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Strap boss connectors to waste pipe – push fit or solvent weld?

Discussion in 'Plumbing and Central Heating' started by gasket, 7 Dec 2020.

  1. gasket

    gasket

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    Hi folks.

    I’m after a bit of advice. I’m just changing the bathrooms in my house (built early 1990s) and I’ve come across strap boss connectors fitted to the soil stack for the first time. Each of the bathroom components has connections to 110mm soil pipe via a strap boss connector.

    Now – one boss, for the sink, is very easily accessed (pictured), the other -for the bath - is accessed at full-arm length through the floor (not pictured - its the same except the pipe has a larger OD).

    IMG_9698.jpg IMG_9697.jpg IMG_9696.jpg

    In each case, the current waste pipe (weird sizes: 35mm OD on the sink and 42mm OD on the bath) is still fitted in the boss. But in both cases they’re very wobbly, they rotate and appear to be simple push-fit insertions – which is confusing as most of the ones I’ve seen online are solvent weld. I've not disturbed them any more than that for now.

    Now, I have a couple of choices and would welcome some advice.

    1) If the bosses are a push fit, then the neatest thing to do is to remove the pipe, clean around the seal and then refit the completed pipework in one assembly - as I can reposition and glue them on the bench in advance ready to slide in.

    2) The second option is that I can keep the existing waste stubs in situ, including their connection to the strap boss, and connect the new wastes onto these. In this case, I could either leave them alone, or I could go belt and braces and seal around the outside of the joint with Plumber’s Gold or a similar silicone sealant.

    The risk is that these will not be accessible after the bathroom is fitted, so I need to be certain that these will be leak free (and airtight on the top as they go straight into the main soil pipe). A leak at this point would cost a lot of money to put right once the bathroom is fitted.

    Could anyone advise the safest approach (including “just leave it alone”) for me to tackle this? I’ve fitted plenty of waste pipes before (compression and push-fit), but haven’t yet done any solvent welding nor have I ever dealt with a strap boss.


    Thank you.
     
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  3. terryplumb

    terryplumb

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    Your pic on the far right shows a push fit elbow , so the pipe into it is not a solvent weld size . Your measurements are confusing as the above quoted pipe can't be 35 mm ?????
     
  4. Jackrae

    Jackrae

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    Your close-up of the boss tends to suggest the pipe diameter is too small since there is normally very little clearance between the collar and the inserted pipe. There should be a specified diameter shown on the boss collar.
     
  5. gasket

    gasket

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    Hi all - thanks or the replies.

    I've had a good look at both bosses and should be able to clarify now. For a starter, everything is push-fit.

    So - I've caused confusion with the waste pipe size. I've been back and checked the OD of the existing waste pipes (one from the bath, not pictured and the one from the sink, pictured above).

    The sink does have an outside diameter of 35mm, which (according to this site) makes it "32mm or 1¼" Domestic Waste pipe Push Fit (rubber ring seal)".

    The bath is the weird one. There's a few lengths of pipe connected by push fit elbows. The bit of pipe I measured had an OD of 43mm (not 42mm) as previously advised. There's two elbows before reaching another bit of pipe that engages with the boss. That piece is 41mm OD.

    So the bath looks like a bit of a cowboy job: they've connected a piece of solvent-weld pipe from the bath trap to a push fit connection (yes, it did leak) before continuing back to the soil pipe boss with the correct sized stub of 41mm OD pipe. This ties in with everything else I've found in that bathroom as being done to a poor standard.

    Now that I've plucked up the courage to remove the waste pipes from the bosses I can confirm that the strap bosses are both pushfit connections (I didn't expect that). The bosses are single-piece (no separate collar or rubber adaptor) and are of a push-fit design (you can see the rubber lip inside).

    The bosses appear to be the same range from a common manufacturer. The sink boss is clearly labelled "1 1/4" / 32mm" the larger boss for the bath has no markings on it at all - however there are other pipes and bosses that date from the house build that are clearly labelled "1 1/2" 40mm" and have a measured OD of 41mm.

    InkedIMG_9728_LI.jpg InkedIMG_9727_LI.jpg strap boss bare.jpg

    So onto the fix.

    The sink is easy - its a female push-fit boss, so that means I have to use 35mm OD waste pipe (labelled as "32mm" but I'll measure this in the shop). There's only one elbow needed, and its easily accessible so I'll just use a push-fit elbow.

    The bath is fractionally more difficult - I'll use 41mm OD push-fit pipe to engage with the boss. From there I will need to have a 90-degree elbow and a 135-degree elbow. That's four joints that could leak in inaccessible places.

    So, my last two questions:

    1) Bearing in mind that I have to use push-fit pipe to engage with the boss, what would be the most reliable fittings to use for the 90 & 135-degree elbows in the inaccessible place? I think that a compression fitting would be best, unless I change to solvent-weld-sized pipe mid-point via a reducer or compression coupling. I am happy to add some Plumber's-Gold or similar in addition to the standard fitting- for a belt-and-braces seal.

    2) I have to re-use 30 year old push fit seals that are part of the strap-bosses. One points straight up, but the other is horizontal on the vertical stack. Is there anything I can do to ensure that these seals hold and that the rubber hasn't perished? The seals look OK, but I'd feel better if there was some sort of compound helping the seal.

    @terryplumb : thank you for the pointer on the solvent weld pipe size. I didn't know they had differed ODs and may have reused some of the mis-matched pipes had you not pointed this out.

    @Jackrae : thanks for the tip on looking at the collar for the specified size. The unusualy large gap is actually OK, its just a weird design as the ID of the hole that holds the seal is a tight fit on the pipe.
     
