Strengthening roof rafters

[AI80]

I appreciate this thread is really old but instead of starting a new one i thought i would try posting here in the hope someone may reply as its on similar subject.

I have similar requirement to the original poster, where engineer has specified existing 3x2 rafters be upgraded with 6x2s bolted to existing. I'm wondering if Atelectrical was able to do this without removing the roof?? I can't work out how to get the new rafters in place, even with just a seat cut instead of birdsmouth at the wall plate end, the other end of the new rafter clashes with the ridge board when trying to lift in to place.

Any suggestions??

***mods note*** new thread split from this old thread :
https://www.diynot.com/diy/threads/strengthening-roof-rafters.361552/

 

[Deleted member 174758]

TBH to install a Velux you're going to need to double or even treble (sister) spars either side of the window opening as well as install multiple sistered trimmers between them (above and below the Velux) to carry the terminated spar ends. That will mean having the tiles/slates off for a goodly part of the roof as well as replacing the felt or membrane, probably over the whole roof (as well as modifying the sarking boards, if any). In view of this it sounds like less effort to simply take the whole roof off and redo it, especially in view of the fact that the purlin(s) will either need to be dropped to accommodate the deeper new rafters (to avoid raising the roof line whilst avoiding bird mouths in the rafters over them) or the purlins will need to be replaced and repositioned to avoid crossing the window opening. Or am I reading this wrong? What does your S/E say?

 

[AI80]

No you're not reading it completely wrong and i understand your points, i probably should have stated what has already been done which would help understand it a bit more as i think I've got over the sequence issue with purlins.

I've got calcs from a structural engineer for the spliced steel beams, the dwarf wall, new deeper joists and deeper rafters etc., the existing structure is a TDA truss type with 4 trusses at approx 1.8m centres. Plan is just to use it as storage space and not actually a habitable space. I used the SE once before for beam calcs when i removed a load bearing wall and he was great, quick turn around and responded to queries swiftly, however its not been the same this time! Took ages to give me calcs and hard to get hold of for any queries which i had loads of - one being sequence of construction and the response was 'as per standard building practice'!

The beams have been installed, the dwarf wall built above the beam is now supporting the existing rafters (photo hopefully attached!) And since the pic was taken horizontal ties have been installed with M12 bolts and spiked washers (toothed plate connectors) from the stud wall back to the existing rafters and ready to be bolted to new rafters (everything else in the photo is existing).

The next step is to install new rafters, however when the wall plate end of the rafter is slid in to place with a seat cut, the opposite end clashes with the ridge board when trying to raise it in to position. If i cut this end so it passes the ridge board it is then too short to butt up against the ridge board. I've tried going in diagonally but am restricted by the next rafter so still clashes with ridge board.

Obviously trying to avoid having to strip the existing roof as adds a huge cost so any other suggestions before going down this route would be appreciated!!

 

[Deleted member 174758]

Well, about the only thing I can think of would be to install the sistering rafters in sections. The idea is to stiffen the existing rafters and transfer loads onto the building structure where the existing rafters do

So, for example, the lowest sistering piece would need to have the seat cut to the wall plate done with the other end being square cut to fit tight to the face of the purlin and then a 50 x 50 bearer would need to be screwed to the face of the purlin to carry the square end of the sistering rafter. The existing and sistering rafter would also need to be connected in situ (on heavier material this would require M10 or M12 bolts, nuts, washers and dig washers). I have done joist sistering this way many times with the 50 x 50 bearer (C24 softwood) getting screwed with 6.0 x 100mm screws at 50mm centres, minimum length about 400mm (to get 7 screws per joist) - but that was a horizontal installation and required S/E calcs to get approval and in your case I would definitely want to run it past the S/E

 

[AI80]

Thanks jobandknock, i think i understand what your saying, however the plan is that the purlin and existing trusses that can be seen in the pic will be removed in the final layout (which is what gives me the extra space for the conversion) so bearers screwed in to the purlin to support rafters doesn't sound like an option.

Although from what you've suggested, it's made me think of something similar but not sure if acceptable from building control point of view.

The way I've supported the existing rafters on 4x2 c24 studs (as per photo), could i do the same with the new rafters and stop them short i.e. from the ridge board to a new set of studs next to the existing instead of all the way down to the wall plate? And still bolt the existing rafters and new rafters.

 

[AI80]

Just thought I'd put an update following BC visit.

Basically he was happy for the new rafters just to go to the new dwarf stud wall and not all the way to the wall plate. So I'm happy with that!