Guide 4 - Tips on Drains and Waste Installation for Wetrooms

For a DIYer the Wetroom drain can be the most daunting aspect of the project.  However it is straight forward with a DIY Wetroom kit you can be sure the critical components are all compatible and easy to put together.

Each kit comes complete with detailed instructions, specific to the supplied components. This guide illustrates the general considerations and practices involved.


See the full range of wetroom kits


See all the available types of drains


Building Regulation Relating to Drainage    

As with most things building related, drainage is covered by regulations.  These are stipulated in “Building Regulations part H” in particular Part “H1”;  “foul water should be properly disposed of to maintain a decent standard of sanitation”, “Such a system should minimise the risk of blockage and prevent foul air from permeating the building”.  Installations should be approved by your Local Authority Building Control

For the full regulations, download a copy from the government planning portal

Be assured that the drain supplied with the DIY Wetroom kits conform to all necessary standards so the “risk of blockage and prevent foul air from permeating the building” bit is covered.

The clause about “foul water should be properly disposed of to maintain a decent standard of sanitation”, is down to design of the pipework and connections.


Working out the wetroom drainage waste pipe runs

Note:  This is rough guidance only, check the regs for full details and alternatives

1. If on the first floor or above the waste pipe from the wet room should discharge in to a soil stack.  On the ground floor it can go into a gully.  (If you are replacing an old bath you should be able use a similar old style discharge in to a hopper) So work out where the pipework needs to go.  When connecting to soil stack check where you are allowed to make the connection and have a look at a “strap boss” to make the connection easy.

2.  The wetroom waste pipe should be 40mm minimum diameter solvent weld,  with a maximum horizontal run of 3m.  If you need a longer run then an air admittance valve should be added to the pipework making runs up to 10m possible.

Maximum waste pipe run length

The regulations are set for baths and not specific to showers. The sizing requirement is mainly in place to prevent a syphoning action that would drain the water from the trap therefore allowing noxious air to enter the room.  The flow rate is much less from a shower compared to a bath so the chance of a syphon action occurring is much less.  Some of the DIY Wet kits also come with a NOOD (no-odour trap) which prevents sewer air entering the room even if they run dry.  

3.  The gradient on the pipe should be between  18mm and 44mm per metre.  This can be increased for short lengths but if you need to drop any real height use a short section of vertical pipe.

maximum and minimum gradient for waste pipes

4.  Long runs of pipe should be supported so it doesn’t sag and so the gradient is maintained.

5. You should have a mean of clearing any blockages in the pipe.  For this reason it is best to keep the inaccessible pipe work under the floor as straight as possible (use “swept” bends if necessary) and provide an accessible rodding point (or a joint outside that you can dismantle).   You can also use a “Flexible Rod Drain Un-blocker” through the body of the wetroom drain so it should be fairly easy to comply with this requirement.

Bends and rodding of waste pipe


Wetroom Waste Pipes that need to pass through Joists

On some installations the drain pipe has to pass through the joists in floor. (Note: NEVER notch the top of a beam).  It is possible to drill the required 45mm hole through centre of joists at least 180mm deep but needs careful planning to ensure the floor isn’t weakened when you move away from the joist centre to give the required fall.  

There are no exact specifications are available to cover all possibilities to be able to say what will be safe.  The loading on the floor (e.g. bath position), the strength of the timber distance from the centre of the room and supporting walls all need to be taken in to consideration. (Check : Building Regulations, guidance note 7 or Part B)

Joist drilled to take waste pipe from shower

What we can say is there will be a solution even if this involves, doubling up joists,  Flitching the beam (attaching steel plates to either side) or reinforcing the joist with builders straps.

We suggest contacting your Building Control Office before proceeding. In many cases it should be possible to over-engineer the solution rather than having to get structural calculations done.

An alternative could be to drop the pipework to the room below and box it in. This is easily done with bottom outlet drain. Alternatively the pipework can run parallel between the joists and then drop down next to a wall in the room below.

Wetroom waste pipe to room below


See all the available types of drains


All tanking /waterproofing product are available from the DIYwetroom store


Other wetroom guides here

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