Plastering over long chases of 20mm oval plastic conduit

[DaveyH]

Hello all,

I am about to start plastering over many long pieces of 20mm plastic oval conduit that I have chased into the walls following a rewire. Each piece of conduit contains a single run of 2.5mm twin and earth cable down to the socket back boxes. Here is a picture of one of them.

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• DaveyH
• 19 Mar 2009
• 1

When finished, each conduit will be covered by at least 5mm of plaster. Question 1, Is this enough, or do I need to chisel the chases out a bit more? I know it’s probably a case of the deeper the better, but I am hoping a 5mm covering of plaster will be enough because the walls are an absolute swine to chase as their made of rock hard engineering bricks.

As my walls are made of engineering bricks I plan to use 2 coats of plaster. The first coat will be thistle bonding plaster this is because from what I have read elsewhere on here engineering bricks are low suction so this is the correct plaster to put on them. This coat will cover the conduit with 2mm of plaster. I will then let the bonding plaster set for a few hours and I will then add a 3mm layer of thistle multifinish to bring the chases level with the rest of the wall.

Question 2, do I need to PVA the engineering bricks prior to laying on the bonding plaster?
Question 3, do I need to PVA the plastic conduit prior to laying on the bonding coat?
Question 4, another alternative I read on the screwfix forums was to put self adhesive plasterboard jointing tape on the conduit instead of PVA, is this a good idea?
Question 5, what’s the best way to hold the conduit in place? Some people say clout nails either side of the conduit hammered into the mortar joints between the bricks, other places say use pink grip, yet another suggestion was to use galvanised steel banding.

Thanks in advance

 

[Richard C]

5mm should be OK but is getting a rather shallow. I prefer to use steel caps over cables, they take up less depth, are easier to fix & if some clot hits em with a masonry drill in the future it won’t penetrate that easily & may just save the cables underneath if Mr (or Mrs) clot realises what’s going on; but it will still go through if you push hard enough for long enough.

Chases can be difficult to hide when blending in & unless instructed otherwise, I prefer to fill out flush with Bonding, reinforce tape the joins & re-skim the whole wall; you’ll never know anyone’s been in there. Always PVA the lot anyway but if your just blending in, leave the Bonding at least 2mm below the surface. Not tried the trick with reinforcing tape, as I said, I always use steel capping. Stagger fix along the edges with masonry nails in where the mortar meets the bricks.