Guide 2 - Wetroom Floors and Drainage

When you have designed your ideal layout it is time to start working out the details. The most important factor is the floor and the drainage. This can be the most complicated part of the process but DIY Wetroom is here to make it easier to choose the best solution for your situation.

Wetroom Floor Design

There are many ways of making a practical and reliable wetroom floor.  All techniques involve creating slope in the floor towards the drain and “tanking” or sealing the floor and the walls to ensure no water can make its way through to the building structure.

Basics of wetroom floor drainageWaterproof Wetroom Floor formed to provide a gentle slope towards the shower trap and drain


The easiest way to perform both these requirements is with a Wetroom Kit. These come with all the necessary bits including a matching drain so you can have confidence in the job you are doing.  The variety of kits caters for virtually all situations and designs.  However DIY wetroom also supplies components for bespoke areas so the options are unlimited. 


See all the DIYwetroom Kits


For this guide we will keep things simple and explain just the kits.

In order to select a kit, decide on:-.

1. The size of the “wet area”

2. What the existing floor structure is. 

Level access or a flat floor is the ideal that most people are after in a wetroom.  If for some reason this isn’t possible then a raised area can always be created.  The DIY Wetroom guides show how the level access is possible in almost all situations.

Size of the Wet Area

From your design you should be able to see the area of the floor that needs to slope down to the drain. This area should cover the area where most of the water falls directly from the shower head or from your body.  In our Design PDF we assume an overhead shower will give a wet area of approximately 500mm radius from the shower head.   The size of this area can be cut down by using a screen . If the screen is supported above the floor the sloping area should extend past it by around 100mm.

Drip and spay area of wetroom shower

What is the Existing Floor Structure?

Suspended timber floors

Most uppers floor in a building are suspended timber floors. These are timber beams with either floor boards or chipboard sheets over the top. The ground floors of many pre-war buildings are of a similar construction.  These floors give an accessible cavity in which the drain and associated waste pipe can be located.  If you can see the floor it is often possible to work out exactly where the joists are and which direction they run to work out a position for the drain and the pipe run.

Floor boards or  chipboard over joistsDirection of floor boarding indicates joist direction to help plan waste pipe layout

Solid Floors

Solid floors are normally a concrete screed. In older buildings this may be a little as 50mm deep direct on to the earth possibly with a damp proof membrane under it. (note: If the damp proof membrane has to be breached to get the necessary depth it must be repaired.) 

Later buildings would have a sand and cement screed over a concrete slab and modern builds will have insulation in the floor as well.  In all cases on a ground floor it will be possible to cut and breakout enough concrete to recess the drain and waste pipe.  

If it is a solid suspended floor the structural elements should not be cut through although parts of the top screed can be removed.  

If the floor contains wet under floor heating then great care should be taken. It is likely that the wet room floor will have to be raised slightly to accommodate the drain and waste pipe. 

If fitting a standard drain and waste on to a solid floor restricts your perfect design a solution is available. An ultra slim pumped drain is available these also allow a wet room to be installed in basements, below normal drainage levels. 


Wetroom Kits for Suspended Timber Floors

The easiest type of kit to install is the Maxxus structural deck.  This is a solid reinforced deck that can span joists so the existing floor boards or chipboard can be cut away and the Maxxus deck fixed in their place to give a level floor. Various sizes are available for use with a square/round or linear drain. The Maxxus is also suitable for vinyl covering instead of tiling.

Wetroom deck structural set into timber floor

Structural wetroom deck set in to existing timber floor.


Read more about structural wetroom decks


Other decks require structural support between the joists so these are normally laid on top of the existing floor, the rest of the floor is then raised by overlaying chipboard or tile backer board.  This will raise the floor level by a minimum of 18mm but is a good way to ensure the complete floor is stable for tiling especially if the floor is floor boards.

Wetroom floor former overlaid on to timber floor

Non-structural underlay former over existing floor


Read more about underlay wetroom decks


It is possible to support a foam cored deck of this type after removing the floor boards or chipboard with a bit of joinery to produce a flat surface flush with top of the joists.

Under boarded wet room deck

Fully supported underlay former set in to timber floor


Wetroom Kits for Solid Floors

Any kit can be used on a solid floor as the floor will give all the necessary structural support. For this reason the low cost foam cored kits are usually chosen.


Note: All decks can be cut down in size to fit your exact space. 


View the full range of deck kits available


Drainage for Wetroom Floors

The waste pipe from the Wetroom drain is almost always 40mm plastic pipe (solvent weld, not pushfit).  This has to be routed in such a way that it has minimum joints and angles and gives a minimum fall over its length of 18mm per metre.  All the DIY Wetroom drains and trap comply with British Standards and Building regulations, you must ensure that the connection to the waste stack or drain also complies with building regulations.  

The DIY Wetroom drain traps are all accessible from above by removing the grid so they can be cleaned and cleared from above. Once fitted no access to the underfloor area is required.

Routing the Waste Pipe

This is a consideration for choosing the ideal kit.  For suspended timber floors it makes a much easier installation if the drain and waste pipe fits between two joists .  If the joist run in the wrong direction then the Joists will have to be drilled. The fewer that need drilling, the easier the job (for more details see “Tips for Wetroom Drains”).  With a solid floor shorter runs mean less depth and length needs cutting a breaking out.

DIY Wetroom kits are available with various drain outlet depths. These give a waste pipe centre lines between 27mm  to 80mm below the Deck. Vertical outlet options are also available.

Taking in to account the minimum fall of 18mm in 1m the illustration below shows space needed over a number of pipe lengths.

Minimum fall on waste pipeMinimum fall of wetroom waste pipe


Choosing the Right Wetroom Kit


View the full range of deck kits available , choose the type and size that will be perfect for your project.


For technical queries or bespoke formers please contact us.


All kits, tanking /waterproofing product are available from the DIYwetroom store


Other wetroom guides here



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