What Lintel to use ?

4 Feb 2010
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United Kingdom
I am after some advice in terms of what is standard practice regarding specifying lintels.

BC are involved, I’d just like some constructive input from you guys on the forum before talking to them.

I am building a 2 storey extension, and I have 2 windows in the ground floor wall. The windows are 1250 wide and are in the wall that is perpendicular to the floor joists. The Building Inspector has said that he is OK with the joists being built into the wall, so that is what I will be doing.

Construction is Brick outer skin, thermalite block inner, Plasterboard/skim finish.

First question – Concrete or steel?

If I use concrete, is it a case of putting one lintel in each skin, and a cavity tray above?

Second question – Loading

The loading of the outer wall is pretty straightforward. It is a gable end wall, that goes up another 5m or so.

However loading of the inner wall will include the load from the floor joists, and the board/skim finish.

If I use concrete, would I use a thicker one on the inside than on the outside ? (They only come in 2 thicknesses don’t they)

If I used a standard steel lintel one, then run one course of blocks, would I be OK sitting the joists on top of that course? Or should I be looking at a heavy duty lintel that extends over the top of that course of blocks as well, so that the joists actually sit in the top of the lintel.

The concrete route seems to be significantly cheaper than going with steel ones, and the rest of the house has got concrete lintels visible painted white, so cosmetically it isn’t really a problem.

65 x 100 x 1800 for outer
140 x 100 x 1800 for inner
= £30 ish

Standard Steel (Catnic CG90/100 type) 1650mm = £60 ish

Heavy Duty Steel (Catnic CZ90/100 type) 1650mm = £95 ish

Any advice or tips will be gladly received.

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If you're installing a lintel in an external cavity wall, then you will need insulated steel lintels. You only use concrete lintels for internal walls and/or single (brick) skin garages.

As for the actual lintel to use, I leave that to a professional (i.e. strucutral engineer) regardless if the lintel will be carrying a light or heavy load. I'm not qualified to deal with loadings.

You may find some lintel manufacturers will be able to tell you what lintel to use and they may even be able to provide you with a calculation to back it up.
If your cavity is 100mm, use Catnic standard open back CG90/100.
Your window is odd size, we use 1200 windows and 1500 lintel. If it was me I would still use a 1500 lintel for your window.
Frame height should be 2100, storey height should be 2400. So will have course of block and block slip above lintel. Pitch your joists straight on this.
Thanks for the feedback guys, just a couple of other questions......

Frame height should be 2100,

By that, do you mean the top of the window frame ?

........So will have course of block and block slip above lintel.

What do you mean by 'Block Slip' ?
Do you mean 1 couse of blocks and one course of bricks - which equals 300mm ?

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Apologies, what we call a block slip is a slang expression for a coursing block or sometimes called a coursing brick. They are the same size as a normal brick 65x215x100mm. All block manufacturers make them.

Nine times out of ten, you have a 2400 storey height above DPC.
Window frames are normally 1200 high with the exception of a kitchen window that is going above a sink unit, and this will be 1050 high, to give you a 150mm up stand to your cill above worktop.
We set top of window frame (lintel height) to 2100 above DPC (nine course block and coursing block cut to 150mm long to give lintel bearing) then course of block and course of coursing block above lintel to give 2400. Rest of wall ten course of block, then two course of coursing block to give 2400.
We do not do it this way though, We run Stihl saw down block to give 140mm cut to go round top, and then cut the waste again to give 65mm coursing blocks to use elsewhere. Blocks being the price they are, we waste nothing.
Any other problems, come back.
Thnks Oldun

That all makes perfect sense, and will make my life a lot easier moving forward.

Yes my windows are a weird size, but I am trying to get my extension to match the existing house so I have taken my window measurements from my existing ones.

This is by far my biggest project to date, and I have learned a lot from making small mistakes.

Constructive replies, like yours, to my posts on this forum are helping a great deal though.


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