Walk onto any building site in Kenya and you will hear “screed” and “concrete floor” used as if they mean the same thing. They do not, and mixing them up leads to cracked tiles, hollow sounding floors and expensive rework.
A concrete floor slab is the structural base that carries loads and passes them to the ground or the frame. A cement screed is a thin layer of sand and cement placed on that base to give a level, smooth surface ready for the final finish.
In simple terms.. concrete carries the load, screed prepares the surface. One is specified by the engineer, the other is controlled by the mason and the site supervisor.
This guide explains how the two differ, how each is mixed and laid, which standards apply, and the mistakes that ruin floors on real sites. Whether you are an engineer, a mason, a tiler or a quantity surveyor measuring the work, it will help the whole team speak the same language.
Quick Answer: A concrete floor is a structural slab, usually 100 to 150 mm thick, that carries loads. A cement screed is a thin sand and cement layer, usually 25 to 75 mm thick, laid on top to give a level, smooth surface for tiles, timber or other finishes.
What is a concrete floor slab and what does it do?
A concrete floor slab is a structural element. On the ground floor of a typical Kenyan house, it is a ground-bearing slab sitting on compacted hardcore, a layer of blinding sand and a damp-proof membrane. It is designed to carry people, furniture, walls and equipment without cracking or settling.
The truth is, the slab is only as good as what sits under it. If the hardcore is poorly compacted, no amount of screed on top will save the floor.
Typical site practice for a domestic ground slab:
- Hardcore 150 to 225 mm thick, compacted in layers.
- Blinding sand 25 to 50 mm to protect the membrane.
- Damp-proof membrane, usually 1000 gauge (250 micron) polythene, lapped and taped.
- Concrete grade C20 to C25 as shown on the structural drawings, with mesh such as A142 where specified.
Concrete quality is governed by BS 8110 and BS EN 206, and the Kenya Building Code sets the minimum requirements for floors. The structural engineer decides the thickness and reinforcement, and the site team must follow the drawings.
What is a cement screed and why is it used?
A cement screed is a layer of cement and sand, mixed with just enough water to be workable, laid on a concrete base or on a suspended slab. It does not carry structural load. Its job is to give you a surface you can finish.
Screed is used to:
- Bring the floor to the exact level shown on the drawings.
- Create falls to drains in bathrooms, kitchens and balconies.
- Give a smooth, flat bed for tiles, vinyl or timber flooring.
- Cover pipes and conduits laid on top of the slab.
The industry reference is BS 8204-1 for concrete bases and cementitious levelling screeds. Cement used on site should comply with the relevant Kenya Standard, such as KS EAS 18-1.
Honestly, most tile failures that get blamed on the tiler start with a weak, dusty or badly cured screed.
Table 1: Concrete floor slab compared with cement screed
| Feature | Concrete floor slab | Cement screed |
|---|---|---|
| Main purpose | Structural support and load transfer | Level, smooth finishing surface |
| Typical thickness | 100 to 150 mm (ground slab) | 25 to 75 mm |
| Typical mix | C20 to C25 with coarse aggregate | 1:3 or 1:4 cement to sharp sand, no coarse aggregate |
| Reinforcement | Mesh or bars as designed | None, or light mesh in thick screeds |
| Designed by | Structural engineer | Specified by architect or engineer |
| Controlled by | Engineer, supervisor, cube tests | Mason, foreman, supervisor |
| Main reference | BS 8110, BS EN 206 | BS 8204-1 |
Which types of screed are used and how do they differ?
Not every screed is laid the same way. The type depends on the base, the thickness you need and whether there is a membrane or insulation below.
A bonded screed is laid directly on a clean, prepared base and bonds to it. An unbonded screed sits on a separating layer such as polythene. A floating screed sits on insulation and is not attached to the slab at all.
Table 2: Screed types and recommended thickness
| Screed type | Typical thickness | Where it is used | Key point |
|---|---|---|---|
| Bonded | 25 to 40 mm | Ground floors, new slabs, tiled areas | Base must be clean, rough and damp, with a bonding slurry |
| Unbonded | 50 to 75 mm | Over a membrane or old slab | Thicker, needs mesh in larger areas |
| Floating | 65 to 75 mm | Over insulation, upper floors | Edge strips needed to isolate from walls |
On most Kenyan residential jobs you will use a bonded screed on the ground floor and an unbonded or thin bonded screed on upper floors. Always check the specification before the mason starts.
