Is it neccessary to cup the back of skirtings and architraves?
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Tagged: Architrave, Masonry., Skirting, Timber on Masonry
- This topic has 3 replies, 3 voices, and was last updated 5 years, 6 months ago by Byron.
I was taught that any piece of timber that was placed flush against a masonry wall needed to be cupped on the back. Think of timber skirtings, architraves, etc. The theory behind this is that even if the wood is stable, and properly cured to have a low moisture content it will still never be 100% ‘dimensionally stable’. Fixed to a wall, one side of the timber will always be exposed to more moisture than the other. Any warping that happens needs to be controlled, so the back of these elements is grooved and cupped so that the timber will warp convexly onto the wall as this is never noticed. If it warps away from the wall unsightly gaps between the wall and the wood open up.
I’ve noticed that many of the locally available pre-profiled sections available at big brand stores do not include any cupping on the back of the profile. A local company that machine profiles wood also do not cup the back of their profiles and seemed oblivious to the need to do this when I asked them about it. I’m suspicious as the suppliers that I speak to obviously have a vested interest to convince me that the way that they do things is correct, and that I am just being over-fussy.
I wanted to check what the general feeling is on this forum, where I think there are people who really know about wood and are not just trying to sell it. Is it sensible to cup the back of skirtings, or am I just doing things for the sake of tradition?
Your comments and opinions are apreciated.
A pal of mine, who is now a project manager/foreman in the building industry, but served his time in the mid 70’s as a carpenter, talked to me about this just recently…
They’d been having some issues with skirting ” misbehaving” after fitting… Turns out the problem was related to what you mentioned…
When he was trained he was told to treat/paint the back of skirting board before fitting to stop it absorbing moisture from just one side.
I don’t know what is “normal”
When I’ve has need to make skirting in recent years I’ve not made any allowance for it, (as it hadn’t occurred to me). However, So far I’ve had no issues. The fact that I was working with 1″ thick skirting perhaps has a bearing?
Matt12 November 2017 at 9:39 pm #364686
It’s pretty good practice to prime the back of nailed on trim before installation so that wood absorbs and releases moisture more evenly. Oil ( alkyd) based or shellac based pimers will work. On outdoor work it’s pretty much mandatory.
you can also run a saw cut on the back side so the wood won’t cup.
Easiest with a table saw.
Thanks for your comments Larry & Matt.
I suppose that priming the backs of skirtings would explain the availability of some commercial skirtings without groves on the back face. If the primed backs of skirtings isolate the timber from moisture there is no need for groving the back face. Any swelling of the timber will be on the outside. I really like the idea of running the back of the skirting over a table saw too (thanks Larry). I had assumed that they needed to have a more significant groove cut during profiling.
Ive obviously never primed the back of a skirting board, but many skirtings Ive stripped have had a black coating on the back (usually with grooves also), which I finaly understand. I expect that the choice between grooving and priming could be economically driven and depend on the type and curing of the wood used as its probabaly not neccessary to do both.
When discussing this, one of my collegues wisely asked me who I trusted more: the guys I learnt from, myself, or the skirting profile salesmen.
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