Guest post: Preparing a subfloor (underfloor)

Wood You Like not only gives out FREE advice on this FAQ-blog, you can also frequently find us doing so on various (UK-based) DIY-forums (DIY-not, E-build, Trust Experts, Green Building Forum etc). Other professional and dedicated to quality and service trade-persons are doing the same.

It's with great pleasure we can announce a first guest author post by one of these professionals:

Preparing a sub floor (underfloor)

Wooden sub floor

Floorboards, you need to use a long straight edge to determine how smooth your floorboards are. If they are reasonably smooth, i.e. 1-2mm run out, you need to over board with exterior grade plywood of 6mm thickness. If you have bad cupping of the boards you need to step up the thickness of plywood to bridge across the cupped boards. I.e. 12mm +. You can also sand the cupped boards flat and refer back to using the 6mm plywood.

Chipboard This type of sub floor should be over boarded with minimum 6mm plywood.
(Note Wood You Like: if you plan to install a wooden floor using the secret nailing method)

How to install plywood

Plywood should be left to acclimatise in the area to be prepared for a minimum of 48hrs. To fix the plywood to the sub floor it needs to be stapled with divergent staples, nailed with ring shank nails or screwed down. If thicker than 6mm it must be screwed. Always fix the boards working from the centre out wards. Your staples, nails etc should be at 100mm centres and 50mm apart around the edge + no more than 10mm from the edge. You should leave 5mm gap between the board joints. 

Concrete/screed

Use a long straight edge to determine dips and high spots of the sub floor. Any high spots should be ground down. Any low spots should be marked out on the sub floor. If the dips in the base are deeper than 3mm they should be filled with repair compound. All sub floors must be primed! Next you need to apply a suitable levelling compound. All compounds should be laid at a minimum of 3mm thickness and no deeper than 6mm. Mix all compounds as instructed on the bag.

Which smoothing/levelling compound should I use?

Water based: Very good levelling and flow. Also dries to a very hard surface. Down side is that it’s not forgiving with mistakes when mixing, damp or poor priming. This will result in failure.
Acrylic based: Dries to a very hard surface. It’s also difficult to make mistake mixing. Down side is that it does not flow as well as water based compounds.
Latex based:  Easy to mix and very forgiving but not as strong as other compounds, poor shear strength and does not flow very well.
Water based flex: Designed for problem floors which flex. It can be used over floorboards. Down side is the price and must be kept above 3mm deep.

Testing for sub floor moist.

To British standard the amount of moister in a concrete base you wish to lay on, must not be more than 75% relative to humidity. To test this you need a working damp proof membrane and a cured concrete base if new. The test should be done with a hygrometer. This is a box which is sealed to the floor for on average of 48-72hrs. Different mixes and depths take different times to take reading. More than one area of the sub floor must be tested. New concrete will need at least a month per inch to dry! I.e. 6” needs 6 months drying time.
(Note from Wood You Like: other moist measure equipment (tested and approved to be used for floor installation preparations) the mentioned above measures the percentage of moist content in the screed.  If this type of equipment is used the reading should not exceed 2% moist content.)

Surface epoxy dpm.

If your underfloor has a failed damp proof membrane or no damp proof protection at all, you can install an epoxy coat to act as a surface damp proof. This will work up to about 92% relative to humidity moister content, depending on which product is used. You can also use a surface membrane to fast track fitting your new flooring on new concrete sub floors. Make sure you follow instructions supplied with the product you choose.

Matt Bourne (August 2007)

Comments

[ian]

I have a floor with no DPM oak blocks are starting to lift in places. the blocks or laid on bitumen what is the best method to relay the loose blocks or is it best to remove all blocks are put a waterproof layer down before laying the blocks.

[Karin H.]

Hi Ian

Thank you for emailing us further information about the age of your floor.
We suspect, if there hasn't been a large leak or other 'wet-'accident, the old bitumen has become very brittle and is loosing its bonding.
Best solutions (not the quickest) is to lift all blocks, remove as much bitumen (from blocks and underfloor) as possible and then to re-install the blocks using a modern parquet adhesive.

Hope this helps

Wood You Like Ltd

[Satinder Dogra]

Hi,
I am in the process of laying a new 120mm solid oak T&G flooring in my through lounge but I cannot decide what would be the best underlayment. The sub-floor is suspended wood floor with floor boards and a 6mm ply. I looked at fixing the oak boards using secret nailing/screwing and also floating. Secret nailing/screwing was my prefered choice but I don't know what underlayment I should have between the subfloor and the oak T&G boards.

The other option was to float the floor. In some other posts I read that the boards could be dry fitted but held together at the perimeter using expanded foam in the 10mm expansion gap.

I would also like to know the pro's and con's of using self adhesive underlay as an alternative.

[Karin H.]

Dear Santinder

when secretly nailing you don' use any extra underlayment.
And never, ever fill the expansion gap with anything, specially not with expanding foam!

Wood You Like Ltd

[Amit]

Hi,

I am having trouble in getting my floor leveled. Got the builders to remove the floor joists and lay concrete layer. I went on holiday post this and when i returned, builders suggested this was topped with insulation, and then followed by the UFH pipes. They say it was then topped by Latex screed, which by the time i came back was very unleveled and had cracks all over. To get it to level, builders use self leveling compound wherever it was needed (in large parts), but to my dismay just after 10 days, the floor has started cracking and unleveling again because of these cracks. I am so damn frustrated with all this leveling thing, and ......
(Question has been published as full blown article here)