Slip membrane for large steel?

[Peter500]

I'm putting a large steel (6m span) in my garage extension to support the roof. I found this (https://www.nhbc.co.uk/binaries/con...ce/6.1/lintels-over-wide-garage-openings-.pdf) online and I'm trying to understand it.

From what I understand, I put two pieces of DPC under the steel, but I can't understand what it's saying about a cavity tray on top. The steel will only sit on the inner skin, so I can't understand why the outer skin would need a slip membrane too

I'm wondering whether I should wrap the whole thing in two pieces of DPC or put two pieces under it and then one on top. I don't know whether the flange of the universal column would also need to have a slip membrane but it doesn't mention those, only the top and bottom.

 

[George_engineer]

If it's a single leaf wall then you won't have any use for a cavity tray, as there is no cavity.

Just the two slip sof DPC below the steel are needed - however personally I've never specified this.

 

[Peter500]

It is a cavity wall, but I still can't see why I would need a tray as the steel won't stick into the cavity. It's only supporting a roof, so there will only be a 500-600mm blocks above it.

 

[^woody^]

A cavity tray is for protecting from water penetration, not movement.

 

[SFK]

Image here might help:

Wickes UPVC Wall Weep Vent - 10 X 65mm | Wickes.co.uk

Wickes UPVC Wall Weep Vent - 10 X 65mm

www.wickes.co.uk

 

[Peter500]

Yeah, I know what a cavity tray is as I've done them for my Windows etc., but just can't understand what it has to do with thermal movement

Steels are done now, just a couple of pieces of DPC below the steel.

 

[George_engineer]

A cavity tray is for protecting from water penetration, not movement.

I think they mention it because it will act as a slip membrane, even though it is put there for moisture reasons.

 

[Peter500]

I think they mention it because it will act as a slip membrane, even though it is put there for moisture reasons.

Ah, so should I put another but of DPC on top the steel when I put the bricks on it?

 

[George_engineer]

Ah, so should I put another but of DPC on top the steel when I put the bricks on it?

See what the lintel manufacturer says. Most of them say yes, in order to direct any moisture from the cavity outwards.

 

[Peter500]

See what the lintel manufacturer says. Most of them say yes, in order to direct any moisture from the cavity outwards.

It's in the middle of a garage, not an external wall. It's also not a lintel, it's a steel universal column. The question is whether I need a slip membrane on top (a single piece of DPC).

 

[George_engineer]

In that case no. You don't need it for moisture reasons.

It's a bit hard to say whether there needs to be provision for thermal movement just looking at one beam in isolation. I don't think I'd bother. As beams aren't usually bedded in (like lintels are) I don't think it'd be much of a problem as far as the beam is concerned. If it's all internal, thermal effects won't be very significant anyway.

 

[^woody^]

I've never specified a slip plane on such a beam, and neither have I seen it specified by engineers.

And TBH, it is a load of nonsense.

 

[noseall]

I've never specified a slip plane on such a beam, and neither have I seen it specified by engineers.

And TBH, it is a load of nonsense.

Agree. It's like when they rush to install wind posts on every bloody beam.

 

[Peter500]

I think I won't bother with DPC on top then. I think it's good to have it under the steel though as it should help take out any unevenness in the concrete.