How to Prepare Your Site for Flow Screeding

DIY Flow Screed Preparation: How to Prepare Your Site for Flow Screeding

 

This page is designed to help you prepare your building site properly for a liquid / flow screed pour. If you follow the steps below, you’ll reduce the risk of delays, failures, cracking and costly rework.

If you’d rather have it handled end-to-end, you can also book GoFlow’s professional site preparation service at the bottom of this page.

Download this preparation guide as a PDF

Before You Get Started (what this guide assumes)

  • You have a suitable sub-floor and you’re ready to prepare it for screed.
  • You can create a fully watertight “tanked” area before the screed arrives.
  • If you’re installing underfloor heating (UFH), you’re fitting it in a way that won’t float or move during the pour.

If any of that sounds uncertain, pause and seek our advice before you continue. A screeded floor is only as good as the preparation work.

STEP 1: Confirm the sub-floor is fully dry
 

First of all, make sure that your sub-floor is fully dry. Reactions can occur where there is excessive moisture content.

Step 2: Clear and prep the substrate (clean, smooth, no hazards)

Your goal here is simple: nothing sharp, loose or messy that could puncture membranes or compromise the finished floor.

  • Clean the sub-floor thoroughly.
  • Remove sharp edges and any debris that could puncture the membrane.
  • Check walls and floor edges for cavities or gaps.
  • Fill or block any voids where screed could push through and leak out.

Take this seriously. If screed escapes under the membrane or insulation, it can undermine the whole floor build-up.

Step 3: Tank the sub-floor (first DPM layer)

Use a minimum 500 gauge DPM and create a watertight area.

  • Lay the DPM across the entire sub-floor.
  • Tape all joints.
  • Push the membrane tightly into the wall base.
  • Lap the membrane up walls / studwork around 5–6 inches above your finished floor datum.
  • Fold internal and external corners neatly and tightly (imagine you are wrapping a present).

You’re aiming for a waterproof “tray” with no holes or gaps for screed to escape.

If you are installing UFH and have a floor build-up, this still applies. You should tank underneath your insulation.

Step 4: Install insulation properly (no rocking, no voids)

  • Lay insulation boards tightly over the tanked sub-floor.
  • Make sure there are no voids underneath.
  • Avoid PIR boards rocking. If they are unsteady, blind underneath (for example with sand) and refit until stable.
  • Seal joints between boards if needed (strong duct works well).

    DO NOT USE FOIL TAPE IN ANY AREAS WHERE IT WILL COME INTO CONTACT WITH SCREED. THIS WILL CAUSE A CHEMICAL REACTION THAT WILL COMPROMISE THE INTEGRITY OF THE SCREED.

Step 5: Install the top tanking membrane (second DPM layer)

This is the final watertight layer between your insulation/sub-floor and the screed.

Important: Do not staple any UFH pipes until this membrane is installed.

  • Install the membrane like the first layer.
  • Tuck it tightly into corners.
  • Tape joints.
  • Lap it up walls around 5–6 inches above your screed datum level.

Why it matters: screed is heavy. If corners aren’t tucked tight, the weight can rip the membrane away from the walls, creating voids and cavities.
You’re aiming for a waterproof “tray” with no holes or gaps for screed to escape.

Step 6: Underfloor heating installation (secure it so it cannot lift)

If you have UFH:

  • Prefer clip rails and/or staples.
  • Avoid standard “egg crate” systems with liquid screeds (they can be extremely buoyant). Low profile crate systems are usually fine if properly secured.

Pipes must be secure. Even full pipes are buoyant.

 

  • On bends/loops: aim for strong fixing (the guide illustrates fixing with multiple staples).
  • On straights: place a staple at least every 400mm.
  • If using clip rails: add staples between rails for added security.

Step 7: Expansion edging and crack reduction (don’t skip this)

Install a minimum 8mm perimeter edging strip around the perimeter of every area to be screeded.

This applies whether or not you have UFH.

