Gluing (?) wood to concrete

[Peter Percival]

I have inherited a shed made of cast (or moulded, whatever the word is) concrete for the lower walls on which the rest is built of wood. (It has a made-up as it was constructed look about it.) The wood is rotten and I need to replace it. How can I attach the new wood to the concrete? I could use screws--those things that expand against the hole they are in when an internal screw is tightened--but that will leave a crack 'twixt concrete and wood where the rain might get in. So I need (I think) an adhesive cum sealant. Any suggestions?

[Brian Gaff]

I've never managed to get concret and wood to glue successfully, so I'd say sealant and screws as per your idea is best. Brian

[Peter Percival]

Thank you. What sealant would you recommend?

I wrote: > I have inherited a shed made of cast (or moulded, whatever the word > is) concrete for the lower walls on which the rest is built of wood. > (It has a made-up as it was constructed look about it.) The wood is > rotten and I need to replace it. How can I attach the new wood to the > concrete? I could use screws--those things that expand against the > hole they are in when an internal screw is tightened--but that will > leave a crack 'twixt concrete and wood where the rain might get in. > So I need (I think) an adhesive cum sealant. Any suggestions?

[newshound]

Not quite enough description. Do you mean the base is made of those pre-cast panels which sometimes whole garages are made of? These are thicker at the edges where there is reinforcing rod, and thinner in the middle.

Or are you saying that the walls were cast in place with formwork which was subsequently removed. How thick are they?

And how is the wood fixed? Does it overhang the concrete, or is it mounted on it.

You *don*t generally want an expanding thingy in the concrete, that is asking for cracks unless the thickness is substantial. But if you rest something like a wall plate on top of the concrete, then hang panelling / T&G / Feather Edge outside, you could prevent it blowing off in the wind by having the panel overhanging the concrete, and bolted to it with normal coach bolts going through holes in the panel below the rebar. For adhesive / sealant, any sort of external mastic / frame sealant should do. But I would want some sort of mechanical fixing as well against wind forces.

[Rick Hughes]

you could use resin fixings, no risk of cracking concrete and hole is fully filled

[John Rumm]

I would be wary of using expanding fixing on thin section concrete - there is a risk of just splitting or spalling it. You could drill the concrete and insert threaded rods into resin, then use those for mechanical fixing. Finally finish with a silicone sealant to make the join waterproof while allowing for natural movement of the wood.

[Peter Percival]

Yes, I think so.

Five inches.

Mounted on it, and so rotten (some of it just dust) that I cannot tell how it was attached.

[Peter Percival]

Excuse my ignorance, but I've never heard of resin fixings.

[Steve Firth]

[snip]

You drill a hole, inject a liquid resin and hardener then insert a threaded rod and wait for the resin to harden before using the rod to bolt down the attachment.

Another method is to use non-expanding masonry screws. These need an exact hole drilling so you need an SDS drill and the appropriate bit.

Also available as "Multi-Monti" from Screwfix and other places.

[Peter Percival]

That sounds like a good idea. Let's suppose I fix the wood to the concrete thus, there will be a gap. It there something like the stuff one puts round the edge of baths (silicone something-or-other) that I can fill that gap with, but suitable for out-door use?

Otoh "exact" is a bit worrying.

[Steve Firth]

When we built our barn/garage it was done by building an oak frame on dwarf walls. The wall plate was screwed to the wall using Monti screws. The only material between the wall plate and the wall is plastic DPC. The lowest weatherboard covers the gap between wall plate and wall. A better solution than sealant IMO.

If you must use sealant I suggest either polysulphide or MS Polymer. I've used both and they do a good job of stopping moisture penetrating wood if you get an even coat on the wood surface that will mate against the wall.

More exact than you will get with an ordinary hammer drill. An SDS is ideal, use screws long enough to ensure that there's 40mm or so of screw in the concrete.

[Jim K]

SDS for 6mm holes? nah ordinary hammer drill will suffice

Jim K

[Nthkentman]

I have inherited a shed made of cast (or moulded, whatever the word is) concrete for the lower walls on which the rest is built of wood. (It has a made-up as it was constructed look about it.) The wood is rotten and I need to replace it. How can I attach the new wood to the concrete? I could use screws--those things that expand against the hole they are in when an internal screw is tightened--but that will leave a crack 'twixt concrete and wood where the rain might get in. So I need (I think) an adhesive cum sealant. Any suggestions?

I use it for a multitude of tasks.