Protecting the flat below: waterproofing, pressure tests and lease consent for a new London bathroom - and who pays if it leaks

In a terraced house a leaking shower tray rots your own joists. In a London flat it comes through somebody else's kitchen ceiling, and the conversation that follows involves your neighbour, the managing agent, at least one insurer and, if you are unlucky, a solicitor. Because so much of the capital's housing is flats and converted houses, waterproofing is not a detail of a London bathroom refit - it is the main event.

This guide sets out the standards a competent fitter should be working to, the tests that should happen before the walls are closed up, the lease and water-company paperwork that is easy to forget, and what to do if water gets through anyway. It deliberately does not repeat our guides to choosing a trustworthy bathroom fitter or what a full refit involves. Sources were checked on 22 October 2026.

Why this matters more in London

The Association of British Insurers says escape of water is one of the most common types of domestic property damage claim, with insurers paying out around £1.8 million every day for it. The ABI's figures for 2025 show insurers paid almost £3.4 billion across more than 560,000 home insurance claims, with the average claim up 15% to about £6,000. Those are national figures covering every cause, not just bathrooms, but they make the point: water damage is expensive, common and, in a block, rarely confined to one home.

A leak from an upstairs bathroom also tends to be slow. Failed grout, a poorly sealed waste or an untanked shower wall can seep for months before a stain appears below, by which time the damage to ceilings, electrics and timber is far greater than the original fault.

Waterproofing: what the standard actually says

The relevant British Standard for wall tiling is BS 5385-1:2018. The Tile Association's briefing on the 2018 revision quotes its key provisions:

• Tanking in wet areas. "In wet areas, e.g. showers, wet rooms and steam rooms, substrates should be protected with a suitable proprietary tanking membrane system" (clause 6.1.1.3).
• Gypsum plaster "should not be used in wet areas unless additional protection in the form of a waterproofing tanking system is used" (clause 6.1.2.9).
• Boards: "A tanking system should be used for boards used in wet areas, e.g. showers and wet rooms."
• Plasterboard, "including moisture resistant plasterboard, is generally unsuitable for wet areas" without a tanking system (clause 6.2.3.4).

In plain English: green "moisture-resistant" board and a few coats of grout are not waterproofing. Ask your fitter which proprietary system they will use - a liquid membrane or sheet system, with the manufacturer's tapes at board joints, corners and pipe penetrations - and ask for the product data sheet. A competent fitter will name it without hesitation.

Floors over timber

Most London flats above ground level have suspended timber floors, which flex. Tiles do not. The adhesive maker BAL's technical note on sub-floor deflection gives the commonly accepted minimum before ceramic tiling as L/360 of the span (L/720 for natural stone) and describes stiffening with noggins, screwed plywood and moisture-stable tile backer boards, citing BS 5385-3:2024. A floor that moves cracks grout, and cracked grout lets water into the ceiling below. If the quote says nothing about the floor build-up, ask.

The pressure test before the walls are closed

This is the single most useful question you can ask. Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 requires that a water system can withstand at least 1.5 times its maximum operating pressure, and sets out the test that satisfies the requirement. For systems containing plastic pipe - now very common - one deemed-to-satisfy method holds the test pressure for 30 minutes and then monitors for a further 150 minutes, passing only if the pressure drop stays within set limits and there is no visible leakage. Paragraph 13 adds that every water system "shall be tested, flushed and where necessary disinfected before it is first used".

So agree in writing that:

1. new or altered pipework will be pressure tested before boarding and tiling, with the gauge readings and times noted;
2. you, or someone you trust, can see the test or a dated photograph of the gauge;
3. wastes and traps will be run and checked for weeping with the bath and shower filled and discharged;
4. any concealed valves have an access panel where the manufacturer requires one.

A plumber who is an approved contractor under a scheme such as WaterSafe can certify that work complies with the water fittings rules - useful evidence if a dispute arises later.

Two notifications people forget

The water company

Regulation 5 of the Water Fittings Regulations requires notice to, and consent from, the water undertaker before installing certain fittings. In a bathroom the relevant ones are a bath holding more than 230 litres (measured to the centre line of the overflow) and a pump or booster drawing more than 12 litres per minute from a supply pipe, alongside some showers and bidets with ascending sprays. Large freestanding baths and power-shower pumps are increasingly popular; ask whether yours needs notifying.

The freeholder

The Leasehold Advisory Service's guide to alterations and home improvements explains that a lease may prohibit alterations, allow them with written consent, or require a formal licence - and that you may need permission "even if you already have planning permission or building regulations approval". Examples it gives include replacing carpets with wooden floors and installing underfloor heating that passes through shared floors. A licence may set conditions such as approving your contractor or requiring proof of compliance. Unauthorised alterations, it warns, can make a flat harder to sell, remortgage or insure.

Write to the freeholder or managing agent early, enclosing the scope, drawings, the contractor's details and insurance. If the lease requires consent, do not start until you have it in writing.

Red flags specific to waterproofing

• "The tiles are waterproof" or "we'll just use good grout" as the answer to a tanking question.
• No mention of the floor substrate on a tiled floor over timber joists.
• Reluctance to pressure test, or a plan to tile the day after first fix.
• Silicone used to hide gaps rather than as a movement joint at changes of plane.
• No proof of public liability insurance - the cover that matters if water reaches the flat below.
• A request that you "sort out the lease side later".

If water gets through

1. Stop the water. Isolate the bathroom or turn off at the stopcock, and stop using the shower or bath. Warn the neighbour below about electrics in the affected ceiling.
2. Record everything: dated photographs above and below, the time it was noticed, and all messages.
3. Tell your insurer, the neighbour's insurer if known, and the managing agent. Many blocks have a single buildings policy arranged by the freeholder. The ABI notes insurers may appoint a loss adjuster, usually within about 24 hours in normal conditions.
4. Notify the fitter in writing and give them a reasonable opportunity to investigate and put the defect right.
5. Know your rights. Under section 49 of the Consumer Rights Act 2015, a trader must perform a service with reasonable care and skill. If they do not, you can require repeat performance at no cost within a reasonable time, and if that is impossible or not done, a price reduction, up to a full refund in appropriate cases. Damage to the neighbour's property is a separate claim, usually against the fitter's public liability insurance.
6. Get an independent opinion on the cause before any re-tiling destroys the evidence. Leak detection specialists and independent surveyors can help.

If the fitter is a TrustMark-registered business, TrustMark's complaints process is another route. Citizens Advice's consumer helpline (0808 223 1133) can advise on next steps.

A short checklist to attach to the quote

• Named tanking system for all wet-zone walls and, where relevant, floors, installed to the manufacturer's instructions and BS 5385.
• Floor substrate stated, including any overboarding or uncoupling layer.
• Pressure test before closing up, with recorded readings; wastes tested under use.
• Water company notification checked for large baths and pumps.
• Lease consent or licence obtained before work begins.
• Public liability insurance certificate supplied; product guarantees for membranes, trays and valves handed over.

None of this makes a bathroom look any better on the day it is finished. It is what makes it still look good - above and below - five years later.

Sources (checked 22 October 2026): ABI, Burst pipes and water leaks; ABI, property insurance payouts 2025 (17 February 2026); The Tile Association, changes to BS 5385-1:2018; BAL TN 07.24; Water Supply (Water Fittings) Regulations 1999, Schedule 2 and regulation 5; Leasehold Advisory Service, Alterations and home improvements; Consumer Rights Act 2015. Information only; not legal or insurance advice.