  6. terryplumb

    terryplumb

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    I would use solvent weld pipe and fittings on all the baths inaccessible
    Waste run,upto the boss , to here I would add a compression coupler /to a short piece of push fit pipe to go into the boss.
     
  7. Hugh Jaleak

    Hugh Jaleak

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    We tend to refer to waste pipes according to internal diameter, so 32mm (1.25 ") for Basins, Bidets, Single bowl Urinals etc, 40mm (1.5") for Bath, Sink, Washing machines etc, and 50mm (2") for longer runs or where several appliances are using the one run. The External diameter varies somewhat depending on type of pipe, Polypropylene for Push Fit/Compression waste is thicker wall than solvent weld, and different manufacturers products may not be compatible, even if the same size. Solvent Weld however, is standard across all manufacturers, along with the 110mm soil and underground drain.

    I'd be very wary now you've removed the old pipework from the bosses, the rubber seals tend to harden and 'set' in position to suit the original pipework, disturbing rubber seals in this situation invariably leads to them leaking when the attempts are made to remake the joint. However, as replacing the bosses isn't going to be an easy task, you may have to look at making them seal!

    Firstly, as some of the joints are going to be inaccessible, I'd rule out any possibility of those leaking and use Solvent Weld pipework. Done properly it is almost guaranteed never to leak provided it is not subjected to external stresses or sunlight. This just leaves the boss connections, I think you have little choice but to insert the new pipework into the existing bosses, and add an appropriate sealant, e.g. silicone, to try and ensure the joints cannot leak. Are these joints going to be able to be got at once the bathroom is fitted, before anything is boxed in/floored/tiled over?
     
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  8. gasket

    gasket

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    Guys - thank you very much.

    Hugh - to answer your question, these areas will be inaccessible once the bath is fitted. It would mean breaking this and the tiles out to access it.

    more: disturbing the rubber, I thought long and hard about this - what I found was that whoever replaced the bathroom previously had already replaced the pipe all the way to the boss and it was very floppy in the seal: relying largely on gravity to keep it dry. For that reason I felt it was OK to remove it.

    So, for the entry into the boss I’ll make sure everything is as clean as possible and will use a new piece of push fit pipe. I’ll keep the first 5mm or so clean and will then run a good bead of an appropriate silicone (I tend to use Plumber's Gold) that will seat against the old rubber seal - then will run a finger around the silicone to bed it properly. That should give protection in case the rubber starts to fail (its going into a vertical soil pipe so as long as the pipe doesn't move then gravity should mean the seal stays dry anyway).

    As the first bend will be hanging in mid air, I've bought a rubber-lined munsen ring to rigidly support the pipe before it enters the boss - a piece of M10 allthread back to a threaded bracket which will be screwed to a joist should give a very strong support.

    From there back, I'll then use solvent weld pipework as advised.

    The ideal would be to find a solvent-weld adaptor that has a pushfit sized male end and a female socket into which I could insert the rest of the solvent weld-sized pipe, but if I can't find that then I'll use a compression adaptor between the two.
     
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  10. gasket

    gasket

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    ...of course, I might be making this more complicated than I need to.

    I could just fit a standard bath waste setup like this
    ...and a shallow trap like this (I don't want to cut a recess in the floor)
    ...and then connect the trap straight into the boss with a flexible connector like this

    That way there's only two joints - one at the boss (siliconed) and one at the trap (accessible). Though I've never used a flexible waste and not sure how well they stand up.
     
  11. dilalio

    dilalio

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    Can you identify the brand of bosses?
    You may be able to get new rubber seals for them.
    Also, there is a brand of ring seal that can be converted to solvent weld by removing the rubber seal and the plastic retaining cap that holds it - have you got that product? From memory I think it's osma but MBM!
     
  12. Hugh Jaleak

    Hugh Jaleak

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    Avoid the flexi wastes, no Plumber worth their salt would ever use one. Shallow trap wont comply with Building Regs, when connecting to a stack, Regs require a minimum seal depth of 50mm, you run the risk of the shallow trap being compromised very easily and then unwanted odours in the bathroom.

    I would really consider using solvent weld throughout, especially given you are effectively making your own seal on the boss adaptors, (if you cannot get replacement rubber seals). Using a short piece of push fit seems counter productive to me as you are then introducing another joint into the equation, which is another potential leak point.

    If you really must go with this method, there are solvent weld expansion joints available, basically a solvent socket on one end, and a compression socket on the other. Will cut it down to one extra joint. (Solvent Weld if done correctly will never leak, unless damaged.)
     
  13. gasket

    gasket

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    Hi all. Just thought I'd update this to say the job is done and to offer some thanks.

    I took in all the advice and ended up with a good job.

    1) I used a 50mm trap, rather than a shallow trap. This still left plenty of clearance under the bath and I didn't need to cut into the floor.
    2) I used solvent weld wherever possible (fantastic stuff, first time I've used it and think its great).
    3) I did have to use a short stub of pushfit pipe into the strap boss, the rubber seals just wouldn't stretch over the solvent weld pipe. To join the two, I used a single ended compression coupling (solvent weld the other side).
    4) I supported the pipework using some large rubber-lined munsen rings fastened to brackets with M10 all-thread (historically this length of pipe had been dangling in a large void).

    No leaks and all good. So thank you for your advice.
     
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  14. terryplumb

    terryplumb

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    Thanks for letting us know ,glad your sorted.
     
  15. dilalio

    dilalio

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    Yeah. Thanks for the update.
    Have any photos you can upload?
     
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