Twelve litres a minute

NHBC’s technical guidance note on tiling bath and shower enclosures runs to five pages, and one number in it decides how the wall gets built. Above 12 litres a minute, the structure behind the tiles has to be waterproofed before a single tile goes on. Below it, it doesn’t.

The note is 9.2/06, fourth issue, published November 2024. It has applied to every NHBC-registered home whose foundations were begun on or after 1 January 2025, across England, Wales, Scotland and Northern Ireland. It is a warranty document rather than a building regulation, which matters and is dealt with at the end — but the reasoning inside it is the clearest published account of why some shower walls are built differently from others.

The headline numbers

What the number actually triggers

Under 12 litres a minute — a fixed shower over a bath, or a standard enclosure — NHBC asks for a moisture-resistant substrate suitable for the application. Type H plasterboard to BS EN 520 or BS EN 15283 qualifies, as do proprietary tile backing boards holding an independent assessment. Tiles solidly bedded in water-resistant adhesive, finished with water-resistant grout. A tray or bath with an integrated upstand, or a proprietary upstand kit, carried up behind the tiling. A full waterproofing system is offered as an alternative to all of that, not as a requirement.

Over 12 litres a minute, the sentence changes. Power showers “increase the potential moisture load on the enclosure walls, and additional protection through the use of a waterproofing system should be provided prior to the installation of any tiling”. The substrate table tightens with it: gypsum-based boards become “generally unsuitable for this application unless they hold a satisfactory assessment”.

Two things are worth correcting here, because the trade write-ups have repeated both. The document defines a power shower by its pump — “a power shower incorporates a pump to boost the water flow rate (>12 litres/min)” — so a large gravity-fed rain head is not automatically in scope. And plasterboard is not banned from showers. Type H remains an acceptable substrate below the threshold. What changes above it is the tanking, not the board.

A wet room is not just a bigger shower

The guidance treats wet rooms as a separate category, and there is no flow rate qualifier attached. A wet room is defined as an area where the shower is open or partially contained by a fixed wall or screen, the floor is flush with the adjacent floor, and the water drains through an outlet set into the floor. In every one of them, “the structure behind the tiles should be made watertight”.

The extents are drawn rather than described, and they are more generous than most people expect. Waterproofing runs a minimum of 150mm beyond the end of any fixed wall or screen, and across the floor. In a power-shower enclosure it extends behind the tiling “up to a maximum of 600mm in height” — a band, not the whole room.

Which is the part that reaches the drawing. Losing the step is a decision about floor build-up and drainage. It is also, under this guidance, the decision that puts a tanking system into the specification regardless of which shower gets fitted afterwards.

Three systems, and one board NHBC won’t take

“We’ll tank it” is not a specification. The guidance lists three acceptable product types and ties each to a standard: liquid applied kits, flexible sheet membranes, and inherently watertight boards. Each must be UKCA, UKNI or CE marked to a named European Assessment Document, or hold an assessment from an approvals authority acceptable to NHBC. So the question to ask is which of the three, and to which standard — not whether.

There is a footnote that can save money. Where the substrate itself has been assessed against the waterproofing criteria — watertight boards, in practice — “tanking may be omitted”. The board is the waterproofing. That is a genuine alternative to a membrane rather than a corner cut, and it is worth knowing before someone quotes for both.

One flat refusal sits in the same table: “NHBC do not accept the use of Magnesium Oxide (MgO) building boards.” MgO board circulates as a cheap tile backer. On a warranted new build it is not an option, and we would not put it behind a shower anywhere.

What we’d take from it

The honest caveat first. This is NHBC warranty guidance, written for NHBC’s builder customers, and NHBC says so plainly — the note states it does not constitute advice and that any third party relying on it does so entirely at their own risk. It binds new homes registered for NHBC cover. A bathroom refit in an existing Cheshire house is not bound by any of it, and nothing here is Part G, Part L or a building regulation.

What it is, is a warranty provider’s published view of where bathrooms leak, and the sequence is the transferable part. Two things want settling before wall linings are specified: whether the shower is pumped, and whether the floor is level. Both change what goes behind the tiles. Both cost very little at first fix. Both cost a stripped room afterwards.

So the questions are short ones. Is the shower pump-boosted, and what is the actual flow rate? Which of the three system types is going in, and to which standard? If the answer is watertight boards, are they doing the waterproofing on their own, or is a membrane going over them as well? And if MgO board appears on a quote, ask why.

One last piece of diligence: this is the fourth issue, dated November 2024, and it superseded a June 2024 edition eight months earlier. Confirm the current version before quoting it at anyone.