The Defect Liability Period (DLP) is the window after practical completion when the contractor is contractually obliged to come back and fix defects. Under JCT, NEC and their variants, this is typically twelve months. For waterproofing, that is nowhere near long enough.
What the DLP actually covers
The DLP gives the building owner a right to notify the contractor of defects arising from workmanship or materials non-conformity. The contractor is required to make good. On exposed structures and visible elements, twelve months catches most issues.
On buried waterproofing — green roofs, landscape decks, inverted systems — twelve months is the wrong unit of measurement.
Why twelve months is not enough for roofs
A membrane breach on a landscape roof may not produce a visible leak for two to five years. Substrates hold moisture. Ballast and paving slow water travel. Inverted roofs are specifically designed to trap water above the membrane.
By the time the leak becomes obvious, the DLP has expired. The contractor is no longer obliged to attend. The remedy is now a dispute, not a phone call.
Contractual options
- Extended DLPs. Possible but uncommon and resisted. Worth raising at contract negotiation.
- Collateral warranties. Useful but depend on subcontractor solvency.
- Insurance-backed guarantees. Property Care Association, NHBC and others — conditional on installation to declared standards.
How to protect yourself during the DLP
- Commission an independent pre-handover roof survey to verify the roof is sound at the contractual milestone
- Use buried leak location during the DLP to find breaches that have not yet leaked through
- Install permanent monitoring sensors during construction so that moisture changes are captured the moment they begin
After the DLP expires
Collateral warranties, latent defect insurance and the Defective Premises Act all remain available. They are slower, more expensive and less certain. Engage early; engage independently.
The Vector view
Independent investigation paired with sensor monitoring gives the owner leverage while contractual remedy is still live. Once the DLP is gone, leverage is gone too.
If you’re approaching practical completion on a complex roof, talk to Vector before the clock starts.