Bonded, unbonded and floating screed explained
By Michał Białas · Last updated
A bonded screed is stuck to the slab and can be thin (from about 1 in / 25 mm for sand-cement). An unbonded screed sits on a plastic separating sheet and needs about 2 in (50 mm). A floating screed sits on insulation boards and needs the most, about 2 1/2 in (65 mm) or more. Each method also changes the shopping list: bonding primer, a separating membrane, or edge strip around the walls.
The three methods
Bonded screed is laid directly on a clean, sound concrete slab with a bonding primer or slurry between them. Slab and screed act as one piece, so the screed can be thin. It is the usual choice for leveling an existing slab when there is no insulation or heating to add.
Unbonded screed is laid on a polyethylene separating sheet. It is used when the slab can’t give a reliable bond — oily, painted, damp-proofed or crumbly surfaces — or when the two layers should move independently. Without the bond to help, the screed must be thicker to avoid cracking.
Floating screed is laid on a layer of insulation boards, with a membrane on top of the boards and a compressible edge strip at every wall. It is the standard build-up for radiant floor heating and for upper floors that need impact sound insulation. The screed carries loads on its own, so it needs the most thickness.
Typical minimum thickness and extra materials
| Method | Typical minimum (sand-cement) | Extra materials |
|---|---|---|
| Bonded | About 1 in (25 mm) | Bonding primer or slurry |
| Unbonded | About 2 in (50 mm) | PE separating membrane, area + 15% for overlaps and upstands |
| Floating | About 2 1/2 in (65 mm) | Insulation boards, PE membrane, perimeter edge strip, perimeter + 5% |
Liquid anhydrite screeds can be thinner: typically from about 1 1/4 in (30 mm) bonded and 1 1/2 in (35 mm) floating. Always use the minimums on your product’s data sheet. The screed calculator adds the membrane, edge strip and primer automatically when the method calls for them.
Worked examples
Example 1 (meters): 4.0 × 3.0 m room, bagged sand-cement at each minimum
- Bonded, 25 mm: 12 m² × 25 mm × 2.0 kg/m²/mm × 1.05 = 630 kg → 26 bags of 25 kg; bonding primer 0.5 L → 1 × 1 L.
- Unbonded, 50 mm: 1,260 kg → 51 bags; PE membrane 12 × 1.15 = 13.8 m² → one 25 m² roll.
- Floating, 65 mm: 1,638 kg → 66 bags; PE membrane 13.8 m²; edge strip 14 m × 1.05 = 14.7 m → one 25 m roll.
- Weight on the structure goes from about 50 kg/m² bonded to about 130 kg/m² floating.
Example 2 (feet): 12 × 10 ft room, 80 lb bags of sand mix
- Bonded, 1 in: 1,311 lb → 17 bags; one quart of bonding primer.
- Unbonded, 2 in: 2,622 lb → 33 bags; 138 sq ft of poly sheeting → one 250 sq ft roll.
- Floating, 2 1/2 in = 63.5 mm: just under the typical 65 mm minimum, so the calculator warns. At 2 3/4 in: 3,605 lb → 46 bags, plus poly and about 46 ft of edge strip.
Inches and millimeters don’t line up neatly: 2 1/2 in looks like “about 65 mm” but is 63.5 mm. When a product minimum is given in millimeters, convert and round up.
Height and weight
The method also decides how much the floor rises and how much it weighs. A bonded screed adds the least height, which matters at doors, at stairs and where the new floor meets an existing one. A floating screed adds the screed plus the insulation boards, so check door heights and thresholds before you commit. Sand-cement screed weighs about 2 kg/m² per millimeter (roughly 10 lb per sq ft per inch): fine on a ground-floor slab, but worth checking with a structural engineer on wooden floors and upper stories.
How to choose
- Leveling a sound, clean slab with no insulation: bonded. It is the thinnest and lightest.
- Slab you can’t trust to bond, or over a damp-proof membrane: unbonded.
- Radiant heating, insulation or sound insulation: floating. See tiling over radiant floor heating.
- Little height available: bonded, or a liquid screed with a lower minimum.
Installation checklist
- Bonded: remove dust, paint, oil and loose material; prime or apply a bonding slurry and place the screed while it is still tacky, as the product requires.
- Unbonded: lay the PE sheet with about 8 in (200 mm) overlaps, taped, and turn it up the walls.
- Floating: lay the insulation tight, then the membrane, then fix the edge strip to every wall, pillar and door frame before placing the screed.
- Set screed rails or level pins to the finished height and check them with a laser.
- Cure as the data sheet says — usually protected from drafts and fast drying for the first days — and plan the drying time before the finish (see screed drying time).
Frequently asked questions
Why does a floating screed need to be thicker?
Because it rests on compressible insulation instead of a rigid slab, so it has to spread loads by itself. Thin floating screeds crack.
What is the edge strip for?
It separates the screed from the walls so it can expand and shrink without cracking, and in floating floors it stops sound from passing into the walls. Trim the excess after the finish is down.
Can I lay a bonded screed over old tiles?
Only if the tiles are firmly bonded and properly prepared, usually by grinding and a special primer. Many installers lay an unbonded screed instead.
Does the method change the drying time?
Mostly through thickness. A thicker unbonded or floating screed holds more water and takes longer to dry than a thin bonded one.
When can I walk on a new screed?
Light foot traffic is often possible after a day or two for sand-cement and liquid screeds, but check the data sheet. Walkable is not the same as dry: the floor covering still has to wait for the moisture test.