How do you lay a cement screed step by step?
Laying screed looks simple, which is exactly why it is often rushed. Follow a clear sequence and the floor will last. Skip steps and you will see hollow patches within months.
Table 3: Screed laying sequence on site
| Step | Action | What to check |
|---|---|---|
| 1 | Prepare the base | Sweep clean, remove laitance, dust and oil |
| 2 | Set levels | Fix screed guides or battens to the finished floor level |
| 3 | Apply bonding slurry | Neat cement and water paste, brushed on just before screeding |
| 4 | Mix the screed | 1:4 cement to clean sharp sand, low water, damp not wet |
| 5 | Place and compact | Press down firmly, no loose pockets |
| 6 | Level and float | Straightedge between guides, then wood float to close the surface |
| 7 | Cure | Cover with damp sacks or polythene for at least 7 days |
The mix matters more than most people think. A good screed mix feels like damp sand that holds its shape when squeezed. If water shines on the surface, the mix is too wet and the screed will shrink and crack.
Use clean, sharp sand. River sand with silt or clay, or sand from a contaminated pit, weakens the bond and gives a dusty surface.
Screed also needs time to dry before finishes go on. A common rule of thumb is about one day for every millimetre of thickness for the first 50 mm, and longer beyond that. Tiling over a wet screed traps moisture and leads to tiles popping.
What mistakes ruin screeds and concrete floors on site?
These are the problems that show up again and again on Kenyan sites, and almost all of them are preventable.
- Too much water. It makes placing easier but cuts strength and increases shrinkage cracks.
- Screeding on a dusty or oily slab. The screed will not bond and sounds hollow when tapped.
- No curing. Screed that dries in the sun in a day or two loses strength and turns powdery.
- No movement joints. Large floor areas need joints, normally at door thresholds and about every 4 to 6 metres, lined up with joints in the slab below.
- No falls in wet areas. Bathrooms and balconies need a fall of about 1 in 80 towards the drain, set out before screeding.
- Using screed to hide a bad slab. A 100 mm screed to fix a slab that was poured out of level is wasteful and cracks easily.
This is where the whole team matters. The structural engineer controls the slab level, the plumber fixes drain positions, the electrician finishes conduits before screeding, and the supervisor confirms levels before any screed goes down. When one trade is out of sequence, the floor pays for it.
Table 4: Quick site checks before and after screeding
| Stage | Check | Who checks |
|---|---|---|
| Before | Slab clean, services fixed and tested | Supervisor, plumber, electrician |
| Before | Levels and falls marked | Foreman, engineer |
| During | Mix ratio and water control | Mason, foreman |
| After | Hollow sound test by tapping | Supervisor, tiler |
| After | Flatness with a 3 m straightedge | Supervisor, clerk of works |
For the quantity surveyor, remember that screed is measured by area and stated thickness in the bill of quantities, while concrete slabs are measured by volume. Mixing up the two items leads to pricing errors.
Frequently Asked Questions
Q: Can I tile directly on a concrete floor without screed?
Only if the slab was finished to a very flat, tile-ready surface, which is rare on ordinary sites. Most slabs have level variations of 10 mm or more, and tile adhesive cannot correct that. A thin bonded screed gives you the flat bed that tiles need.
Q: How thick should a cement screed be?
A bonded screed is normally 25 to 40 mm, an unbonded screed 50 to 75 mm, and a floating screed about 65 to 75 mm. Going thinner than 25 mm on a bonded screed leads to crumbling edges. Always follow the project specification.
Q: What is the best mix ratio for floor screed?
Most sites use 1:4 cement to sharp sand by volume for general floors and 1:3 where a stronger, harder surface is needed. Keep water low so the mix is damp, not wet. Measure with a gauge box, not with shovel counts.
Conclusion
A concrete floor and a cement screed do different jobs. The slab carries the load and is the engineer’s responsibility. The screed makes the surface level and finish-ready, and it depends on good workmanship, clean sand, low water and proper curing.
Get both right and your tiles, vinyl and timber will sit flat and last for years. Next time you are on site, tap the floor, check the levels, and ask which layer you are really looking at. That one habit will save you money and rework.
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