Shower trap clearance underneath shower tray

Discussion in 'Plumbers' Talk' started by diymostthings, Oct 11, 2018.

  1. diymostthings

    diymostthings Well-Known Member

    Fitting a shower tray (concrete ground floor) for the first time. The instructions require:

    (i) a plinth (for access to the waste)

    (ii)18mm marine ply as a deck,

    (iii) then the tray itself is set in mortar on the ply (say 15mm thick mortar).

    Problem: all the shower traps I have looked at have a dimension between the bottom flange (which seals against the underneath on the tray) and the top of the compression nut (which allows the waste pipe to be connected to it) of between 12mm and 15mm. The total thickness under the tray (ply and mortar) is a minimum of (18mm + 15mm) 33mm. Therefore the waste pipe leaving the trap will foul the underneath of the plywood - and couldn't be fitted anyway as the fitting would be half way up the thickness of the plywood!

    Am I missing a trick here? What do I do? Grateful for and comments/advice/rolling about on the floor laughing etc.
     
  2. dobbie

    dobbie Well-Known Member

    Build a platform with 4x2 and the ply on top with a removable panel to get to the trap in future if needed.
     
  3. diymostthings

    diymostthings Well-Known Member

    Thanks dobbie - yes I was going to do that anyway - that's not the problem. The problem is clearance between the base of the shower tray and the waste pipe connection on the trap - with the ply and mortar bed underneath the tray, this is too thick for the waste to emerge in the clear under it. I appreciate the advice - I had a job to explain the problem..
     
  4. dobbie

    dobbie Well-Known Member

    You dont need a 1/2" bed and cut the hole in the ply bigger than the waste,the bit where the pipe connects faces down slightly and will miss the ply.
     
  5. CGN

    CGN Well-Known Member

    Be a rebel like me and dig out sufficiently then bed the tray on tile adhesive.
     
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  6. diymostthings

    diymostthings Well-Known Member

    Thanks again dobbie - yes I noticed that the connector is biased slightly downwards - however, even without the mortar thickness, the pipe securing nut on the waste pipe compression fitting would still foul the ply underneath. Even if the pipe would just clear, it would be impossible to turn the nut. I think I'll have to go for 12mm ply...

    I was trying to follow the instructions to the letter but it just shows that the people who write the instructions don't actually fit them!

    Best wishes.
     
  7. just pumps

    just pumps Active Member

    You are totally missing the point, come back and re read the answers tomorrow.
     
  8. Heat

    Heat Well-Known Member

    Use heavy plywood.
    Cut the hole in the plywood enough to allow the trap and it’s nut to completely clear the plywood.
    Use obtuse bends or other bends to allow the pipe to fall at the point it meets the trap at same angle.
    The waste fitting is designed to be completed BEFORE you fit the tray.
    So just needs accurate measurements.
    I suggest you test fit the tray before final connections and bedding
     
  9. fillyboy

    fillyboy Well-Known Member

    Rather than a round hole in the plywood, think keyhole shape, circle with 6=8 inch slot coming away in the direction of the waste, that should clear the compression fitting.
     
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  10. Heat

    Heat Well-Known Member

    Exactly the above.
    I use a large hole saw, centred on where the round waste is. Then I use a jigsaw or a multi tool to cut the extra square bit out to make a key hole shape.
    Some wastes will have the outlet lower. Strangely McAlpine do different versions.
     
  11. fillyboy

    fillyboy Well-Known Member

    I've said it before on here (and I'll say it again).
    I struggle to see how the shower tray manufacturers say it must be bedded on mortar otherwise the warranty could be invalid (I prefer CGN's suggestion of tile cement), but on the other hand they are happy to supply a plastic leg kit?
     
    Heat likes this.
  12. Heat

    Heat Well-Known Member

    I think cement bed is stupid, especially on top of wood.
    Flexible tile adhesive is much better and it will allow good thin bedding, unlike sand and cement that would really need to be a fairly heavy bed.
    The shower tray manufacturers have not a clue.
     
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  13. dobbie

    dobbie Well-Known Member

    Have bedded loads of them on powdered tile cement,bedded on 3/4 ply and have never had a problem.
     
    KIAB and Heat like this.
  14. KIAB

    KIAB Well-Known Member

    Can't beat flexi floor adhesive for bedding shower trays.
     
  15. diymostthings

    diymostthings Well-Known Member

    Thanks for the tip - my guess is that they specify a mortar bed to ensure that every part of the tray is supported (but is that necessary??) Tile cement can have voids (according to "youtube")
     
  16. diymostthings

    diymostthings Well-Known Member

    Thanks Kiab - do you know the logic behind using flexi (as opposed to rigid). I would have thougth that the more rigid the better considering the forces that will be made on the tray in use...
     
  17. diymostthings

    diymostthings Well-Known Member

    OK Dobbie - I'll seriously consider this - sounds good...
     
  18. diymostthings

    diymostthings Well-Known Member

    Thanks Heat - That's good advice - if the waste pipe clears the base as it leaves the trap it will clear it well at the edge of the tray,
     
  19. diymostthings

    diymostthings Well-Known Member

    I think they want the tray to be 100% supported but i agree with you---why cant it be 95% on tile cement?...
     
  20. Heat

    Heat Well-Known Member

    The tray manufacturers ask the impossible.
    No chance of tray moving or cracking as long as there are no big voids below tray, especially on corners.
    Flexible tile cement will support the tray well if it is done right.
    It really isn’t extremely flexible, but just takes some slight flex rather than crumbling. So basically is as solid as sand and cement mix, but better imo
     

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