Recover Over Wet Lightweight Concrete on a Cold-Weather Deadline
A 1978 warehouse, 36 m × 24 m, has a roof assembly of two plies of SBS modified bitumen over 50 mm lightweight insulating concrete (LWIC) on corrugated steel deck. Slope is 1/8 in/ft to six interior drains. After a 60 mm rain, the tenant reports eight ceiling stains in the retail area. An infrared scan shows wet zones around two drains and along a 12 m seam. The owner wants a recover-50 mm polyiso adhered with low-rise foam and a new SBS cap-to avoid shutting down production. Work must be done at night in five days, with forecast temperature −3 °C and wind 30 km/h. Two HVAC curbs are 200 mm high; one is rusted at the base. The owner's representative says, “Everyone recovers over LWIC; just glue it down.”
1. Is a recover acceptable here? What verification would you require before ordering materials?
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A recover is not acceptable as proposed. The infrared scan and ceiling stains indicate moisture in the existing assembly, and LWIC is friable and highly moisture-sensitive. Encapsulating wet LWIC traps water against the steel deck, promotes corrosion, and can blister or debond the new membrane. The −3 °C forecast also means low-rise foam may not rise, cure, or bond properly.
Before ordering, require:
- Core cuts at each stained area and at the infrared anomalies, plus moisture-content readings of the LWIC and any insulation.
- Verification that the steel deck is not corroded through; remove a small section if necessary.
- Pull tests for the proposed adhesive or mechanical attachment, since LWIC may not provide reliable uplift resistance.
- A re-scan after opening the roof to confirm the wet boundaries.
If moisture is isolated, remove the wet LWIC and membrane to the deck in those areas and replace with new insulation and cover board; do not attempt to dry LWIC in place. If moisture is widespread, the scope must change to tear-off. The owner's schedule does not override the need to remove trapped water.
2. If the owner accepts limited tear-off, outline the sequence and the critical details at the drains and the rusted HVAC curb.
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Sequence: protect the interior and HVAC openings; remove the wet zones to the steel deck; inspect and treat corroded deck; install new insulation and cover board; install new membrane; then flash drains and curbs before general membrane completion.
At drains:
- Cut back existing membrane in a square around the drain.
- Install a new drain sump with tapered insulation so the membrane slopes to the drain.
- Use the drain manufacturer's clamping ring and a new strainer; do not rely on sealant alone.
- Reinforce the membrane with a compatible ply or fabric and strip in according to the membrane manufacturer's detail.
At the rusted HVAC curb:
- Remove rust and evaluate the curb's structural condition. If the curb is deteriorated, rebuild or replace it rather than cover it.
- Raise the curb so the finished membrane and flashing have adequate height above the roof surface, and coordinate with the HVAC contractor.
- Install a new curb cap and counterflashing; use a compatible sealant only as a secondary seal, not as the primary weather barrier.
Water-test or flood-test the drains and curbs before demobilizing.
3. What adhesive and weather controls would you require at −3 °C and 30 km/h wind, and why?
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At −3 °C, many low-rise polyurethane foams will not expand or cure correctly, and cold substrates can cause adhesion failure. The 30 km/h wind increases heat loss and can lift lightweight insulation before bonding.
Require:
- A cold-applied adhesive or mechanically attached insulation system specifically approved by the manufacturer for the actual substrate temperature and wind uplift pressure. Do not allow standard low-rise foam unless the manufacturer confirms it is rated for −3 °C and the substrate is preheated as required.
- Mechanical fastening through the existing deck where the LWIC cannot provide pull-out resistance; verify fastener length and deck engagement.
- Temperature monitoring at the substrate, not just ambient air, and records of adhesive batch, application rate, and cure time.
- Wind breaks or temporary ballast for insulation boards until the adhesive cures, and no application during rain, snow, or when the deck is wet.
- Keeping adhesive and membrane rolls stored warm and dry until immediately before use.
If these conditions cannot be met, the work should be postponed. A rushed cold-weather recover risks widespread debonding and blow-off.