'We've Got the Contractor's Guarantee' - What It Covers, and When the Belief Costs Developers Millions

Direct answer

If you have a contractor’s guarantee, you have something real. A guarantee from the specialist contractor who designed and installed the waterproofing carries their workmanship liability and, in most cases, some design liability too - and a contractor that has guaranteed the workmanship has normally accepted responsibility for the design it built. The belief that costs developers money is not “we have a guarantee”; it is one of three things behind that sentence. First, that the guarantee in the file is actually a supplier’s or manufacturer’s guarantee, which covers the product and very little else. Second, that any guarantee is a substitute for having the waterproofing designed properly in the first place. Third, that the guarantee will still be worth something in ten years, when the company that gave it may not exist. A contractor’s guarantee is a good backstop. It is not a strategy, and it is only as solvent as the company behind it.

Full explanation

“We’ve got the contractor’s guarantee” is a sentence that closes conversations about waterproofing risk. Sometimes it should. A specialist waterproofing contractor who has designed the system, installed it and put their name to a guarantee has taken on real liability, and on a well-procured project that is exactly the position a client wants to be in. The problem is that the sentence gets said in a lot of files where it is not true in the way the speaker thinks.

Which guarantee do you actually have?

The first thing to check is who signed it. There are two very different documents that get filed under the same heading:

• A contractor’s guarantee is a promise from the company that designed and/or installed the system to return and remedy defects. It covers workmanship, and where the contractor produced the design - which they usually do on a design-and-build waterproofing package - it normally carries design liability as well, alongside the design liability they owe under their subcontract.
• A manufacturer’s or supplier’s guarantee typically covers the product: that the membrane, additive or board meets its stated properties. It almost always excludes design, installation, damage by following trades, ground movement, water-table change and consequential loss. Since most basement leaks are design or installation failures, a product guarantee rarely responds to the leak in front of you.

A great deal of the “we’ve got the guarantee” confidence in the industry rests on the second document while believing it is the first. As we set out in supplier warranties vs independent waterproofing design, a supplier’s paperwork is product information, not design, and its guarantee follows suit.

What a real contractor’s guarantee does and does not do

Where the guarantee genuinely comes from the specialist contractor, it is a useful safety net: defective workmanship, and usually defective design, within the guarantee period, will be put right by the party best placed to do it. What it still does not do is prevent the defect. Waterproofing risk is decided in the design and procurement decisions that determine whether the right system was specified for the conditions and whether the interfaces were coordinated. A guarantee is silent on all of that until after the water arrives. That is why CLW’s preferred model pairs the contractor’s guarantee with an independent outline design and specification, review of the contractor’s detailed design, and construction monitoring: the guarantee is the backstop, and the independent work is why you rarely need it.

Read the exclusions carefully, because they tell you which case you are in. A guarantee that excludes “design inadequacy” is telling you the contractor did not accept the design. One that excludes “building movement” and “changes in the water table” is telling you the design risk assessment was never theirs. The narrower the cover, the more the design responsibility sits with someone else - and the more important it is that someone was actually appointed to hold it.

A guarantee is only as solvent as the company behind it

This is the failure mode developers consistently underestimate, and it applies to the best-drafted contractor’s guarantee as much as the worst. A guarantee is a promise from a specific legal entity. Below-ground water defects are characteristically latent and seasonal: as we explain in why the defect liability period expiring is when your real risk begins, a leak may not surface until a full wet cycle or a rising water table finds the defect, which can be years after practical completion. Construction is a sector with significant contractor turnover. By the time the stain appears, the company that signed the guarantee may have been restructured, sold, renamed or wound up. A ten-year guarantee from a contractor that ceased trading in year three is not protection; it is a reminder of who used to be responsible. An insurance-backed warranty addresses this, but insurers usually want evidence of competent design before they will stand behind one.

The contractor cannot be the only line of assurance

Leaning on the guarantee alone usually goes hand in hand with letting the contractor set their own performance target and self-certify their own waterproofing. This asks the party with the strongest commercial interest in programme and cost to also be the quality gatekeeper, on a concealed element that cannot be inspected once it is covered. It is precisely the scope gap and orphaned-responsibility pattern that recurs in dispute work. The remedy is not a stronger guarantee; it is an independent designer setting the target, reviewing the contractor’s design and verifying the install at hold points before the work disappears.

What actually protects the developer

A contractor’s guarantee earns its place as one layer in a sound arrangement. The foundation is waterproofing design that is right before construction starts, with a clear owner. On top of that sits construction monitoring, so avoidable errors are caught on site. Around both sits a documented, defensible record of grade, strategy and inspection - the thing that turns a leak into a recoverable, often preventable event rather than an argument about whose fault it was. The guarantee is the backstop you hope never to test. Make sure it is the contractor’s, make sure you know what it excludes, and make sure you have not made it the only thing standing between you and the water.

Want to know what the guarantee in your file actually covers - and who signed it? Put its terms and your scheme to the Waterproofing Wisdom agent, CLW’s specialist AI trained on BS 8102:2022 and two decades of these failures, or talk to us about an independent read before you rely on it.