MortarThePoint

I'm intrigued by your mixture of water pipe sizes. You appear to have thinner and thicker water pipes. I am currently planning my plumbing so it would be good to understand.

It's our main bedroom en-suite so high likelihood of basin and/or toilet being used at same time as shower. (and yes that second bit horrifies me too).

I didn't like the sound of the extra 16seconds of wasted hot water so have punched a hole in the sole plate of the wall and the hollowcore concrete, below the word SHOWER, and the hot water feed can come up through there. I now wonder if the cold should come up there too as will turning the basin tap on affect the shower temperature?

I'm trying to plan the plumbing of the en-suite. Thanks to @Nickfromwales I know what to do with my basin waste and I show that it brown and black below (brown in the room, black is in the wall). I then need to route the hot and cold water feed (shown combined as red below) around the room to the various places. It's going to come in to the room in the corner where there is a trench in the screed along the top wall to the bath. It's going to then have to come above screed to reach the right stud wall before feeding the basins and continuing on to feed the shower. The shower and bath wastes have holes in the floor underneath the shower and bath. The feed may pop up under the bath rather than the corner, but that doesn't change much. Where a right angle is shown in pipe below there may need to be an elbow or tee. Is it a good idea to clip the fed pipes to the OSB or does that make it noisy? I am expecting to use something like push fit JG SpeedFit for the water feed, but am solvent welding the wastes. The length of pipe round the room is about 12m so if it's 15mm OD / 11.5mm ID (JG Speedfit pipe datasheet) the pipe volume is about 1.2litres. A shower running and 6l/m total and 3l/m each of hot and cold would take 24 seconds to pass that volume through. It's a bit of a shame because if I could get there direct it would only take about 4m of pipe and so take 8 seconds for the shower to heat up. Is 15mm the correct size to use to feed a shower? Bathroom wall layouts as viewed from outside the room:

We aren't considering any particularly fancy tiles so weight probably not a concern. Do you think it reduces the bond strength of the tiles to plaster though? I'm just thinking it is easier to ask he plasterer to do a whole room that mark out where he shouldn't plaster.

I can understand the concern if the crack isn't supported by material behind, but ultimately if you just treat it like a join surely(?) it can't cause any problems

I have two sheets of plasterboard that snapped whilst being carried. They didn't get dropped, but the snap is straight across and the sheet folded to near right angle. I saw in the BG book it says not to use damaged sheets, but how do you feel about this? They are moisture resistant and for bathrooms where they will be over OSB. On one sheet the crack is square to the edge of the sheet so much like how you'd join a sheet. On the other it's at a slight angle so may be no good. If using the one that's square, I could cut it clear and leave a small joint for the plaster skim.

BG give the guidance below when it comes to wall tiling. I presume an 11mm sheet of OSB counts as equivalent to a 12.5mm plasterboard and so it's OK to use 12.5mm over it?

Thanks this is all really helpful. So sounds like the winning recipe is: Studs at 400mm c/c (70mm C-studs in my case) OSB (in my case 11mm) onto studs, held 15mm off the floor screed 12.5mm moisture resistant plasterboard onto OSB, held 15mm off the floor screed Plaster skim (Thistle Multifinish or is there a better choice for bathrooms?) Tanking solution (any recommendations?) CT1 shower tray against the plastered wall and tile above as needed Elsewhere, use cementitious tile adhesive to fill the 15mm gap when tiling the floor

I've sat the 11mm OSB on offcuts of 11mm OSB so am a bit shy of the 15mm. I haven't fitted the plasterboard yet though so could try cutting 4mm off though could be tricky.

I was hoping for some tips on using OSB under plasterboard in bathroom walls. How far off the screed should the bottom of the OSB sit? Particularly next to the shower tray should I raise it clear of where the top of the shower tray will be and use either smaller OSB offcuts on the studs as packers or perhaps use 11mm thick timber there? Is it worth treating the bottom of the OSB with anything? How far off the screed is it normal to raise moisture resistant plasterboard? Note both of these are distance off screed not off top of tile. I plan to tank the plasterboard when done. Is that considered a good approach or will it create difficulties for tiling or plastering?

