Domakihub How to Lay a Screed Floor: Steps, Mix, and Pro Tips
Get a flat, solid sub-floor ready for tiles, LVT, or underfloor heating with sand-and-cement screed.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
A screed floor is the smooth, level layer of sand and cement mortar laid over a concrete slab before your final flooring goes down. Getting it right matters whether you are renovating a home in Athens, refurbishing a holiday rental in the Aegean, or fitting out a hotel room — a poor screed telegraphs every imperfection through tiles and LVT for years. The video above shows the mixing and laying process hands-on; this article walks you through every stage so you know exactly what to expect and how to prepare.
What Is Floor Screed and When Do You Need It?
Floor screed is a semi-dry or wet mortar mix — typically one part cement to three or four parts sharp sand — applied at 40–75 mm depth over a structural concrete base. It creates the flat, load-bearing surface onto which ceramic tiles, natural stone, engineered wood, or LVT can be bonded. You will need a fresh screed any time you are doing a complete room gut-out, installing underfloor heating pipes or electric mats, correcting a badly uneven slab, or raising the floor to a new datum level.
In Greece, where concrete-frame construction is the norm, the structural slab is rarely finished flat enough for direct tiling. Screeding is therefore a standard part of almost every renovation. For areas larger than roughly 30 m² or where the screed goes over a damp-proof membrane (especially in basements or ground-floor slabs close to soil), a licensed civil or structural engineer should confirm the slab condition and specify the mix — this is particularly important in older buildings in seismically active zones.
Materials, Tools, and Practical Considerations
You will need Portland cement (CEM II 32.5 or 42.5 is typical in Greece), clean sharp sand (not beach sand — salt contamination destroys strength), clean water, a plasticity additive or SBR bonding agent, timber or aluminium screed rails, a long aluminium straightedge (rule), a float, a mixing trough or forced-action mixer, a wheelbarrow, and a spirit level. For underfloor heating installations add polystyrene edge insulation strip around the perimeter and insulation board beneath the screed.
Timing matters in the Greek climate. Laying screed in peak summer when temperatures exceed 35 °C causes the surface to dry too quickly, leading to shrinkage cracks. Schedule the work for early morning, keep windows shaded, and mist the slab with water before laying. In winter, protect the fresh screed from temperatures below 5 °C for at least 48 hours. Allow a minimum of one day of drying per millimetre of depth (so a 50 mm screed needs roughly 50 days) before laying moisture-sensitive floor finishes.
How to Lay a Sand-and-Cement Screed: 5 Steps
- 1
Prepare and prime the base slab
Remove all dust, oil, and loose material from the concrete slab — a stiff brush and vacuum are usually enough. Check for any structural cracks wider than 0.5 mm and fill them with a cementitious repair mortar before proceeding. Apply a generous coat of SBR bonding agent (or a cement slurry of equal parts cement and water) to the entire slab surface. Do not let it dry completely — lay the screed while it is still tacky. This bond coat is the single most important preparation step; without it the screed can delaminate.
- 2
Set screed rails at the target level
Fix timber battens or proprietary steel screed rails across the room at 1.0–1.5 m intervals. Use a laser level or long spirit level to set them all to the same datum height — this becomes your finished screed surface. In a typical room renovation, the target height is dictated by door thresholds, step heights, and the finished floor finish thickness. Double-check your datum against adjacent rooms before mixing any mortar; adjusting rails once the screed is down is not an option.
- 3
Mix the mortar to the right consistency
Combine one part Portland cement with three to four parts sharp sand. Mix dry first until the colour is uniform, then add water gradually. The correct consistency for a semi-dry mix is the classic 'snowball test': squeeze a handful — it should hold its shape without crumbling and without releasing water when you open your hand. A mix that is too wet shrinks and cracks; too dry and it will not bond. If using a forced-action mixer, a batch time of 3–4 minutes gives a reliable result. Avoid mixing more than you can lay in 45 minutes.
- 4
Lay, compact, and rule off
Tip the mortar between two rails, spread it roughly to height with a trowel, then compact it firmly by hand or with a tamper before ruling off. Place the aluminium straightedge across the two rails and draw it towards you in a sawing motion to strike the surface level. Fill any low spots immediately with extra mortar and rule again. Work backwards towards the door so you are never walking on fresh screed. Keep the surface 2–3 mm proud of the rails at this stage — slight settlement and compaction will bring it to datum.
- 5
Float and cure the surface
Once the screed has stiffened enough to walk on without leaving deep footprints (typically 2–4 hours depending on temperature), use a steel or plastic float to close the surface and remove rail marks. For a smooth finish suitable for LVT or resin, work the float in circular passes. For a keyed surface ready for tile adhesive, a slightly open texture is fine. Cover the finished screed with polythene sheeting or a curing compound for at least 72 hours to slow moisture loss and prevent surface cracking. Resist the urge to walk on it heavily for the first 24 hours.
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FAQ
- Can I lay screed over underfloor heating pipes?
- Yes — this is one of the most common reasons to screed a floor in a Greek renovation. The pipes or electric mats must be fully fixed, pressure-tested (for water systems), and held in place so they do not float up into the screed. Use a minimum 65 mm screed depth over the pipe centres, and add polystyrene edge strip around the perimeter to allow for thermal expansion. Do not run the heating until the screed has fully cured — typically 28 days for sand-and-cement.
- How thick does the screed need to be?
- A bonded screed (glued directly to the slab) can be as thin as 25–40 mm. An unbonded screed over a damp-proof membrane needs at least 50 mm. A floating screed over insulation (for underfloor heating or sound insulation) should be 65–75 mm. Going thinner than these minimums risks cracking under foot traffic. In Greece's older concrete buildings, check with a structural engineer if you need to add significant weight — a 70 mm screed over a large area adds roughly 120 kg/m².
- Do I need a building permit to screed a floor in Greece?
- In most cases, a standard screed applied within an existing building does not require a building permit in Greece, as it is considered routine maintenance or interior renovation. However, if the screed is part of a larger structural intervention — for example, raising a ground-floor slab, modifying a load-bearing element, or changing the use of a space — you may need to notify the relevant local authority (Poleodomia) and involve a licensed civil engineer. Always confirm with your local municipality before starting work on a listed building or in a protected zone.