Extra U-bend/Drilling a joist

[Paul]

I’m replacing the shower tray. The new one can be fitted with/without a 10cm stand. I’d like to fit it without but the existing drainpipe is too high in the cavity* to do so without building in a u-bend.
Is this a bad idea?

(*I can’t change this without ripping up the entire - tiled - bathroom floor. I would much sooner fit the stand.

[mrcharly-YHT]

Normally a u-bend lowers the connection height a little so I'm not sure what you have currently. Can you give us a photo?

You really want a u-bend, or some seriously horrible smells will come back up this pipe.

[Paul]

No photos I’m afraid. New tray is not due until next week.

There will be a trap to stop smells.

I realise I didn’t mention the most important bit: the reason for the additional, proposed, vanity-necessitated u-bend is that there is a joist in the way.

The drain in the new tray is in a different place to the old one. If I use the stand I can run the connection over the floorboards. But if I don’t use the stand the connection to the drain pipe has to pass through a joist.

My research says that it’s okay to drill through a joist as long as the hole is not more than 1/3 of the depth of the joist (it won’t be) and is in the middle of the depth of the joist (among other requirements which I can comply with).

It’s the fact that the hole has to go through the middle of the depth of the joist that means I need a u-bend. If I could cut a notch or channel in the top, immediately below the floorboards I’d be laughing.

So, I’ll have an extra u-bend in the drain pipe. Do you think it will cause any problems?

Alternatively, do you think I could get away with drilling a channel through the top of the joist? It will be under the shower tray so there’s no danger of anyone hammering a nail or screwing a screw into it. Does drilling through a joist anywhere other than the middle cause other problems? Can it weaken the joist more than drilling through the middle?

[Ham]

Someone that knows what they are talking about will be along in  a minute, but I wouldn't have a problem notching the joist. If it's under the shower tray the immediate load will be spread, the overall strength of the floor will  not be compromised and I wouldn't have a millisecond's worry.

[jsabine]

Meh. Problem is that the top of the joist is in compression and the bottom is in tension (or the other way around, I'm very much *not* Someone That Knows), so a notch, especially one deep enough for a waste pipe, compromises the strength and stiffness very much more than a hole in the middle.

Chances are, most joists will be oversized enough that you'll get away with it (especially as it's one joist, and more especially if it's a short span), and it's vastly more likely to result in a bit more flex than in your entire bathroom plunging into the floor below, but I'd probably avoid it if I could.

[Canardly]

https://www.diydoctor.org.uk/projects/Notching_joist.htm

[Paul]

https://www.diydoctor.org.uk/projects/Notching_joist.htm

[Aunt Maud]

Sorry mate, but it's on the neutral axis or not at all for such a big hole. If you want to be in with the building regs there are considerations to be taken.

Any other holes in the joist and the distance to the bearing.

See....

http://www.welhat.gov.uk/CHttpHandler.ashx?id=9100&p=0

And see canardlys' post.

[canny colin]

In the past I used to work for a plumbing firm doing walk in showers with porcelain tiled floor.  The waste pipe holes were never in the right place . So I just  plated timber joists with angle iron and  coach bolts  . I have also used 4 layers of 18 mm ply ( 2 each side) glued pinned and coach bolted to the joists . The longer the length the better. Noggins ie  timber  same depth as joists fixed at right angles each side of  the hole also help put some strength back in to the floor . Strengthen the joist first before cutting the hole / notch .

[Moleman76]

It seems that the "where can I make a hole" question has been answered.

"Are two U-Bends acceptable?" - not so much. 

I think the answer is no, unless you can fit the sort which has an integral removable cleanout plug and can provide access to said plug.  One u-bend (trap) is needed to put a barrier of standing water between you inside your house and the sewer gases lurking beyond.  However, from time to time that one can become obstructed and not drain, and has to be accessed for removal of whatever didn't pass through.  Having two traps in succession means the far one will be hard to reach for cleaning.  AND, downstream from the trap (each trap) you need a vent pipe connection, or fluids passing through the pipe on their way to the sewer main may 'suck' the water out of one of the traps, breaking the seal against those gases.

So, unless you can get a plumbing expert to sign off, two traps in the same waste line is a no-no.

If, however, you've got several fixtures draining into a common sewer pipe (one that's got a vent on the end of it), no problem with each having its own trap / u-bend as long as the distance from the fixture to the main pipe is reasonably short.  That's the way it's supposed to be - sink/lavatory has a trap, shower has a trap on its drain line, etc.