Liquid screed depth
The thinnest liquid screed is 25 mm, bonded to a slab. In a house, floating on insulation, it's 35 mm for calcium sulphate and cement-based liquid screed, and 45 mm for cement-based liquid screed. Over heating pipes, at least 25 mm of cover.
Where the floor zone is deeper than these, put the extra into insulation, not screed. A cement-based liquid screed shouldn't be more than 80 mm.
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Every minimum in one table
| Construction | Calcium sulphate | Cement-based liquid screed | Cement-based liquid screed | Sand and cement |
|---|---|---|---|---|
| Bonded | 25 mm | 20 mm | Not given | 25 mm (40 specified) |
| Unbonded, on a membrane | 30 mm | 30 mm | 50 mm | 50 mm (70 specified) |
| Floating, house | 35 mm | 35 mm | 45 mm | 65 mm |
| Floating, commercial | 40 mm | 40 mm | 60 mm | 75 mm |
| Over heating pipes | 25 mm (30 nominal) | 25 mm | 25 mm | Within the floating depth |
What the words mean
- Bonded: stuck to a prepared concrete slab. The thinnest option, and the least risk of curling.
- Unbonded: laid on a membrane over the slab, not stuck to it. Used where the slab is contaminated or has a damp-proof membrane on top.
- Floating: laid on insulation, separated from the walls by a perimeter strip.
Working out your depth
Start from the finished floor level on the drawings. Take off the floor finish and its adhesive, then the screed, and what's left is insulation. With heating, the screed is the pipe plus the cover: a 16 mm pipe and 25 mm of calcium sulphate gives 41 mm. Never go below the floating minimum, whatever the sum says. The floor check does the sum for both binders.
Thicker isn't better
Every extra 10 mm above 40 mm adds about 20 days of drying, on the rule of 2 days a mm past 40 mm. Put the spare depth into insulation, which also helps the floor reach the U-value it needs.
Liquid screed poured by the contractor who takes your call
Calls on this site ring JAM Floor Screeding: James Mulhall, based in Portlaoise, 25 years laying floors. Lines open Mon to Sat, 7am to 6pm. These are his jobs.
“JAM were punctual, professional and efficient and did an outstanding job on pouring our new floor as part of a renovation. Very highly recommend!”
“Excellent work, laser level finish, neat and tidy worker, highly recommend.”
“Top class work by james and his crew, highly recommend”
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Liquid screed depth questions
What is the minimum depth for liquid screed?
25 mm bonded and 35 mm floating in a house for calcium sulphate; 45 mm floating for cement-based liquid screed.
How deep is liquid screed over underfloor heating?
The pipe plus at least 25 mm. With a 16 mm pipe, about 41 mm, and never less than the floating minimum.
Can liquid screed be too thick?
No more than 80 mm for a cement-based liquid screed: every 10 mm past 40 mm adds about 20 days of drying. Add insulation instead.
How deep is a sand and cement screed by comparison?
65 mm floating in a house, 75 mm elsewhere, 50 mm unbonded and 25 mm bonded.
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