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.