Slab / substrate moisture transfer

Mechanism

ATFA PCRM notes that adhesive bonding of flooring to concrete slabs relies predominantly on the bond; moisture migrating from the slab to the wood/adhesive interface can cause the floor to separate from the slab. Moisture uptake causes cupping, tenting and gapping; when floors are laid on plywood over a slab, severe moisture effects can take in excess of a year to dissipate. Concrete slab moisture meters are either capacitance type or use relative humidity measured above or in the slab; in-slab RH probes give the most accurate measure.

Risk profile by species

Mitigations

• Verify slab RH before installing. In-slab RH probes or capacitance meters; ATFA PCRM flags both options and identifies RH probes as the most accurate.
  Source: atfa-timber-flooring-problems-causes-remedial-measures-pcrm-2020-wayback.pdf p.195–198
• r540
  Apply Bona R540 single coat for up to 90% RH substrates. Apply two coats of R540 for up to 95% RH substrates. Coverage is affected by substrate porosity.
  Source: backup__Bona_R540_Specifier_Paragraphs.txt
• r540r820
  For fixing timber to a cementitious screed with moisture barrier requirement, use Bona R540 under R820 adhesive per R820 specifier paragraph.
  Source: backup__Bona_R820_Specification_Paragraphs.txt
• Address cold construction joints and slab additions: ATFA PCRM notes these joints can leak moisture and must be sealed before timber laying.
  Source: atfa-timber-flooring-problems-causes-remedial-measures-pcrm-2020-wayback.pdf
• Ensure subfloor is dry and well ventilated; moisture content of the floor must match expected in-service moisture content.
  Source: atfa-timber-flooring-problems-causes-remedial-measures-pcrm-2020-wayback.pdf

Bona product guidance

• r540
  Bona's silane-based moisture vapour barrier. Rated for up to 95% RH with two coats; can be overlaid with R848T or R820 adhesive. Stated cure and recoat rules on specifier paragraph.
  Source: backup__Bona_R540_Specifier_Paragraphs.txt
• r820
  Silane-terminated elastic flooring adhesive. Used as sole fixing for narrow boards (depth-to-width ratio > 1:6) and with R540 for moisture-loaded substrates. Coverage rates given per board profile in the specifier paragraph.
  Source: backup__Bona_R820_Specification_Paragraphs.txt

NCC / standards references

• NCC Volume Two 3.3.1 Dampness in Buildings (moisture barrier / DPM requirements under slab)
• ABCB Condensation in Buildings Handbook 2019 (vapour permeability and condensation risk)