Always refer to BS 8203:2017 Code of Practice for the Installation of Resilient Floor Coverings
Following extensive consultation with industry experts we have detailed some suggested procedures to help achieve a professional and reliable installation of plywood in the subfloor preparation process. Hanson Plywood welcome the help, support and involvement of all suppliers of subfloor preparation products in ensuring SP101 can be used appropriately and successfully with all types of materials.
Please note that SP101 is stamped on the face side (better side) on 5.5mm only as this is the most common thickness used in the flooring industry.
All other thicknesses – 3.6mm, 9mm, 12mm, 18mm, 25mm are stamped on the reverse of the panel as these panels are also used in industries that require a clean unstamped face.
5.5mm – stamped face up.
All other thicknesses – 3.6mm,9mm,12mm,18mm,25mm stamped face down.
The single most important factor affecting the performance of plywood in a subfloor is moisture, and with this in mind, we pay particular attention to this aspect in our guidance notes.
In common with other wood-based panel products, SP101 plywood is hygroscopic and its dimensions will change in response to changes in humidity.
However, wood tends to shrink/expand much more across the grain than along the grain and the cross – laminated structure of plywood means that the longitudinal veneers in one ply tend to restrain the perpendicular veneers in the adjacent ply.
Accordingly, the dimensional movement of SP101 is quite small; typically, a 1% change in moisture content increases or decreases the length and width of plywood by about 0.15mm per metre run. The corresponding change in thickness is likely to be in the region of 0.3% to 0.4% per 1% change in moisture content. These figures should be taken as a guide only.
When overlaying a subfloor with plywood, a moisture test of the subfloor should be carried out using a wood moisture meter. A reading of the relative humidity in the air and the room temperature also needs to be carried out.
Once all three readings have been taken, the installer should be able to make a decision on whether the timber subfloor is at the correct moisture content in relation to the humidity and temperature of the room.
Prior to installing, a moisture test should be carried out on the plywood underlay and a reading taken. If it is within + or – 2% of the timber subfloor the plywood can be installed.
I.e.; if the subfloor has a reading of 10% moisture content, the plywood would need to have a moisture content of between 8% mc – 12% before it can be installed. If the reading is not within 2% of the subfloor, the plywood should be left in the room to acclimatise until it is within equilibrium of the subfloor.
Note on Moisture Meter Information
Basic moisture meters have only one scale, which is pretty crude and is usually based on readings for Pine; whereas more sophisticated meters such as the Protimeter Timbermaster have a range of 8 or 9 different scales for different wood species. Therefore, we recommend acquiring a meter that can be calibrated for different woods.
Readings can then be taken with the corresponding scale for the correct type of wood – in both the subfloor and the plywood underlay. If these types of readings are not taken, then the inaccuracy of a “basic” meter can actually be more than the “plus or minus 2%” as recommended; and so the REAL m/c’s of the floor and the plywood may be considerably more than 2% adrift from one another.
When pushing the meter pins into plywood, it is possible that they may hit the glueline and give a “false” reading. Care should be taken to ensure pins are not pushed too far into plywood to enable an accurate reading of the face veneers.
Before laying your plywood underlay sheets, the subfloor should be thoroughly cleaned and checked for removal of any debris allowing sheets to lay as flat as possible.
The correct thickness of plywood panel should be selected dependant on the quality of the surface to be overlaid and the overall height of the finished floor.
It is recommended, where possible, that sheets are laid perpendicular to floor boards with the joints in the plywood staggered.
Typical fixings recommended are ring shank nails and countersunk screws.
Advice on suitability of fixing process should be taken with regard to the type of covering to be applied and any further surface preparation required. For best results fixings should be countersunk.
Fixings should be spaced at a maximum 100 mm centres around the perimeter of each plywood sheet, 12 mm from the edge and at a maximum 150 mm within the sheets.
When using levelling or seam filling compounds which contain water, the plywood underlay, and associated subfloor components, will again need to be allowed a period to dry out to reach a moisture equilibrium. Once this is achieved, further finishing of the floor covering installation can take place.