plastering over metal conduits

[piapiapiano]

Is it okay to plaster over metal conduits?

We've just had our house re-wired. My husband has started re-plastering in lime and it's occurred to us that the lovely metal conduits for the wires might be a problem.

I'm really hoping that someone's going to say it's okay.

 

[d49xc]

My house has lime plaster walls throughout.
It also has metal conduits in the walls - usually sightly recessed into the brickwork
These are old style steel conduits - black painted.

It appears the conduits were simply plastered over when the job was done (1930)
Obviously I would have no idea whether a different grade of lime /limemix was used over the conduits

They are totally invisible and even when the walls are bare fore re-wallpapering the only way you'd find where they are is by using one of those pipe/stud detectors.

 

[Penners]

I'm not aware of any problem in plastering over metal conduit. I would, however, make sure that the conduit is painted in a oil paint or lacquer, to make sure that you don't risk any rust discolouration bleeding through the plaster.

I should add that I have no personal experience of this problem - it's just a thought that occurs to me.

 

[Flyfisher]

I can add some personal experience to Penners' excellent suggestion.

We have various lengths of conduit throughout the house, some is surface mounted and some is embedded within the plaster, both lime and gypsum in various places. One section of wall had suffered particularly badly from moisture caused by a leaking gutter and downpipe (now fixed) and the conduit had rusted and stained the plaster as a result.

But in areas where the walls are in good condition, the conduit shows no signs of being adversely affected by being embedded in plaster.

 

[middi]

Cable conduits were ( and are) regularly " buried" in render although nowdays corrugated plastic conduit is used, especially on the continent. This creates a better "key" for the plaster to adhere to especially where more than one conduit are run parallel.
It is advisable to cover metal conduit with 20mm plaster or more, this will allow better insulation from heat variants within a room therefore thin covering of plaster is not susceptible to cracks due to minor metal expansion.
Lime plasters will need to be "worked" an trowelled over the metal pipes before they are comp[letely dry as they may show signs of cracks over these pipes due to the faster shrinkage of plaster at either side of pipes. The deeper the pipes the better.

Middi

 

[MatthewC]

I'm not aware of any problem in plastering over metal conduit. I would, however, make sure that the conduit is painted in a oil paint or lacquer, to make sure that you don't risk any rust discolouration bleeding through the plaster.

The new conduit fitted by my electricians was anodised (or some similar treatment) and so shouldn't rust (it says here).

It is advisable to cover metal conduit with 20mm plaster or more... The deeper the pipes the better.

I just hope that we have enough mortar over the top.

 

[skier-hughes]

We were told by our lime supplier that we should use stainless steel under all lime plaster.

 

[MdB]

My feeling would be that lime once it is carbonated wouldn't attack steel but equally it wouldn't protect it either the way cement does. As it is good at transporting moisture, there is a danger of the steel rusting, expanding and cracking the lime. Just my thoughts - definitely not an expert.

 

[middi]

My feeling would be that lime once it is carbonated wouldn't attack steel but equally it wouldn't protect it either the way cement does. As it is good at transporting moisture, there is a danger of the steel rusting, expanding and cracking the lime. Just my thoughts - definitely not an expert.

Technically the above is totally correct, however, It would take over 100 years at least before any real damage to galvanised pipes should occur.

Middi

 

[Penners]

We were told by our lime supplier that we should use stainless steel under all lime plaster.

Well, that's probably the best (and most expensive!) solution. But if the mild steel conduit is already in place, then I still say that a good painting with oil paint (or better still Hammerite) will be a reasonable protection against rust.

 

[piapiapiano]

Thanks for all your replies. I realise I haven't been here much since my first little flurry of posts, but that's what happens when you have a baby I suppose! We're getting close to finishing our cottage restoration though so I'll be back in another month or two with before-and-after photos. I hope you're all going to approve of our PVC windows! :lol:

 

[skier-hughes]

We were told by our lime supplier that we should use stainless steel under all lime plaster.

Well, that's probably the best (and most expensive!) solution. But if the mild steel conduit is already in place, then I still say that a good painting with oil paint (or better still Hammerite) will be a reasonable protection against rust.

I'd rather do without the hassle of fixing, painting, waiting to dry, plastering, would much rather just fix and plaster.......... well just watch someone fix and plaster.... besides, by the time they've spent all that ime painting, with the average hurly rate of electricians or plasterers, it's cheaper to fit stainless!!!!

 

[Penners]

But piapiapiano's conduit has already been fitted.

 

[skier-hughes]

True.

 

[Andrew Bodenham]

Galvanised conduit should not cuase any problems, although if it has been threaded into mounting boxes or junctions then the threaded portions may not be galvanised and so will be at risk of rusting intially whilstthe plaster remains damp. It would be sensible to paint these areas with an anti-rust primer / cold galvanising paint.

The relatively large dimensions of metal conduit means that it will form a significant line of weakness within the base layers of plaster; future drying or thermal movement cracking may well be concentrated at the conduit. To guard against this you could take the opportunity to embed some scrim tape (e.g. fibreglass mesh plasterboard tape) into the base coats over / around the conduit to provide some reinforcement against possible future cracking.

 

[piapiapiano]

The relatively large dimensions of metal conduit means that it will form a significant line of weakness within the base layers of plaster; future drying or thermal movement cracking may well be concentrated at the conduit. To guard against this you could take the opportunity to embed some scrim tape (e.g. fibreglass mesh plasterboard tape) into the base coats over / around the conduit to provide some reinforcement against possible future cracking.

Ah. That's good to know! My husband actually did this last week, on the basis that it couldn't do any harm. What a clever boy he is.