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Lintel selection help


 
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glangri2

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PostPosted: Wed Dec 07, 2011 11:49 pm Reply with quote

My extension is now water tight, and I am fast approaching the point at which I will need to break through into the existing house.

I am making 2 doorways through what was the external cavity wall, but is now an internal wall.

The floor joists run parallel to the wall, so the only load bearing will be the masonary above.

Should I be putting in a normal steel cavity lintel, or would 2 concrete lintels (one in each leaf) be OK?

Will it be necessary to use an acro/strongboy to support the brickwork above the opening?
When I cut a new front door in, I used an acro & strongboy, but it really didn't seem necessary. When I was taking the bricks out to create the opening, none of the ones below the strongboy fell down, or even sagged.


Also, I am thinking that I will probably make the outside leaf opening 200mm wider than the inside one, and then building a block return to close the cavity off, before putting the door lining in.

Cheers
Gary
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RedHerring2

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PostPosted: Thu Dec 08, 2011 9:33 am Reply with quote

Perhaps this diagram will help with your query about support while inserting the lintel:


You'll see that you could remove three bricks from the lowest course, but only two could fall from the next upper course, then only one above that.
So when you remove several bricks you end up with a natural arch, albeit brick shaped.
If you can manage with any bricks that might need replacing above the lintel, assuming there aren't too many to substantially weaken the wall, then you may not need support while you insert the lintel.

You can get a steel lintel for two skins and cavity with insulation included in it. http://www.catnic.com/libraries/document/Lintelproductselector.pdf
I would have thought that this is easier than using two concrete lintels.
Unless you want/need to match other existing.

I, personally, would use a cavity closer and make both skins the same width.
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Static

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PostPosted: Thu Dec 08, 2011 9:44 am Reply with quote

Go with 2 concrete, as normal cavity lintels arent designed to take any significant loading on the outer leaf (normally just arched brickwork) and any loading from your new extension may cause troubles..
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glangri2

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PostPosted: Mon Dec 12, 2011 5:42 pm Reply with quote

RedHerring2 wrote:

Unless you want/need to match other existing.


That isn't an issue, becasue it is going to be an internal wall, and will be plastered over.

RedHerring2 wrote:

I, personally, would use a cavity closer and make both skins the same width.


My theory was that if I made the hole in the outer skin 200mm wider, I could get a nice perpendicular 85mm deep angle grinder cut in the outer brick work from the extension side, and then once the bricks were out, I could do the same with the inner skin, again from the extension side............. thus minimising the mess in the existing house.. Going out by 100mm on each side, means that I could use standard blocks to close the cavity off.

Yes, Cavity closers would be easier, but last time I cut a hole through a cavity wall, it was nigh on impossible to do make hole in the inner skin tidy, because I was cutting from the outside of the building.

The cavity walls are Brick/Brick rather than brick/block, and cutting the inner skin by perforating the outline with drilled holes would take an age and probably get through several drill bits.


Finally, cost comes into it.
A 1200mm Steel SD90 is in the region of 45, whereas a couple of concrete ones will only be about a quarter of that.
I've got 2 doors to do, and saving 75 is worth doing !
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