Improving Structural Integrity of Brick Garage
Folks were very helpful in my recent insulation thread here: https://www.greenbuildingadvisor.com/question/help-with-detached-garage-hip-roof-insulation
That thread may have some useful information that I’ll recap:
-1937 detached garage with brick walls (8″ thick) on concrete slab in historic neighborhood, zone 4 in Maryland.
-Slate hip roof, 7/12 pitch, with 2×6 rafter ties 48″ OC about 12 inches above the top plate. 1x hangers supporting the rafter ties.
-Rafters themselves are 2×6 24″ OC with the hip rafters being 2×8.
-Interior dimensions are 17.5 x 16.5 give or take a few inches.
The plan re: insulation was to add additional rafter ties to every rafter, so there will be a 2×6 24″ on center going both width wise and length wise. I will use 2×4 vertical hangers to support the ties. I’d then use 2″ rigid foam with R30 fiberglass batts on top.
Although I don’t think the dead load weight of the additional rafter ties, foam, and fiberglass will present a problem for the roof, I’d like to beef up the structure to the extent possible.
Quick aside, I do have some inquiries out to structural engineers to see if I can find someone for a quick consult, but if they are like other contractors I’ve been working with they are swamped and not especially interested in my small job. So I’d like to educate myself as best as I can acknowledging that you all are not here to see the roof.
So my question has a few dimensions.
First re: the placement of the rafter ties. I see that in many cases the rafter ties are placed directly on the top plate and nailed to the corresponding rafter and top plate. This shifts the vertical load (of the ties themselves, insulation, or whatever else) to the walls rather than the roof. Would it be advantageous to install the new rafter ties on top of the top plate rather than a foot up like the current ties? Once I installed the new ties 24″ OC, I would remove the old ties. The ties going in the other direction would sit perpendicular on top of the new ties and affix to the rafters. I would still use 2x4s as vertical hangers.
Although this would lower the ceiling so-to-speak, it might be more structurally sound. Are there other options to consider? Is there a way to leave the rafter ties a foot up but somehow transfer the load to the top plate? My searches have come up empty.
On other note, I measured the space from the front edge of the top plate to the rafter and there’s about 7.5″ meaning I *could* use 2x8s as rafter ties if there was an advantage. However, I don’t plan on creating a walkable space above, just enough to hold the insulation and rigid board (likely Thermax or similar exposed interior board).
Second, currently there are no collar ties. While I’m not sure they are needed, I do see one of the rafters is pulling away from the ridge beam. I figured adding 1x6s or maybe 2×4 or 2x6s in the top 1/3 to serve as collar ties could strengthen the structure without much downside.
Pictures forthcoming. Thank you for your time!
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