handle and quick to install, underlay gives you a flat, durable and smooth surface. It also resists moisture, mould and doesn't swell if it gets wet, so it's ideal for your bathroom, laundry or any other wet areas.

Tools and materials

Tools

Materials

Steps

1Measure up to work out the size your underlay sheets

The underlay sheets are 1200mm wide and need to be cut to size. Measure the distance from one wall to the opposite wall, so you know what size the underlay sheets need to be. When laying out the pieces on the floor, remember to leave a 3mm gap between the walls and other sheets for silicone. Next measure the width of your doorway, keeping in mind that the underlay should end halfway under the door. Then measure 1200mm from the edge of the doorway and mark the floor for the first sheet.
Person using measuring tape across tiles.

2Measure the underlay for cutting

After you have your floor measurements, transfer and mark out those lengths on either side of the underlay. Now draw a straight line between the two points across the underlay. Use a spirit level to make sure all your lines are straight.
Person marking wall with pencil and spirit level.

3Cut the underlay to size

Make sure you have someone to help you hold the sheet while you cut it. It's important to wear safety gear while you handle and cut the underlay, so put on your gloves and dust mask. Use the fibre cement sheet cutter to cut across the line you have drawn.

Person cutting tile underlay with fibre cement sheet cutter.

4Measure and cut the underlay to fit under the door

Lay the piece of cut underlay on the floor, and make it flush to the doorway. Then mark 3mm in from the edge of the doorway. Draw an arrow onto the underlay to show the direction you need to cut, either left or right of the doorway.

Measure the width of the doorway and halve it. Then mark that distance out from the 3mm mark on your underlay. Use your level to draw a straight line from this mark to the top of the sheet. Then cut the underlay and lay it flush against the doorway.
Person laying large piece of ceramic tile underlay on the floor.

5Measuring and cutting the rest of your underlay

Measure the distance from one wall to another and cut your underlay to size. Make sure you leave a 3mm gap between the walls, so you can silicone seal it later. Remember you don't want to install underlay where a shower or toilet may be, so cut the underlay to suit. After you've cut all of your underlay to size, lay it on the floor.

Sheets of ceramic tile underlay on the floor.

6Nail the underlay into position

Now you're ready to nail the underlay into the floor. There are places marked on the underlay to make it easy for you to hammer in the nails. Make sure you space the nails out evenly apart along the edges of your underlay.

Person using a hammer to hit a nail into tile underlay.

7Seal up your underlay with silicone

Use your caulking gun to apply a wet and dry silicone sealant to the gaps between the walls and the underlay. This will make sure that no moisture rises up between the underlay.

Caulking gun being using to apply silicone sealant in between tiles.

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Health & Safety

Please make sure you use all equipment appropriately and safely when following the advice in these D.I.Y. videos. You need to be familiar with how to use equipment safely and follow the instructions that came with the equipment. If you are unsure, you may feel it is safest to consult an expert, such as the manufacturer or an expert Bunnings Team Member.

Grave health hazards are linked to asbestos, which may be in homes built up to 1990. Health hazards may result from exposure to lead-based paints in older materials and copper chromium arsenic (CCA) treated timber. For information on the dangers of asbestos, lead-based paint and CCA treated timber and tips for dealing with these materials contact your local council's Environmental Health Officer. You can also use a simple test kit from Bunnings to indicate the presence of lead-based paint.