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Floor Screed

The floor in the new extension is currently the top of the fairly rough over site concrete which was poured before any of the structure was built. The level is around 3 inches lower than the floor in the original part of the house and now needs to be screeded to bring it up to the same level.

Heating pipes set on concrete slab Insulated water pipe

As you can see in the pictures there are a number of pipes running across the concrete. These are mostly heating pipes and have been run in push fit plastic. They have been secured in position with pipe clips plugged and screwed direct to the slab. These require no special treatment before laying the screed as the plastic will not be degraded by the cement. In addition, there is a copper pipe running across the floor which supplies a new outside tap. This would be liable to corrosion from the cement in the screed so has been protected with a plastic lined felt type lagging. This is simply threaded onto the pipe as it is laid. The radiator tails have been protected in the same way.

Mix on site lorry Screed mix being loaded into barrow

As you can imagine, there is a fair amount of sand and cement required to screed a floor of this size and the plasterer has arranged a ‘mix on site’ service from a local company. The lorry arrives first thing and parks up right outside the job. On board the lorry are the sand and cement as well as a mixing unit. The beauty of these services is that you can have exactly the mix and quantity you need made up on site and, as the plasterer pointed out, it’s a lot quicker than using a standard builder’s concrete mixer.

Barrowing screed to work area Screed stored on tarpaulin

The driver sets the machine up and begins mixing the screed – a cement and sharp sand mix in the ratio of 1:3. A dry flake additive is also added to the mix. The plasterer explained that this is a retarder which will slow down hydration of the cement and therefore the setting time. It’s actually a really hot day so this is particularly important. When the first batch is ready, it gets discharged from a short conveyor belt mounted under the hopper straight into a waiting barrow. The first half dozen barrow fulls are wheeled straight round to the work area allowing the plasterer to get started. The rest is offloaded onto a large plastic tarpaulin on the front drive. This allows the delivery to be completed relatively quickly rather than having to wait while the barrows are ferried back and forth to the back of the house. Access is tight so this would have slowed things down too much. Once it’s all been offloaded, the pile is kept shielded from the hot sunshine with a plastic sheet to minimise drying out.

Sand and cement spread out over first area Levelling screed with steel straight edge

The sand and cement is a semi dry ‘biscuit’ mix and gets roughly shovelled out. The plasterer starts by spreading the mix along one edge of the floor, where the old and new floors meet, as well as along the side wall. A band of about 18” or so it laid in each direction. This will form the ‘base’ level from which the rest of the floor can be levelled. Making sure that the mix is fully compacted first, the screed is levelled up with the floor edge of the old part of the room. Then, using this level as a guide, a long steel straight edge is drawn across the surface of the surrounding area. This pulls away any excess mix leaving a flat section which is then checked for level. The straight edge actually has a spirit level vial mounted in it allowing the same tool to be used.

Using float to smooth screed Checking screed with spirit level

The surface is fairly rough, but, once he’s happy with the levels, the plasterer begins smoothing it. The tool for this is a plastic float which has a slightly rough surface and rounded edges. The reason for the rounded edges soon becomes apparent – it’s used in a circular motion and the edges ‘float’ across the work rather than dig in. Continuing to work in this polishing fashion, the plasterer quickly produces a smooth level surface on the first area. Any low spots get filled in as he goes. A little extra mix is spread over the hollow and worked smooth again with the float.

Levelling return section Float used to smooth screed

Another ‘strip’ of screed is laid on the return wall back towards the French doors so there is now a ‘U’ shape area of screed around the perimeter. Joining the ends of this shape, the plasterer runs a band from front to back of the room.

Levelling central strip Levelling main infill with straight edge

With this in place, the middle section is infilled by shovelling in more of the mix so that it’s roughly the right depth. Spanning the straight edge between the previously laid strips, the finished level can be achieved. The straight edge is drawn across so that it sweeps away excess material. This area then gets polished up with the float. Filling any hollows as he goes, the plasterer has this first section complete. The result so far looks great – and the plasterer has a knack of making it look far easier than it is.

Main area being floated Main area almost complete

While the first section was being screeded, one of the others has been busy barrowing through the remainder of the mix from the front drive to the other half of the room. The process of screeding this section is the same as before. A band is built up along the edges to create ‘U’ shape around this side of the room, carefully checking levels as he goes. Another front to back strip is laid like the one earlier and the area between here and the edges gets infilled, levelled and smoothed.

Lastly, working from the original floor back towards the new French doors, the middle section is built up. Apart from the obvious reason of using the original floor level to work back from, working this way means he’s not left boxed into a corner. More of the mix is shovelled into the middle area and scraped back with the straight edge as before. The plasterer floats each section so that it’s smooth and level as he goes.

Finished screed Finished screed close up

The whole process has been surprisingly quick – especially when you see how much sand and cement was needed.

If you live in the Hertfordshire area and are looking for a professional building contractor, you can get in touch with G L Smith and Sons via their website:

G L Smith and Sons