Fitting wood floors over different subfloors

Concrete subfloors

Subfloor must be flat

Make sure the concrete slab is flat to within 3mm in 2m radius.

If the slab is out of specification, consider grinding, floating or both. Many high spots can be removed by grinding, depressions can be filled with approved leveling compounds, and slabs can also be flattened using a self-leveling concrete product.

When sanding or grinding concrete, care must be taken to minimize the amount of silica dust produced. Best practice would include using dust-collection devices, or applying water to the concrete before sanding. Approved respirators should also be used to minimize the amount of silica dust inhaled.

Subfloor must be dry

For more information, please see moisture testing.

Slab must be:

Minimum 3000 psi.

Free from non-compatible sealers, waxes, and oil, paint, drywall compound etc.

Check for the presence of sealers by applying drops of water to the slab, if the water beads up, there may be sealers or oils.

Do not attempt to glue a wood floor over a chalky or soft concrete slab.

Burnished, slick steel-trowel slabs and power floated slabs may require screening with a 30-grit abrasive and using Ted Todd Primerfast as a primer.

Specifications for lightweight concrete

Make sure the concrete is well bonded to the sub-floor. Check for hollow spots, cracks and loose areas.

As with on-ground concrete sub-floors make sure the concrete is clean, flat to specification and dry.

Over lightweight concrete (less than 3000 psi), only float engineered floors directly over the sub floor.

Rule of thumb: Draw a nail across the top; if it leaves an indentation, it is probably lightweight concrete.

For wide solid boards a ply sub floor can be installed over a concrete screed.

Nominal 15mm Class 1 Exposure ply wood sub floor panels.


Solid subfloors

Subfloor must be flat

Make sure the boards are flat to within 3mm in 2m radius. If the boards are out of specification, consider sanding. When sanding, care must be taken to minimize the amount of dust produced. Best practice would include using dust-collection devices. Approved respirators should also be used to minimize the amount of dust inhaled.

Subfloor must be dry

For more information, please see moisture testing.

Specification

Solid board sub flooring should be: 19.05mm x 140mm (1×6 nominal), Group 1 dense softwoods (SYP, Doug Fir, Larch, etc.), No. 2 Common, kiln-dried to less than 15% MC.

Solid-board subfloors should consist of boards no wider than 150mm, installed on a 45 degree angle, with all board ends full bearing on the joists and fastened with minimum 8d rosin-coated or ring-shanked nails, or equivalent.

Some types of wood flooring should not be installed directly over solid-board subflooring:

15mm solid and engineered flooring must have a 12mm or better plywood underlayment installed over solid board subflooring.

Parquet flooring cannot be installed directly to solid-board subfloors. A parquet installation over solid-board subflooring requires 9mm or better underlayment panels, nailed on 150mm minimum grid pattern using screws, ring-shanked nails or staples.


Mixed subfloors

For areas with mixed sub floors, most commonly extensions where by the ground floor has a suspended floor and the room is extended with a concrete screed it is important to level the entire area and to ensure the ridge between the two areas is removed.

Only use one fitting method to install the floor in the area unless the floor is separated by the use of a T section at the join of the subfloors.

The best way to install the floor is to ply the entire area (see Panel Products Subfloors) and either nail or glue the flooring to the ply.

Refer to moisture testing section to ensure the tolerance for moisture is met and the correct use of vapour retarders for each area.


Panel product subfloors

Subfloor must be flat

Make sure the panels are flat to within 3mm in 2m radius. If the panels are out of specification, consider sanding. When sanding, care must be taken to minimize the amount of dust produced. Best practice would include using dust-collection devices. Approved respirators should also be used to minimize the amount of dust inhaled.

Subfloor must be dry

For more information, please see moisture testing.

Specification

For panel products subfloors, check for loose panels and re-nail or screw down loose panels securely.

Ensure that there is proper expansion space (3mm) between the panels. If the subfloor panels are not tongue-and-grooved and if there is not sufficient expansion space, use a circular saw to create the specified space. Do not saw through joints on T&G subfloors.

Also check for delaminated or damaged areas and repair those areas as needed. Make sure the subfloor is free of debris before beginning installation.

Acceptable panel subfloors: Building regulations A structure:

On truss/joist spacing of 406mm o/c or less, the industry standard for single-panel subflooring is nominal 15mm CD Exposure 1 Plywood subfloor panels (Class 1 EXPOSURE 1) or 18mm OSB Exposure 1 subfloor panels, 1200mm X 2400mm sheets.

On truss/joist spacing of more than 406mm up to 488mm o/c, the standard is nominal 19mm T&G Class 1 EXPOSURE 1 Plywood subfloor panels, (Exposure 1), 1200mm X 2400mm sheets, glued and mechanically fastened, or nominal 19mm OSB Exposure 1 subfloor panels, 1200mm x 2400mm sheets, glued and mechanically fastened.

Truss/joist systems spaced over more than (488mm) o/c up to a maximum of (610mm) require nominal 22mm T&G Class 1 EXPOSURE 1 Plywood subfloor panels, (Exposure 1), 1200mm X 2400mm sheets, glued and mechanically fastened, or nominal 25mm OSB Exposure 1 subfloor panels, 4’ x 8’ sheets, glued and mechanically fastened or two layers of subflooring. Or brace between truss/joists in accordance with the truss/joist manufacturer’s recommendations and with recognised building codes. Some truss/joist systems cannot be cross-braced and still maintain stability.

For double-layer subfloors, the first layer should consist of nominal 19mm class 1 Exposure 1 Plywood subfloor panels , 1200mm X 2400mm sheets or nominal 19 OSB Exposure 1 subfloor panels, 1200mm’ x 2400mm sheets. The second layer should consist of nominal 12mm class 1 EXPOSURE 1 plywood subfloor panels, (Exposure 1) 1200mm X 2400mm sheets. The 12mm plywood should be offset by 12mm panel in each direction to the existing subflooring. The panels may also be laid on a diagonal or perpendicular, with 3mm spacing between sheets. Nail on a 305mm minimum grid pattern, using ring-shank nails or staples.

Fastening and spacing specifications

Follow the panel manufacturer’s recommendations for spacing and fastening.

Typical panel spacing and fastening requirements for truss/joist systems call for a 3mm expansion space around the perimeter of each panel, with panels fastened every 300 mm along intermediate supports.

Edge swell should also be flattened. This can usually be accomplished by using an edger sander.


Over joists

Only acceptable for 20mm solid or engineered floors

Subfloor must be flat

Make sure the joists are flat to within 3mm in 2m radius

Subfloor must be dry

For more information, please see moisture testing.

Specification

Joists must have centers at no more than 450mm. If outside of this parameter, refer to panel product subfloors.


Tiled Subfloors

Subfloor must be flat

Make sure the tiles are flat to within 3mm in 2m radius

Subfloor must be dry

For more information, please see moisture testing.

Specification

Engineered and solid flooring can be installed directly over existing ceramic tile, terrazzo, marble and granite.