OSB Flooring Installation and Fixing

OSB is light, strong and goes down dry. That makes it a good choice where a wet screed would put too much weight on the structure or hold up the work. Typical situations include:

• A loft or older house with a timber joist floor that needs a new structural deck
• A renovation where old floorboards or parquet come up and the surface underneath is uneven
• A cabin, garage workshop or utility room where the floor needs to be finished quickly
• A dry floor build-up with levelling fill and insulation in a flat or timber-frame house

Older floors often have several layers, such as parquet stuck down with bitumen adhesive or old vinyl tiles. These usually come up before the OSB goes down. If you suspect asbestos in old floor tiles or adhesives, do not break them up yourself. Only a licensed specialist contractor should remove that kind of material.

The base decides whether the floor stays flat and quiet in a few years' time. It must be clean, dry and solid. Unevenness of more than roughly 2 mm under a two-metre straightedge should be corrected before fitting.

Timber joists

Joists need to be sound, free of rot and level with each other. If one sits proud, it is planed or trimmed down, or the others are packed up to match. Joist spacing determines the board thickness. With old floors that sag or have damaged joist ends, it is wise to have a structural engineer check the load capacity. Replacing or strengthening joists is no longer ordinary flooring work, and before that kind of job you should check with your local building authority what approval is needed.

An existing solid floor

OSB can go straight onto concrete, a stable chipboard deck or old floorboards that do not bounce. On concrete at ground level or above a cellar, a polythene vapour barrier comes first, usually at least 0.2 mm thick, so moisture from below does not get into the timber.

Dry levelling fill and insulation

For larger height differences, a dry levelling fill is spread, compacted and screeded flat. Rigid insulation boards go on top, for example high-density mineral wool floor boards or wood fibre boards, and the OSB goes over them. In a block of flats, impact sound insulation matters because of the neighbours below. Before a bigger change to the floor build-up, it is sensible to let the building management or owners' association know.

Thickness depends on whether the board spans gaps by itself or simply rests on a solid base. As a rough guide:

• 15 mm only as a levelling layer on a continuous solid base, never across joists
• 18 mm for joists or battens at closer spacing, usually up to about 40 to 50 cm
• 22 mm or more for wider joist spacing or heavier loads, such as in a workshop

The board manufacturer gives the exact permitted spans, and it pays to stick to them. For heavier loads, a two-layer build-up also works, with the second layer covering the joints of the first.

For indoor floors, OSB/3 is the usual choice because it copes with moderate humidity. OSB/4 is stronger and meant for more demanding conditions. OSB/2 is for dry conditions only and is not really suitable for floors. Tongue and groove boards give stiffer joints between joists, so they are used on floors more often than square-edged boards. A sanded board has a smoother face, which helps under thin coverings such as vinyl.

If you are laying OSB on a stable old floor or on level joists in good condition, you can manage with a cordless drill driver, a circular saw and a spirit level. If you have any doubt about the strength of the joists, get advice from a professional first.

1. Leave the boards lying flat in the room where they will be fitted for at least 24 to 48 hours so they adjust to the temperature and humidity.
2. Clean the base, check it with a straightedge and level out any high or low spots.
3. On concrete, lay the vapour barrier with overlapping joints, and add an insulation layer if needed.
4. Leave a gap of roughly 10 to 15 mm around the edge of the room, using wedges as spacers.
5. Lay the boards with the long side across the joists and the groove facing the wall.
6. Stagger the joints so the end joints of neighbouring rows do not line up. Always place short edges without tongue and groove on a joist.
7. Knock tongue and groove boards together with a rubber mallet over an offcut of timber so the edge is not damaged. With square-edged boards, leave a gap of about 2 to 3 mm between them.
8. Screw about every 15 cm around the edges and roughly every 30 cm across the board, about 10 mm in from the edge. Use wood screws around two and a half times the board thickness in length.
9. Sink the screw heads just below the surface, no deeper.
10. Lightly sand any lips between boards and vacuum the whole floor.

The simplest case is a rectangular room with straight joists or sound concrete. Anything that adds cutting and preparation makes the job longer and therefore more expensive:

• Lots of corners, alcoves, columns and cut-outs around pipes
• Uneven or sagging joists that need packing or trimming
• Lifting the old floor and taking the waste away
• Levelling fill, vapour barrier and insulation under the boards
• Thicker boards, a two-layer build-up or OSB/4 grade
• Carrying materials up to a higher floor without a lift

Most problems only show up after a few months, when the floor starts to ripple or creak. The same mistakes come up again and again:

• Boards pushed tight against the wall, which then bulge in damp weather
• Too few screws, or screws that miss the joist, so the board bounces and creaks
• Fitting on an uneven base, with the waves showing through the laminate later
• Thin boards on widely spaced joists, which sag in between
• Boards fitted straight from a cold store without time to acclimatise
• Short joints left floating between joists on square-edged boards

Before the final floor covering goes on, it is worth walking the whole area and checking:

• Flatness with a straightedge in several directions, with no noticeable waves or steps at the joints
• That the floor does not bounce or creak underfoot, especially near walls and midway between joists
• The expansion gap around the whole perimeter, including at thresholds and pipes
• Staggered joints, with no end joints lined up in one row
• Screw spacing and that the heads sit just below the surface
• A clean surface without dust, splinters or raised edges
• On concrete, that the vapour barrier is in place under the boards