Preparation, Substrate and Surface Assessment
What must be assessed
Before any felt is laid, the substrate must be assessed for soundness, moisture content, flatness, slope, and compatibility with the proposed system. Candidates should understand deck types (timber, concrete, metal, existing felt), the need for vapour control and ventilation, and the role of primers in promoting adhesion.
Moisture and movement
Trapped moisture causes blistering and premature failure. Know how moisture meters and hygrometers are used, why new concrete must cure, and why timber must be at a specified moisture content before covering. Recognise that movement joints and differential movement between substrates must be accommodated, not bridged rigidly.
Traps to watch
- Assuming a clean-looking surface is dry - always test.
- Forgetting that primers need a dust-free, dry substrate and adequate flash-off time.
- Overlooking slope - ponding indicates inadequate fall, not a membrane fault.
- Confusing surface preparation (cleaning, priming) with repair (replacing defective substrate).
Worked rule of thumb
Use the S-M-F-C check: Sound, Moisture-tested, Flat enough for the membrane, Compatible with primer and adhesive. If any letter fails, stop and remediate before laying felt.