Problems boxing in waste pipes in bathroom

you should have boxed it in prior to fitting the wide window architrave and concealed cistern back and top panels.
2x1 fitted to the walls 2x2 at external corner,clad with plaster board and then skimmed.

 

thats a pvc window fitting which will be all glued in place and cause a right mess if undone so that will need to be worked around. it could be cut carefully with a fein multimaster or such to allow for boxing in.

 

Easy peasy squeasy, Jockie Sharon.

All you need are two strips of MDF, ideally at least 9mm thick but 6mm will actually be ok once it's installed - and is a lot more easy to cut.

Use a piece of cardboard with a good straight edge to make a template of that slope - cut it roughly at the ceiling slope angle and then place it tight against the wall and up to the ceiling - use a spacer (anything, a wee block of wood) with a pencil against it to copy the actual ceiling angle on to the top edge of the card slide it up the wall. Cut and check.

Now you have your ceiling angle.

Measure up against the wall from the unit top to the corner where the ceiling slope begins. Mark that measurement on your MDF and then place the card template on this to continue the correct ceiling slope on to the MDF.

On this piece of MDF also measure out the depth of the panel required - coming out from the back wall to the front of the pipes, allowing, of course, for enough to cover the hole in the unit top.

That's the tricky panel almost done. You now have two choices - either trim that PVC architrave back to allow the boxing it to sit tight against the wall (recommended - and since you say tyhe trim is removable, then do this) or else mark the height of the PVC and its thickness, and trim this orf the MDF panel instead.

Do the first option! You should find the PVC trim easy enough to cut with a Stanley knife - but do it in lots of gentle cuts. Or score it a couple of mm deep, and then snap off the rest over a sharp corner. Use sandpaper to clean it up.

The other side of this boxing in is even easier - it's chust a rectangle. You don't even need to worry about bevelling the top edge as it hits the ceiling as decorator's filler will be used afterwards on it all. DO make sure, tho', that it's made wider by the thickness of the panel so's it overlaps the other panel.

Now join the two panels using a batten - 1" x 1" timber will do, but whatever's available. Attach this batten to the sloping side panel (the panel that's to be overlapped...) right at the edge and use PVA and a few panel pins (if needed) to secure it. (If you have clamps, then PVA alone will do chust fine - just make sure it doesn't slide around before it sets.)

Then attach the other panel - again using PVA and the odd pin/screw. Make sure - if anything - that this second panel's edge overlaps the corner and sticks out a gnat's crochet unless you can get it perfectly aligned. Ie - don't stop it short of the corner. Ie - it's easy to trim orf a teeny tiny sticky-out edge to get it all perfect than it is to sand down a whole 'side' to meet the low edge. Make sense?

Now your boxing has been built - hopefully it fits fairly neatly. If it's too big anywhere, then plane/sand/rasp it down. If it doesn't quite meet anywhere, that's not a problem - it's wot decorator's filler was made for.

Finally, fix it in place. Simplest method is to use a thin bead of something like StixAll (you can get it in white...) and run a bead along both wall edges and a thin bead along the bottom edges. Position the boxing in, press gently into place making sure you are pressing from the outside corner straight in to press each edge into the wall - ie do NOT squash the sides of the box in or you'll distort it!

If the box stays there nicely and doesn't fall away at all, then keep holding it and use a scraper - anything will do - to wipe away any excess adhesive from the joins before it sets. Doesn;t matter if there's gaps there - you'll be running a bead of decor filler along all joints afterwards (once the adhesive is fully set.)

Jobbie jobbed.

Use a tube of decorator's sealant/filler to fill any gaps along all the joins. Then paint it the wall colour - emulsion straight on to the MDF (the exposed edge of the MDF might need an extra sanding and repainting as it'll look more 'rough'. But an extra coat and a fine sanding will get this smooth too.

If it ever needs removing, just run a blade along the edges.

Consider whether to add insulation (for soundproofing) around the soil pipe first. DEFINITELY fix down and secure these water pipes or you risk 'hammer time'... Perhaps chust a sleeve of insulation on them will do.

 

I would buy something like Pendock and just fix to the wall with paintable building adhesive like CT1 or similar.
http://www.pendock.co.uk/pipe-casings/
It is pricey but looks the dogs when carefully fitted.

 

Ignore my post - go for that