  • Make sure the edging is secure at top and bottom.
  • If it’s insecure, screed can work behind it and lift it, leaving voids.
  • You can seal the bottom edge with duct tape.
  • If you have wooden studwork or low density blocks, staples can help.

    Door thresholds (create a clean break)

    Thresholds are a weak point and need a deliberate break.

    Options:

    • Quick: strip of hardboard
    • Best: 100mm strips of 10–12mm shutter ply

    Important: Prepare these, but do not install them yourself before the pour (it can block the line of sight on floor lasers). Prepare so that when they are cut to size they wedge between the 8mm foam that should be running around both sides of your door threshold.

     

     L-shapes, courtyards, pool surrounds (stress points)

    Corners and L-shaped areas often crack unless stress reduction is built in. 

    • Use OSB or ply to create the breaks across corners and L shapes. It’s recommended that these be fixed to the sub-floor before preparation as floating shutters tend to move when screed is installed.
    • Make sure they’re secure (screed will push through if they’re not).
    • When creating expansion breaks across L shapes etc. it is best to stick to squares and rectangles ie. break on one side of the L don’t use a diagonal break. 
    • If you have any pipes or conduits rising from the middle of the floor in screeded areas these should also have an 8mm edging strip wrapped around them. 

    Step 8: Final clean (before screed day)

    Once preparation is complete:

    • Clean the area thoroughly.
    • Remove dust and debris.

    A messy site can compromise the finished look and texture of the floor.

    Step 9: Preparation for the day of the screed

    Hold ups on site may result in waiting charges. Protect from direct sun (UV) during curing

    You must ensure direct sunlight does not hit the finished screed during curing, especially through glass (windows, bifolds, roof lights).

    Solar heat gain can speed-dry the screed, causing cracking, blemishing the finish, and compromising integrity.

    Ventilation: night-latch, not wide open

    • Keep windows on the night lock position.
    • Do not leave them wide open (downdraughts can cause cracking).
    • Keep doors closed except for entry/exit for at least 7 days.

    Check your depths

    Ensure you have enough depth available above UFH pipes (or the highest point of the floor).

    • Standard screeds typically require 25mm cover above UFH pipes if installed. If not, minimal depth is normally 30mm for an unbonded floor screed.
    • Lower depth screeds can be possible (down to around 12mm above UFH pipes if installed), but confirm suitability before assuming.

    Water access and washout area (essential)

    On the day of the screed, there must be:

    • Access to an outside tap with good pressure. Please note the outside tap must be outside of the screeded area.
    • A watertight washout area for pump/pipe/tool cleaning.

    This can be:

    • A lined trough made from scaffold boards
    • A lined skip
    • Or similar

    Expect the equivalent of around 1–1.5 wheelie bins full of washout material.

    Keep the site clear

    The working area and the screed area must be:

    • Clean and clear
    • Debris removed
    • Floors hoovered

    Failure to carry out any of the above may mean that we would be unable to carry out your screed resulting in cancellation charges as per our Terms and conditions. Hold ups on site may result in waiting charges.

    Quick checklist (download and print this)

    • Sub-floor is dry
    • Substrate cleaned and hazards removed
    • Voids/cavities blocked
    • First DPM installed, joints taped, lapped up walls 5–6 inches
    • Insulation installed tightly, no rocking, joints sealed if needed
    • Second DPM installed before UFH fixings, tucked and lapped up walls 5–6 inches
    • UFH fixed securely, staples at least every 400mm on straights
    • 8mm perimeter edging installed everywhere
    • Threshold breaks prepared (not installed)
    • Stress breaks planned for L-shapes/corners where needed
    • Site cleaned and ready for screed day
    • Sun protection plan in place (cover glazing)
    • Windows on night-latch, doors closed
    • Outside tap confirmed and washout area prepared
    Click to Download checklist

    Would You Rather Let Us Handle It?

    Free Quote

    If you want the peace of mind of having your site preparation done properly (and quickly) by professionals, we can take care of the full site preparation so your screed pour goes smoothly. Just click the button to get a free quote.