Do I actually need a lintel over a little window?

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Grappling with a little project and having a headache with a window.....
I am putting a little window (building regs fire escape size 600mm wide x 750mm tall) in the gable end of a single storey loft conversion - the room is tiny, 1.7m wide with vertical walls just 1.2m running into skeelings that apex at just 2m high. Consequently the available space to fit the window is pretty tight and I am working in a fairly small triangle....

The problem I have is that the gable wall is a cavity and the exterior is rendered - I need to try and fit all the lintels etc from inside to avoid damaging the render and exterior finish..... so I eventually end up cutting out an exact window sized hole in the exterior finish / outer leaf, into which I slot the window, with everything on the inside / inner leaf already completed.

I will be fitting a lintel to the inner leaf anyway and could use some recommendations for the thinnest / easiest lintel to use here. I basically have to get the lintel as high up the triangle formed in the gable (i.e. as far from the floor) as possible so the bottom of the window is well above the 800mm level from the floor. I was thinking of using something like an IG L10. With the recommended 150mm bearing either end of the lintel, plus the 600mm width of the window, you can see how I have very little room for manoeuvre.....

That got me to thinking how I was going to get a lintel fitted in the outer leaf.... and then I thought maybe I actually wouldn't need one....
The outer leaf of the gable isn't supporting the roof structure and there is only about 1m of blockwork above the 600mm window opening, so I am wondering if the (wooden) window itself would be sufficient to act as a lintel in this case..... What do you think?
 
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An L10 is fine. In fact something smaller would do the job structurally. Don't worry about 150 bearings. 75-100mm is plenty for a tiny load like that as long as the bearing masonry is sound.

PS. This only applies as long as the roof is not supported by a ridge beam.
 
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An L10 is fine. In fact something smaller would do the job structurally. Don't worry about 150 bearings. 75-100mm is plenty for a tiny load like that as long as the bearing masonry is sound.

PS. This only applies as long as the roof is not supported by a ridge beam.

Any suggestions for something smaller?

The roof isn't supported by a ridge beam - there is a ridge board, but the roof is supported by chunky purlins with queen struts and tie beams.
 
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How are you going to fit the lintel into the outer skin?

THAT is one of the questions! Do I actually need a lintel in the outer skin?

If I do, I am going to have to try and fit it from reverse, through the inner leaf.... not easy. I am wondering if the wooden window will be sufficient to act as a lintel in the outer skin.... Its a small opening and the outer skin isn't load bearing... its just the weight of the blockwork above the window that needs supporting and that is only about a meter high. Would the window be enough to support that?
 
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Strictly, it would be load-bearing, and load-bearing elements have to have fire resistance, which a timber window wouldn't have.
But in practice, at 600 wide, it will only be supporting three or four bricks and there must be hundreds of thousands of older houses with walls built off the window frames.
Just do as you propose.
 
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Thanks for your thoughts.

I plan to do some exploratory work to check on the size of the cavity, then let an IG L1 (or similar) into the inner leaf from inside on 100mm bearings, if possible... then cut the window recess out externally and fit the frame directly. I think with some careful cutting it should be possible to get a neat, tight fit and avoid the need for any external refinishing. Fit some cavity closers and board out the reveal and it should be job done.....
 
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