Why catnic lintels for cavity walls?

21 Oct 2014
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West Midlands
United Kingdom
Why use catnic (type) lintels above your windows and doors if doing rendered blockwork and not just concrete lintels filled with the insulation bats and a cavity closure strip?

Surely they do the same thing but the concrete lintel can be rendered on the window reveal whereas the steel catnic could not....
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Your aware though presumably that if doing this method then you need to provided a stepped horizontal dpc above the reveal? The catnic performs this function with the need to build in the seperate dpc.
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The stepped DPC is a piece of DPM that goes from the inner skin (higher up) to the outer skin (lower down) covering the whole window gap + 150mm each side right? So I have to put this in if I use concrete lintels.... I was also under the impression I had to put this in with catnic too or the steel would eventually rot through. Either way, I fail to see where the water is coming from.
Water can penetrate a 4 inch outer skin and run down the inside of the outer wall in driving rain. In theory render stops this, but cracks can develop which allow moisture through.
Theoretically down the cavity. It's an ancient debate, a rendered wall in good/new condition is gonna be fairly safe bet though the cost of a bit of dpc is peanuts. But compared to fixing a damp problem a decade down the line and its priceless. Its a no-brainer IMO.
The coating on the black catnic lintels is its own dpc (ie cavity tray not required). The galvanized lintels (eg IG) do need a cavity tray, as of course do concrete lintels.
Any cavity wall needs a cavity tray above any opening. The lintel type doors not matter.

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