This hints and tips will explain the various uses and techniques for laying a chipboard sub-floor. There are three main uses for chipboard; as the subfloor on timber floor joists, as a floating floor on insulation and concrete and for boarding lofts. The first use, on timber joists, is commonly the technique used on new builds and renovations. If you find yourself with a house with old rotten floorboards then a chipboard sub floor is a quick cheap and solid replacement.

The Product

There are three main types of chipboard on the market.

 The classis 8”x4” sheet. Available at DIY sheds and builders merchants in thicknesses from 12mm to 25mm. This is the cheapest way to buy chipboard (expect to pay from £10 a sheet for 18mm) but it is the hardest to transport, requires the most cutting and has no tongue and grooved edges. I would steer clear of this board for flooring.

I would just like to digress at this point onto the topic of imperial v metric. I am not being old school by using imperial measurements, it is our archaic system that is to blame. Timber sheet materials are sold as 8x4 and they measure 2440mm x 1220mm. Confusingly, plasterboard sold as 8x4 sheets is supplied in the metric equivalent so it measures 2400mm x 1200mm. So two sheet materials nominally the same size are actually quite different so bear it in mind when setting out noggins and studwork. I don’t make the rules (well not yet anyway!)… it is just the way it is.

Loft Boards. Most commonly available in the DIY stores, these normally come as packs of three, each measuring 1220 x 320 x 18mm. They cost less than a fiver for a pack and come with T&G edges to create a strong interlocking floor. These boards are specifically designed for boarding lofts by a DIYer who has to take them up through a loft hatch. As such they are perfect for the job and are much better for the job than buying 8x4 sheets and chopping them up. Any savings in materials will be lost in time spent cutting, and the T&G edges make for a much more professional job.

P5 V313 chipboard. The professionals choice. Available in a standard size of 2400 x 600mm in a choice of two thicknesses, 18 or 22mm. For some time there were two types of board, P4 and P5. Only the latter was moisture resistant so it was dyed green. Now the manufactures have done away with P4 meaning all chipboard is P5 so there is no need for the green colouration. This simplifies matters a little and seems to have given the manufacturers a dubious claim to environmental responsibility by no longer using dye.

These boards have the advantage of T&G on all four sides, and come in large sheet sizes making them very economical to use, retailing at about £6 a sheet for 18mm.

Boarding a Loft

If you can get the V313 into you loft then use it. If not then go with the smaller loft boards. The principle of laying any wooden floor is the same. The key elements are as follows:

• Fix boards at right angles to the ceiling joist
• Join the short edges on the centre of joists
• Stagger the short end joints across the floor in a brick pattern.
• Screw boards down when working in a loft as nailing could disturb plasterwork on the underside of the ceiling. Use screws 2.5 times the depth of the board. I would recommend a 50mm No.4 screw.
• Given the infrequency of use, there is no need to glue the boards in a loft. 

Special considerations for lofts.

At TBE we have always tried to exercise a bit of laissez-faire and give our readers respect as capable workers but the following should be considered as common sense precautions.

•  Are the ceiling joists able to take the weight of the boards and the mountain of junk you are going to put on top of them? If in doubt seek advice or reconsider the aims of the project.
• If your loft has modern levels of insulation you will probably find the insulation is much deeper than the ceiling joists. Do not squeeze the insulation down. Instead counter-batten across the joists with the relevant depth of timber and place the top layer of insulation within the counter battens. Then fix the chipboard at right angles to the counter battens.
• Do not block ventilation at the eaves, trap cables, or even more shocking, screw through any cables.
• Halogen lights. If you have halogen lights these can be problematic as they need ventilation around them to keep them cool. Ensure there is no insulation on top or near them and an adequate air flow around the lights. It would be best to avoid covering the lights at all if possible, perhaps leaving a square of floor out around them.
•  Loft conversions. Boarding the loft is not the first stage in a DIY conversion to get that extra bedroom. To officially (i.e. when the property is sold) add an extra room via the loft it must comply with a host of regulations regarding loft conversions. If you want to go down that route, speak to an architect and don’t even bother to start by putting a floor down.

A Floating floor

This is a great alternative to screeding a floor, and an excellent way of getting insulation into a previously cold floor. You will only use this method in a drastic renovation or extension as it is very disruptive to rip up an existing screed. But if you want to consider it, here is the way to go.

The concrete sub-floor needs to be smooth and level otherwise any bumps will translate through to the chipboard. NHBC guidelines require no more than 5mm deviation under a 3m straight edge. If you have a troublesome concrete floor then apply a self-levelling compound first.  

If the concrete slab does not already have a DPM then one should be incorporated at this stage and lap up the walls above finished floor level. Alternatively, consider a two coat liquid DPM. 

Lay insulation, either Jablite(polystyrene) or Celotex (PU rigid board) type depending on the required U-value. A minimum 25mm is recommended for floating floors but most jobs will specify more than that. The insulation should be staggered and laid carefully to ensure it sits flat without bouncing on off-cuts of insulation or other debris.

Install a vapour and moisture control barrier above the insulation to keep the chipboard dry with any joints in the barrier overlapped and taped.

On top of this the chipboard can be laid out in a brick pattern, making sure the joints in the insulation do not coincide with joints in the chipboard. On all floors an expansion gap must be allowed. Minimum 12mm all the way round will suffice, but increasing if a single run of floor exceeds 6m. Allow for expansion at doorways by using thresholds.

Also consider access for pipes and cables. Glued floors cannot be taken apart once dry so ensure all work below the floor is complete and satisfactory.

Floating chipboard floors should be glued on all the edges using a generous amount of PVA glue. Apply a bead into the groove and on the top edge of the tongue. Once the glue is dry you have a well insulated, perfectly flat floor ready for the final floor covering.

On Timber joists

Fitting chipboard to timber joists is again a relatively simple task. As above, the boards need to have the short ends staggered and joined on the centre of joists. Where this is not possible fix noggins between the joists. V313 can be nailed or screwed. As a personal preference I prefer to use screws but nailing is probably a little quicker. Glue must also be used in the same way as a floating floor but to go the extra mile, put a bead of glue onto the joist. This will help eliminate squeaking.

 When screwing into joists it is always a good idea to mark the centre line so you don’t miss with your screws or nails. It only takes a few extra minutes but is worth it. Marking is absolutely critical where pipes or cables cross joists. In an ideal world, they will pass through holes drilled in the centre of the joists so as long as your screws are not too long you should have no trouble. But often there will have been some notching of the top of the joist. In this instance be extra sure that you have marked the area where the pipes or cables are so you know not to put any fixings in. If you have underfloor heating in trays suspended between the joists then go its time to get really scared! Tempting though it is to walk away, if you think about it logically, if the joists are properly marked up and the ‘no-fixing zones’ are clearly marked you will be fine. Preparation and patience are the key in this instance.

 And finally, written on each board are the words “Lay this side up”. Just a tip….

Chipboard floors are the industry standard and offer a very economical, quick and simple method of creating a perfect subfloor. The floating floor technique offers an excellent alternative to screeding and should be considered in renovations as an opportunity to get insulation into previously cold floors. The world has moved on from floorboards. Many are reluctant to embrace a floor that cannot be taken up again without serious upheaval, but why should a floor need to come up again? If there are further electrics or plumbing to do then the job is not ready for the floor to be fixed. If the services are all in then either leave access panels or accept that they are all present and correct.