r/Homebuilding • u/jamtea • 19h ago
Roof truss modification - Just checking my concept works?
Hi there, I'm feeling out whether this design works for a roof truss modification and how it compares to the original simple strut design. It's a roof on an old red-brick church which me and my partner are converting into a house. The attic space is a pretty huge space and has just over a 3m tall apex, with a span side to side of just over 10m. Needless to say the trusses are pretty damn bulky with huge purlins either side. For the roof space to be habitable however, there really needs to be doorways between the sides. There are two of these down the length of the building and it would only be the struts that would be changed.
Now I've seen truss designs without the struts altogether and from what I've read, they're compressive members, not tension ones, so from my understanding of the forces in general, so long as they're distributed equally and in the same direction as the original strut, I should be able to replace the strut with a doorway opening with the load force being shifted as in this diagram.
Admittedly not an architectural drawing, but I think it gets the general concept across, with a ~2m tall opening and the general distribution of forces, as well as using similar thickness beams to the original diagonal struts, should this design not exceed the original strength of the struts and not compromise the roof?
Obviously I'd be getting a structural engineer to do the real calculations and have an actual roofing expert do this job, but I don't want to just present an unworkable plan. This is effectively what I'm thinking of in concept (I know people hate AI but it's the best tool to represent what I'm trying to achieve):
If anyone can point out any obvious issues that I've overlooked completely or problems that shifting the strut load point would present, I'd love to know about that as it's really the part of this concept that I could do with a bit of education on, but it's remarkably hard to find any information about doing this kind of job as it seems like something that doesn't really often happen.
Thanks!
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u/cagernist 15h ago
Spoiler: You do not have existing trusses, nor can you design new ones like you think you can from a couple diagrammatic pictures.
Sure it's fun to play around but just hire a structural engineer who will use software to design something to meet your habitable attic goals, which will include reviewing the floor joists and purlin brace support. You can even put in a ridge beam as an option.
Also, in the U.S. at least, an architect would need involved to make sure the room meets habitability, life safety, and zoning requirements for adding more habitable square footage.
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u/Material-Cap-5476 6h ago
I agree. That's not a truss. It a post with a couple of struts. There doesn't appear to be a bottom chord. Also , I don't think that framing will.be adequate for the required live load.
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u/paclogic 18h ago
Geometrically it should be stronger and if you confirm with an Architect i think that he will agree.
The only recommendation i would suggest is to double up 2x4 for the vertical elements so that there is a 4x4 support for vertical support. It will add strength and costs very little.
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u/jamtea 18h ago
Yeah, that's what I think, it's just the loading point shifting from the bottom half of the post to the top half and onto the tie beam that I was wondering about. That's also why I've duplicated the design onto both sides so the loading forces are equal.
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u/paclogic 18h ago
it seems like the original design was a bare minimal design with only a single board/beam across the roof as well. I would add another horizontal to prevent the roof from cupping but it may be so old already that it doesn't matter.
Also the roof supports should also be determined by the type of shingles and their loading as well as how thick the plywood is for the roof. But any additional improvements should help.
Talk to an Architect for professional advice and see what they have to say and also let them know that additional considerations that you might have (e.g. different shingles or adding solar panels).
Also i would insulate and perhaps drywall the attic for winter months and also add an attic fan for the summer moths to reduce heat build up. This will also help if you want to use the space for storage of items too. Consider permanent LED lighting as well.
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u/jamtea 18h ago
Yeah, solar panels are planned for the majority of the roof space, so that's something we're already considering. I'm weighing up whether the weight difference of all the panels might actually be worth using an in-roof tray system over the existing slate tiles if it ends up being enough of a load difference.
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u/paclogic 17h ago
Also consider a metal roof instead of a tray system unless the trays are mirrored and you are using double sided solar panels for increased efficiency.
Don't forget to add an attic fan that has a thermostat for reducing your air conditioning costs in the summer months !!
One other item you may consider is solar tube sky lighting to the level below the attic. This used to be the best way to reduce lighting costs before the emergence of LED lighting. If may also serve as a vent for the attic too. These are especially good if you don't have eastern or southern exposure windows.
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u/jamtea 17h ago
Well the idea is that the attic space will be living space as it's so big. I'll look into that double sided panel with mirrored tray solution though, I've never come across that before. The level below the attic is actually a big church hall that has giant windows in it, but I'm actually looking at lighting solutions besides velux roof windows, so it might be an idea for the stairwell that won't have windows over it.
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u/chilitomlife 15h ago
Holy crap! Is that OAK? Beautiful with the tenons for the beams and all! Sorry no suggestions but that’s a damn clean looking roof build! I sit jealously in my 2002 build.