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)