Routing waste pipe past shower tray

[Mike Barnes]

Bathroom reorganisation. I need to route the waste pipe from the basin to the outside wall, but in the way there will be a new corner shower unit. I want the shower tray sitting directly on the floor so there's no appreciable height difference when stepping in or out of the shower.

The house is substantial, about 200 years old, with very uneven walls and floors.

Things I've thought of, not necessarily sensible, are:

• stud the wall behind the shower out a couple of inches and run the pipe though the gap

• cut a channel in the wall, which is single-leaf brick

• cut a channel though the shower base, which is acrylic

• notch the joists and strengthen them as necessary - I will be putting thick plywood and a limestone floor on top of them

Comments?

[Nick Finnigan]

I guess you will have to do that for the shower waste anyway? Can you go from the basin along the internal wall (boxed in) to the shower, then down through the limestone and plywood, through a joist or two and join up with the shower waste?

[Simon Avery]

Hello Mike

Commiserations. I've got to do mine this winter too. :(

OTOH, I've just discovered that the walled-in section I want to put the shower is EXACTLY 1200mm across, which is EXACTLY the width of a standard double tray so I don't need bespoke or homebuilt trays. Not often that something like that happens! Not like it was intentional, either. It's also 900mm deep, another standard size. Worrying.

Ah, that buggers up the normal method of boxing it up 2-3" and running everything through the boxing. Sure you don't want to do this?

Possible, and probably the least hassle. You don't mention the shower feeds, but you may be able to use this for them too, making a neater finish.

Absolutely not.

Absolutely not, for it'll weaken it dramatically, make it bendy and it'll split shortly after.

Unless they're particularly thick joists, I'd not do this as the limestone will be adding a lot of weight and you don't want to be weakening them unduly.

The first seems the best, and worthwhile adding some kind of inspection/access panel if you possibly can (and not doing any tiling, flooring until you're 100% sure it's working properly) I'd use solvent weld for the waste as it doesn't tend to leak like compression sometimes can.

[Matt Beard]

The Building Research Establishment do a "Defect Action Sheet" number DAS 99 on the issue of notching and drilling joists in a way to produce the minimum weakening.

A brief summary is:

Notches can be cut in the top of a joist in an area that stretches from 7% of the span from one support to 25% of the span from the same support. The notch must be no deeper than the lesser of 12.5% of the depth of the joist or 250mm. There is a matching "notch zone" at the other end of the joist.

Holes can be drilled in a zone that starts a distance of 25% of the span from one support and ends 40% from the same end. The zone extends from the centre line of the joist up 12.5% of the joist height and down 12.5% of the joist height. There is a matching "hole zone" at the other end of the joist. Holes must be spaced such that the distance between centres is at least 3 times the diameter of each hole (I guess if the holes are different sizes you take the larger diameter).

They don't say how close notches and holes can get at the 25% point, but I would say "as far away as you can keep them".

This is my interpretation of the document, not an official "you can do it this way". If you want to be sure you could try contacting the Building Research Establishment in Watford or your local planning office (shudder!)

[Nick Finnigan]

That just about rules out notches for waste pipes then.

[Mike Barnes]

Thanks, excellent stuff. The bad news is that that formula allows only a

16 mm notch. Actually I'm pretty sure that there are holes and notches outside those specifications under just about every floor I've ever taken up :-( however I'm talking about substantial old houses where the joists probably started out much stronger than they needed to be.

Thinking on, I wonder if some reinforcement might help. Two steel plates say 600x100 with a 50 diameter hole in the middle, put them either side of the joist, bolt them through, then drill out the wood. Obviously I'd want someone qualified to do the sums but I'm still at the ideas stage.

A previous house had one joist on the ground floor not unlike that. I could see it from the cellar. The end of the joist had obviously rotted so someone had made a new piece about 600 mm long, butted it up to the original, and bolted the two pieces together using steel cheeks.