How to prepare the substrate for a screed

About 30% of a screed's quality comes down to what it is laid on. A dirty, uneven substrate means cracks and delamination. Here is how to get it right.

1. Breaking out the old screed (if there is one)

A new-build skips this step — the slab is bare concrete. In an older flat or a house the old screed often has to come up.

Take it out if:

• It is cracked, delaminating or dusting
• One room is badly out of level
• There is no waterproofing underneath and the room needs it
• You are adding underfloor heating and need the depth

Leave it if:

• It is sound and uncracked
• It is stuck down (no hollow sound when you tap it)
• There is headroom for a new 40–50 mm screed

2. Cleaning the substrate

The substrate has to be clean:

• Clear away all builders' debris and dust
• Remove any old adhesive, paint or oil
• Sweep with a stiff brush
• Vacuum with an industrial vacuum

3. Checking the levels

Use a laser level, or a 2 m straightedge with a spirit level. Check the whole room:

• Up to 2 cm out — fine, the screed takes it up
• 2–5 cm — fine, but you need a thicker layer
• More than 5 cm — consider insulation, which levels things up as well
• More than 10 cm — a separate conversation; semi-dry with reinforcement may be needed

4. Waterproofing

When you need it:

• Ground floor over a cellar, or with no cellar
• Bathrooms, shower rooms, kitchens
• Over underfloor heating pipes (protection against a leak)
• In flats where the slab's condition is uncertain

What to use:

• Cheapest and effective — 200 micron polythene, lapped 10–15 cm and turned 10 cm up the walls, joints taped
• Sheet membrane — torch-on or self-adhesive. Dearer, more reliable
• Liquid-applied — brushed onto the concrete. Good for small areas

5. Insulation (if needed)

When you need it:

• Ground floor over a cold cellar, or with no slab below
• Above an unheated space
• Under underfloor heating (essential, or the heat goes into the slab)

What to use:

• XPS polystyrene — 30–50 mm in flats, 50–100 mm in houses
• Mineral wool — less often, as it does not like damp

Boards go down tight, no gaps, joints staggered. Any gaps can be filled with foam.

6. Edge strip

Fixed around the full perimeter and around columns, piers and door frames. Taller than the finished screed — the surplus is trimmed later.

It absorbs thermal movement and keeps the walls from pressing on the screed. Leave it out and you get cracks in the corners and along the walls.

7. Screed rails

For a wet screed the rails are bedded on gypsum or set on adjustable supports, spaced 1–1.5 m apart.

For semi-dry machine work we use a laser level and datum points. Traditional rails are often unnecessary — the crew sets the level off the laser.

8. Humidity and temperature

Before the pour:

• Close every window and balcony door — no draughts
• Temperature between +5°C and +25°C
• In cold weather, warm the room with a heater first
• In real heat, ventilate — but keep the air moving gently

What NOT to do before screeding

• Do not wet the substrate "for adhesion" — that is a myth; a wet base is worse than a dry one
• Do not screed onto plaster less than a week old
• Do not leave windows and doors open
• Do not run pipes or cables without protective conduit
• Do not screed below +5°C without admixtures

Readiness checklist

Before the crew arrives, check:

1. ☐ Substrate clean, free of debris and dust
2. ☐ Old screed removed (where it had to be)
3. ☐ Waterproofing laid with laps
4. ☐ Insulation laid tight (where required)
5. ☐ Underfloor heating pipes pressure-tested and under pressure (if fitted)
6. ☐ Edge strip run around the perimeter
7. ☐ Room at +5 to +25°C, no draughts
8. ☐ Access for the machine: a way in for the truck, room for the hose
9. ☐ Water and power on site

With all of that in place the crew arrives, works and leaves the same day. With something missing it is a wasted day, extra money and frayed nerves.