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Once all of your insulation is down and the underfloor heating has been laid out, you will be able to begin screeding.
Screeding a floor is the process of laying down a layer of material that will provide you with a flat surface upon which to lay your flooring of choice.
It’s worth bearing in mind that this is not an easy job to get right, and a badly screeded floor can cause problems later on.
However, if you feel confident enough to attempt screeding a floor yourself, we’ll go through the process step-by-step.
For DIYers a semi-dry mix is the only way to go. Its components (sand and cement) can be purchased cheaply from pretty much any decent DIY store and requires only a few basic tools to complete the job.
Liquid screed on the other hand uses a different mix which needs to be pumped out via specialist equipment when covering large surfaces; therefore a contractor is required to complete the job successfully. For this reason we will concentrate on semi-dry screed in this article.
Check out our calculators to get an idea of the quantities you’ll need to get the job done.
Screeding a floor relies on good preparation
A screeded floor is only as good as what’s underneath it, so it is vitally important that everything has been prepared to a high standard before continuing. If you haven’t done so already, be sure to take a look at our guides on installing insulation and underfloor heating before proceeding.
Preparation also includes ensuring that the area to be worked on is both wind and waterproof, as well as having provisions put in place to keep the surface free from footfall for the duration of the curing process.
Step 1 – Tools you’ll need
As with any DIY task it’s easy to overbuy the tools needed to complete a job. Screeding a floor is more about knack than an expensive range of equipment. That said, there are a few things that you’ll need to have to hand:
- Tape measure – to measure the size of the room so you can calculate how much mix will be needed. We recommend the Stanley Powerlock 10m
- 100mm x 50mm timber battens – to make the framework for the job
- Spirit level – to ensure the framework is level. A four foot level works best here
- Shovel – To mix and load. We recommend a Spear and Jackson for the job
- Wheelbarrow (or buckets for smaller amounts) – essential for getting the mix from one place to the next quickly. The BRISTOL EASILOAD will make your life a whole lot easier
- Straight edge (or a piece of excess timber) – to run along the framework for a nice flat finish. If you want a pro tool for this we recommend Refina
- Trowel – used to move the mix into place and give a reasonably smooth finish. Get yours here
- Float – to give that all important super smooth finish. Nothing too extravagant needed here, a Dekton will do the job nicely
- Protection – Gloves and knee protection should be worn throughout, cement burns like crazy when you’re exposed to it for any length of time
Step 2 – The mix
If at all possible, having someone to help you throughout the process would be an enormous advantage. They could be preparing the mix while you perform other tasks or vice versa. It will also help to get the mix down quickly too, as one of you can be bringing it in while the other concentrates on laying it.
The subject of the correct mix is a contentious one. You’ll hear all manner of ratios when asking what the best mix is. We like 4:1 sand to cement, but ask around and you could get as many as half a dozen different answers.
Something else that can cause a great deal of debate is the use of additives in a screed mix. Some offer faster drying times, others higher thermal conductivity and strength, but for the purpose of this article we’ll leave those for another day.
Add water sparingly and test as you go. The old rule of ‘you can add but you can’t take away’ should be followed here. The important thing to remember is that once mixed correctly, the consistency should hold together in your hand when compressed into a ball but then crumble up nicely without leaving wet clumps. There should be little to no moisture escaping when squeezed in the hand.
One of the best tips is to choose your sand wisely. You should go for a good sharp sand with a nice mix of grain sizes to it. Avoid uniform looking sand grains as these won’t bond as well.
Step 3 – Levels
In order to be able to lay a completely flat surface you’re going to need some guidance as you go. The easiest way for a DIYer to do this is to work within sections. These are typically laid out with timber battens as shown below:
A professional will create their own levels using the mix to make a frame. This takes plenty of practice, but timber battens work just as well and make things a whole lot easier for the novice screeder.
Creating these sections will allow you to work in a much smaller area, which makes it far easier to keep the surface flat. In addition, the battens will create runners upon which you can lay your straight-edge when you begin to smooth off the surface.
The amount of battens that you use is entirely down to the size of the room and how comfortable you are with the section size that you’ll be working on, the diagram is purely a guideline. A semi-dry mix is usually laid to a depth of around 65 to 75mm when used for underfloor heating so your battens height should reflect this once in place.
In order for the battens to sit securely you’ll first need to lay a bed of mix for them to be placed on. This bed should be around 3cm deep so you can get the battens packed down and levelled off to the desired height.
When laying out the timber it is important to constantly check that the battens are lying true with a spirit level. Test the level both along and across the battens as soon as two or more are in place.
Selecting a nice straight piece of timber before you begin can prevent a lot of head scratching when it comes to this point! If they are off, a small amount of mix under the dip will allow you to get your level.
Another tip is to make sure that you wet the timber just before laying down the mix. Doing so will make things easier when it comes to removing the battens for filling.
Care must be taken when working with underfloor heating, as a mistake can be an expensive one. It is important that the pipework is uppermost in your mind when screeding a floor so just be aware of where they are whilst working.
Step 3 – Laying it down
Now that you have your mix ready and framework in place, there’s no time for tea! Start at the furthest points away from the doorway so that you can work your way back towards it.
The last section that you complete should be the one directly in front of the entrance to the room (the vertical section in the diagram above).
When you’re ready, start shovelling the mix onto the floor next to the furthest wall. Put down enough to cover about 60cm (2 feet) from the wall, if you lay down any more than this in one go you’ll struggle to reach the edge.
Move the mix around with your trowel enough to cover the area you’ll be working on, compacting the mix as you go. Poor compaction is one of the more common problems novices have when screeding a floor. The constant changes in temperature and cement shrinkage can also play their part, but compaction is the biggest issue so be certain to bed it down properly around the pipes.
Once in place, you can then grab your straight edge.
Place the straight edge at the furthest point away from you and slowly begin to draw it back in a slightly circular, sawing motion along the top of the battens to create a smooth and flat finish.
If you leave any dips or deviations with the straight edge, simply put some mix on your trowel and flick it into place. Flicking it down will help it to bond rather than just placing it. Then you can go over the whole area with your float to give it a final smoothing out.
Leave your straight edge on top of the finished portion and lay down another 60cm worth of mix before repeating the process above. Repeat with all sections, remembering to remove the battens and fill in the gaps as you go. Do this as carefully as possible so that you do not disturb all of your hard work.
Step 4 – Protection
Now that the mix has all been laid and you have a nice flat surface, it’s important to make sure it stays that way.
Cover with a polythene sheet to hold the moisture in and aid the curing process.
Keep anyone and everyone off of the floor for at least 48 hours, preferably longer.
Step 5 – Drying
Drying shouldn’t be confused with curing. Curing actually needs the moisture to remain somewhat constant in order to prevent cracking, so the polythene sheeting should be left down for 7 to 10 days before removing. After that, the true drying can begin.
For a traditional mix, such as the one used here, the drying time is usually one day for every millimetre in depth. So, for a 70mm covering, a 70 day drying time should be allowed. Variables such as weather conditions and temperatures should be taken into account, however.
Once drying is complete you are only one step away from enjoying the benefits of your new UFH system, laying the floor.
Don’t forget to take some pictures while you follow our guide, we’d love to see your work and we might even start up a new page devoted to them. Fire them across to us via social media or email.