Truss Cross members and velux windows

14 Aug 2019
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United Kingdom
Sigh... I thought this was going to be easy.
A portion of my kitchen (about 4m wide by 2m long) is a pitched roof lean-to type construction (that continues down from the main roof) with trusses every 600mm. These trusses appear to be completely independent from the main roof on the rest of the house.

I was planning to install 550mm velux windows between the trusses and box them in with light shafts. However I hadn't taken into account the cross members between the trusses, which I now see after opening a nice ugly hole in the ceiling and getting my head in there!

There is one cross member horizontally that I might be able to work around with smaller windows than planned, but there are also what look like 1x4 members running diagonally across the whole (small) roof.

Is there anything I can do within the realms of DIY to cut and replace these cross members or is everything a non-starter without a structural engineer? I'm trying to keep costs as low as possible.

In addition to the costs of the engineer, I expect any solution might involve opening up the entire space to replace all the trusses, which sounds expensive.
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Sorry... The roof isn't tied in to the rest of the roof at all (can't see how to edit). I also forgot to attach pics. One of the cross members and one external view showing the full roof) the lower roof on the rhs of the image) .

IMG_20190814_214626.jpg IMG_20190520_201045.jpg
These are braces to stop the trusses all folding over like dominoes.

You could potentially relocate them (or parts of them) to acheive the same effect, but not remove them completely.

Generally, you could cut out the offending section, and replace it higher and lower up the slope but with longer sections to span several trusses.
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Thanks. Both of these seem like sensible options. I talked to a structural engineer who seemed to suggest that no engineer would be willing to suggest these kinds of mods and that I'd be better off opening the whole thing up and changing to stick framing. One issue seemed to be that the cross members were more substantial than the trusses themselves.

Not sure how I can proceed DIY given I need building control sign off at the end.
Prefabricated gang nailed monopitch roof trusses with longitudinal bracing and diagonal bracing like yours are usually designed by specialist truss suppliers and they don't allow any modifications to be made to their trusses. However ,I am not convinced an engineer couldn't come up with some solution for replacing the diagonal bracing as you suggest.
ps what is stick framing :?:
You'd think so... Will try some more.

By stick framing I meant "traditional" rafters
By stick framing I meant "traditional" rafters
Never heard them called that before, suppose that's another one for me to add to gobbo and muck - I must learn these technical terms :!:
If you cut out only what you need to and sheath your lightwells with plywood (prior to plasterboarding)using plenty of screws and wood glue then it will be stronger than before.
If you cut out only what you need to and sheath your lightwells with plywood
Correct (sort of).

In a situation where the diagonal bracing is interrupted then plywood diaphragms fixed between rafters to 50mm x50mm framing, is acceptable.

I'd just phone (Matt at Trussfrom, Staffs) and confirm this. There are special bracing conditions that deal specifically with roof lights.
Thanks noseall. I will try to put some better drawings together here then will give Matt a call.

I've managed to get a roofer & chippy round to quote but still not heard back. No SE seems to be interested.

The carpenter suggested sistering the "rafters" on each side of the windows (which seemed a bit pointless, didn't think this was necessary if we're not cutting the trusses), cutting out the horizontal member and diagonal bracing and just relying on the framing for the light shafts. I got the feeling it was a bit of telling me what he thought I wanted to hear.
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OK, put together a better rough sketch with rough planned window install location.

Might be beginning to get some grasp of the lingo.... It's a monopitched truss roof, ~2m span, ~4m wide.

So, I'd first have to cut out the diagonal bracing to the right of the second truss in the picture, and to the left of the fifth truss. I can then add a plywood diaphragm nailed to 50x50mm ledgers nailed to the inside of the third and fourth rafters. (I assume this needs to be as high and low as I can reasonably go?)

Not sure what to do with the longitudinal bracing, or if there is even space to relocate it to (or indeed anything to nail it to where it would make a difference).

After fitting the windows, I'd use plywood sheathing for the angled sections of the light shaft (above and below the windows) affixed to horizontal battens between the trusses, before plasterboarding.

Will give Matt a call to confirm.

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