A screed failure is one of those renovation disasters that reveals itself slowly. You pour it, it looks fine, you tile over it, everything seems great for six months - and then a grout line cracks, and then another, and when you tap the floor with your knuckle you hear the hollow boom that tells you the screed has delaminated underneath.
I have seen this happen twice on projects where the screed was too thin. Once at 20mm over a polythene layer on a retrofit bathroom floor - the minimum for that application is 50mm, and 20mm stood no chance. The second time was a floating screed at 45mm over 100mm insulation boards, which started cracking within months because it should have been at least 65mm.
Screed depth is not a guideline you can stretch to save material. Here is what the numbers actually mean.
The three types of screed and their depths
The required depth depends entirely on how the screed sits on the substrate. There are three configurations.
Bonded screed - laid directly onto a prepared, primed concrete slab with a bonding agent. The screed is mechanically attached to the base and shares the load with it.
Unbonded screed - laid over a polythene separating layer. The screed floats on the membrane and has no mechanical connection to the base. It must be thick enough to be self-supporting.
Floating screed - laid over a compressible layer (acoustic or thermal insulation). This is the most demanding application because the screed has no rigid support.
| Screed type | Minimum depth | Recommended depth |
|---|---|---|
| Bonded | 25mm | 35-40mm |
| Unbonded | 50mm | 65mm |
| Floating (over insulation) | 65mm | 75mm |
| Over underfloor heating | 65mm | 75mm (25mm min over pipes) |
These figures assume traditional sand:cement screed. Anhydrite (liquid) screed has different minimums - check the manufacturer specification.
Why depth matters so much
Screed is strong in compression but relatively weak in tension and bending. A thin screed over an uneven substrate or a compressible layer is being asked to behave like a structural slab when it is nothing of the sort.
When screed is too thin for its application, it cracks under the thermal and structural loads of normal building use. It then starts to move relative to the substrate. Once it delaminates, it is hollow - and a hollow screed telegraphs every crack to the tiles above it.
The minimum depths exist because below them, the material simply does not have enough mass to resist the stresses it will encounter.
Screed over underfloor heating
This is the application where I see the most problems, because there are two competing pressures: the desire to keep the floor build-up as thin as possible to preserve ceiling height, and the need for adequate screed depth for structural integrity.
The minimum rule is 25mm cover over the top of the heating pipes. If you are using 22mm diameter pipes, the top of the pipe is at least 22mm above the sub-base - so your total screed depth to get 25mm cover is at least 47mm. Most specifications call for 65-75mm total to allow for a proper structural thickness around and above the pipes.
Anhydrite (liquid) screed is popular over underfloor heating precisely because it flows around the pipes, eliminating voids that sand:cement screed can leave around the lower surface of a pipe. But it has its own requirements - it must be primed and the laitance removed before any floor covering is applied.
| UFH pipe diameter | Minimum screed depth for 25mm cover |
|---|---|
| 12mm | 37mm (minimum 65mm practical) |
| 16mm | 41mm (minimum 65mm practical) |
| 20mm | 45mm (minimum 65mm practical) |
| 22mm | 47mm (minimum 65mm practical) |
The practical minimum of 65mm applies regardless - the 25mm cover rule is a floor, not a guide.
The total floor build-up
Screed depth is only one part of the floor height calculation. The total build-up from structural slab to finished floor level includes:
- Any insulation layer (typically 50-100mm PIR)
- Screed layer (see table above)
- Adhesive bed for tiles (5-10mm) or underlay and flooring
- The floor covering itself
A typical ground floor with 100mm insulation and 75mm floating screed already adds 175mm before the floor covering. This matters enormously in retrofit situations where you are tying into existing door thresholds and skirting heights.
How to work out how much screed you need
Once you know the right depth for your application, use the Floor Screed Calculator to convert floor area and depth into weight (kg/tonnes) and bag count. The calculator handles sand:cement, anhydrite and rapid-set screed types.
My tips on getting screed depth right
Measure the depth you actually have. Before specifying screed, measure the distance from your structural slab to the finished floor level you need. Subtract the floor covering thickness and the adhesive or underlay. What is left is your available screed depth. If it is less than your specification minimum, you need to re-plan rather than compromise.
Do not try to save height by going thinner. The saving is usually only 10-15mm. The cost of re-screeding when the first attempt fails is enormous - strip out the tiles, hack out the screed, prep the slab, re-screed, wait for drying, re-tile. Going the right depth once is always cheaper.
Mark your depth gauges before you start. Drive steel pins or timber pegs to the correct screed level across the whole floor area before mixing anything. Trying to judge 65mm by eye while moving a screed batten is not reliable enough.
Cure it properly. Polythene sheeting over fresh screed for at least 3 days is the minimum. Screed that dries out too fast from the top down will dust and crack on the surface even if the bulk of the screed is fine.
For accurate quantities, use the Floor Screed Calculator before ordering.