Sub floors can be divided into three basic categories as follows:
1) Concrete or screed
2) Wooden: floorboards, ply or chip board
3) Floor joists or battens.
Whichever of the above sub floors you have, remember that the finished floor will only be as good as the sub floor it is laid on. It is therefore worth the time and effort to get your sub floor right.
Concrete or screed
Concrete or screed floors must be level to within plus or minus 3mm over a two-metre span before any hardwood floor can be fitted. This applies whether you are floating the floor or sticking the floor directly to the sub floor. The screed must also be dry before attempting to fit the floor. There are lots of “old wives tales” of allowing one month of drying time per inch of screed or a minimum of 4 weeks drying time. The truth is that these are at best guesstimates and at worst potentially reckless, if the sub floor is not dry enough you risk the floor failing in the future. The concrete/screed must according to British Standards 8201:2011 be allowed to dry to a maximum of 75% relative humidity before installation takes place. The sub floor must be tested before a hardwood floor is fitted. The British Standards compliant test involves drilling into the concrete to measure the amount of moisture the floor is giving of over a 24 hour period. A surface mounted humidity box can also be used if drilling into the screed is not practical. Simple surface tests will only tell you what is going on on the surface of the screed and not if the screed has dried out to any depth.
For those of you who are in a hurry and cannot wait for your screed to dry out to 75% Relative Humidity, there are now surface treatment products on the market that will allow you to fit a hardwood floor much sooner. Be warned, these products are very expensive!
Wooden: floorboards, ply or chip board
Wooden subfloors of any nature must be level to within plus or minus 3mm over a two-metre span, must be securely fixed and any rotten floorboards replaced prior to installation of a hardwood floor. It is also a good idea to test the moisture content of the wooden sub floor using a protimeter. The sub floor moisture content should be within 4% of your hardwood flooring to avoid over expansion or contraction.
Existing “Parquet Block” flooring is not a suitable subfloor for any hardwood flooring and must be lifted prior to fitting a new floor.
Floor joists or battens
As with any other sub floor, joists or battens must be level to within plus or minus 3mm over a two–metre span. The distances between the joists or battens will determine the amount of fixings that can be used (you can only nail where there is a joist). Solid floors must be fixed every 250mm – 300mm and 20mm engineered floors every 400mm. Most houses in the UK have joists that are 16 inches or 400mm apart.
If you are fixing a hardwood floor to battens on top of an existing concrete or screed sub floor, the concrete/screed must comply with the conditions stated above.