I'd be worried about having the 63x38 CLS studs that way round. The deflection under the same load is 2.7 times as much in that orientation and thickness in the line of the load force is king for keeping deflection down. 600mm centres compounds it further and makes it 4 times floppier than 400mm c/c with 63x38 CLS in the correct orientation. You'd probably be better without staggered studs and using resilient channel or think about using Metal C-studs (also Acoustic studs are an option there). Second Moment of Area: (63*38^3)/12 = 0.29E6 vs (38*63^3) / 12 = 0.79E6 -> 0.79E6 / 0.29E6 = 2.7 Beam deflection is inversely proportional to Second Moment of Area.

Here's what the return will look like in terms of OSB:

I wonder if I should have held the OSB a little higher than 10mm off the screed, but the plasterboard will need to come down about 5mm above the screed I presume.

I've gone with the stripwood approach. Hard to see, but I left a 3mm gap between the two boards by resting the top one on screws sat on the bottom one whilst installing. Also 3mm gap at the blockwork and will do he same at when I add the 15mm plasterboard continuation. I've not taken the OSB to the ceiling as that is 2.7m and there won't be cause for anything up there (will there?). I'll pack the studs at the top with stripwood and strips of OSB just at the studs. I raised the OSB about 10mm off the floor, you can see a strip of OSB under the sheet edge which I will then chop up and use as above. Stud positioning to avoid pipe penetrations is the next key thing. The sink waste is going into the wall 600mm room the RHS end of the OSB (400mm stud centres). It will then pop out in the bottom left corner and I expect I will need to pass water feeds in there too to then take to the sink and continue round, into the return and over the door in the return to the shower. The other route to the shower is along with the shower's waste, but I'd need to cut a trench into the screed for it to continue past the shower trap and onwards to the stud wall. [Note: the stud in the odd position is just moved temporarily

That doesn't work so well with the 1200mm gauge unfortunately

I was worried that it would affect the strength of the junction with the plasterboard stopping in free space rather than on a stud.

I could put 36mm x 4mm stripwood behind the OSB3 at all its studs to make it effectively 15mm:

The sketch above has the issue of the plasterboard needing to come away from the OCB towards the corner. Or am I better off creating a small pocket in the corner to get filled by plaster when the wet plastering is done? They may not look it, but the sketches are pretty much to scale.

I'm looking to include OSB in the Bathroom wall but it's complicated by there being a return. The wall sheathing will effectively be changing thickness at the return which is a small challenge. Also, I may only be able to get 11mm OSB (or 18mm but same problem) which means it's not the same as the 15mm plasterboard. Below is a drawing of what I'm looking at. The green is 15mm or 12.5mm moisture resistant plasterboard, yellow is 11mm OSB3 and grey is 15mm Duraline. All on 70mm C-Studs (BG) in blue.

The AAV could be above the sink and behind the mirror. To service the AAV, I could remove the mirror and an access panel. I suppose it raises the question of how does air enter the wall and does it bring moisture in with it? I know there are small trap based AAVs but suspect they wouldn't admit enough air to prevent the problem in this situation. 9l/s for 1¼" solvent weld pipe https://mcalpineplumbing.com/air-admittance-valves/ventapipe/vpsf-4050-ventapipe-4050

Thanks Nick. The bath is freestanding so it would be a pain to have an AAV there. Could the AAV be in the stud wall with some form of air access to the stud wall? Another option that might be possible is to keep the bath and basin wastes separate all the way to the stack. This is what I was worrying about in terms of an S-trap: Though it would be more like this I guess: Could I solve it all by having the AAV off that initial in wall elbow? If just having the AAV in the wall, I would make sure it is well above the top of the basin.

The pipe goes about 500mm along the floor, joins the bath waste and goes 200mm down through floor before going 1800mm along ceiling void and joining soil stack. I may not pass through floor though and join stack above floor, but that area will be very busy. So I can't going purely horizontally to the stack unless I penetrate the wall at skirting board level which would be strange.

The basin would have a U-trap before the wall penetration so combined with the 90degree in the wall that would make an S-trap. That's bad for symphonic emptying isn't it? What are you thoughts on height?

Hmm, does this second drawing create an S-trap which is a no-no?