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I'm a bit lost in the happy world of bath traps.
I am fitting a bath which (as ever) has not a huge amount of clearance under it. I have noticed that "shallow bath traps" are a thing (e.g. https://www.screwfix.com/p/mcalpine-bath-trap-white-40mm/84362) which on the face of it looks ideal.
But! Building regs ( https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/442889/BR_PDF_AD_H_2015.pdf) say of a 40mm bath waste that one needs a minimum 50mm water seal, which can drop to 38mm if discharging to a gully. The shallow traps I have seen all have a much smaller water seal -- the McAlpine one above is 19mm.
Am I missing something here? Are people fitting shallow traps with an additional trap later to achieve the required water seal? Any ideas?
On the same topic, I have seen that HepFlow "waterless traps" exist. I don't know whether these satisfy building regs or not, but I suspect they might not be ideal for a bathroom under heavy use (they look as though they could gum up and/or fail fairly quickly). Any experience of these?
Can you cut the floorboards?
We have a shallow one on the shower and a compact one under the sink.
if the council aren't coming round to check your diy plumbing why not try the shallow trap and see how it goes? as long as it's easy enough to get back in if you need to change it I'd be giving it a go or like Jamie says put a hole in the floor for more space if possible
Aye, fair point!
I'll have a look at the "hole in the floor" approach. I guess the shallow trap might work but I don't really want to be going back to mess with it again in 6 months if it doesn't work out.
I fitted a shallow P-trap above the floor followed by a couple of 90-elbows and a Hepvo hygiene trap horizontally under the floor. Solvent-weld pipe between and to outside
There was a connection problem that was solved with the threaded part of a water tank outlet (I think) under the floor, so I can't look, but it's been doing the job without smells for 20 years.
The trap depth you mention isn’t a building regulation but is in guidance. If the shallower trap performs the same function then it will meet the regulation behind this guidance.
I don’t recall the wording of that particular regulation but it will be something like ‘foul drainage system must safely carry all foul waste to x without causing damage to the building or health of its occupants’
Id highly doubt any building control surveyor will look at the depth of seal traps
What Daz said
Approved document H states that traps should retain a 25mm water seal or equivalent under working or test conditions.
Your shallow seal trap wouldn’t hold up to a test being applied to the waste system, however the number of times in my 30 year plus plumbing career I have been asked to perform one is less than one a year, and almost always was before appliances fitted anyway.
The shallow seal trap should work fine unless you’ve got any pressure that builds up in the pipework, don’t ever put traps in series, recipe for disaster.
So yes the shallow seal trap will in all probability be fine and I wouldn’t worry about it.
snip...don’t ever put traps in series, recipe for disaster
Not a challenge, my one experience doesn't equate to your 30 years of experience, but what's the issue?
Our bath waste has a slight fall to an external soil pipe that vents above eaves-level
Thanks folks 👍
I am back on the bathroom bashing today so I'm going to see if I can squeeze a 50mm seal trap in without too much swearing and if not I guess it's shallow trap and hope for the best.
The pipe run won't be awfully long I don't think, probably under 2m to the stack.