Project 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.

Continue to step-by-step instructions
This D.I.Y. Advice is part of a series How to lay floor tiles

Step by Step Instructions

1 Measure up to work out the size your underlay sheets.
2 Measure the underlay for cutting
3 Cut the underlay to size
4 Measure and cut the underlay to fit under the door
5 Measuring and cutting the rest of your underlay
6 Nail the underlay into position
7 Seal up your underlay with silicone
  • Step 1. Measure 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.
  • Step 2. Measure 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.
  • Step 3. Cut 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.

  • Step 4. Measure 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.
  • Step 5. Measuring 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.

  • Step 6. Nail 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.

  • Step 7. Seal 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.

Tools and Materials

Tools

  • Caulking gun
  • Dust mask
  • Fibre cement sheet cutter
  • Gloves
  • Hammer
  • Knife
  • Pencil
  • Spirit level
  • Tape measure

Materials

  • Galvanised clouts
  • Sealant
  • Sheets of tile underlay
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