Gambrel Roof

Hi Permies,

Just wondering if anyone has advice to maximize usable attic space and head room with a gambrel roof for a 14'x14' structure. Originally we were going to have 2.5' knee walls with a gable roof, but have since concluded that knee walls can't support a roof.

I'm specifically wondering if we can have a structurally sound and long lasting roof with plywood gussets (secured with screws and glue) only, and no collar ties. The more simple the design, the better. We are racing the weather unfortunately.

Thoughts?

You don't need collar ties if you upsize the rafters properly. Collar ties are a way to save money by using smaller rafters.

Edit to add--and tie in the floor joists properly!! Bolted to the rafters.

I'm wondering the reason why you think you can't do a 2.5' knee wall on a gable roof?

Excellent question Jay!

I am a complete novice at building, and currently I'm working overtime at my weekday job, and the rains have arrived in full force so I'm scrambling to research the best/fastest options.

We were planning on the knee walls for the last 8 months, and we're relying on my carpenter dad for designs and a lot of the labor. He said that he recently concluded that the knee walls couldn't (reliably) withstand the lateral force of the gable roof (rafters would be attached to the knee wall top plates). Possibly because we've been planning on 2"x 8" rafters to accommodate R-38 insulation "to code". We are hoping to rely on good insulation and passive solar for heat. Maybe we should just take our chances with thinner R-23 insulation and use 2"x6" rafters, especially since we have a bunch that were intended for the knee walls. I would love any and all opinions on the insulation and rafter size!

R. Scott, thanks for the tips! Do you think 2x6 rafters would be secure with plywood or osb gussets, and no collar ties? What about 2x8? This article supports the use of gussets instead of collar ties, and that would be my preference! Otherwise I foresee years of banging head on collar ties.

I am trying to visualize how to use bolts to connect rafters to the joists. I've been thinking we should cut a "joist seat", basically an L shaped cut-out that rests on the subfloor, and toe-nailing with screws (toe-screwing? haha). Maybe bolts would be stronger? I'm having trouble finding photos or diagrams that show those connection points clearly, so it's difficult to evaluate options.

Any and all tips, questions, comments, links, photos are much appreciated!!

On a side note, although not technically required, we are striving to build according to International Building Codes...so that makes things extra fun

if it were to much lateral force on the knee wall. It would be to much lateral force on the original wall.

I would put some ceiling joists in. That would eliminate any force pushing outward on the wall from the rafters.

if you are wanting to have the underside of your roof rafter as the ceiling. You can still put a few ceiling joists in and let them be exposed.

Another possibility is to raise the height of the ceiling joists further up the roof rafters. Like an "A". Were the horizontal line is your ceiling joist and the diagonals are your roof rafters. The bottom of the legs of the "A" are what would be touching the top plate on your knee wall.

Then move the horizontal line up by another 1/3 and the inside of your house would look just exactly like the inside of a of a house built with a gambrel roof would look.

if I were going with shear speed of build I would just build a single pitched roof on a 4:12 or what ever pitch you would like.

you can get by with 2x6 on 24" centers but you need to use 3/4 plywood as decking.

anything less than 3/4 decking and I'd go 16" centers on your roof rafters.

1/2" on 24" centers creates to much sag and bounce.

Thanks Jay, this is very, very helpful.

The gambrel roof (or knee walls) would be attached to ceiling (ie attic floor) joists, which will be toe-nailed into the upper-top plates of the first floor walls. We intend to use the attic as an additional room. I probably should have specified in my original post. Also, we will be using metal roofing.

I believe we've been planning on 16" spacing for the rafters anyway (that's also what we used for the walls), though I haven't yet bought the plywood so I could go for the 3/4" and 24" spacing.

Maybe 16" spacing, 2x6 rafters, and 3/4" plywood gussets and decking would be a solid way to avoid the need for collar ties?

are you planning to use a ridge beam or no? If not the ties can go up 5 1/2 inches higher.
What are you planning on finishing the inside with?
If Sheetrock the collar ties will help by giving you something flat to finish the mud on. Otherwise you'll be left with a sharper than 45 and your corner tool can finish it and it'll be annoying to get perfect.

My preference is to always put them. Even if they are pushed all the way up with only 4-6" of level underside exposed.

3/4 plywood with rafters on 16" centers is a bit of over kill. Unless you have a lot of snow load or plan on hanging out on the roof a good bit.

it maybe code up there though I do not know.
Down here the general rule is 3/4 on 2ft centers or 1/2 on 16" centers.

The reason knee walls would be problematic is that the roof rafters would be pushing out on them at the top. A gambrel roof solves this issue both by having a rigid joint at the change in slope (plywood gussets or whatever), and by sloping inward some so the angled load is transmitted more directly to the floor framing. As long as the framing is solid (and joists running crosswise to the ridgeline), this will easily be able to handle the spreading load. You could actually get the same effect with kneewalls and a gable roof by making gussets at the kneewall-to-rafter joint.

How tall are you planning for the ridge? A 7' run at 12/12 pitch gives a 7' rise, plus 2 1/2', making 9 1/2' at the ridge. You could add a couple of collar ties 2 1/2' down with the ceiling following the rafters, and have 7' clear and a spacious feel to the ceiling. Gussets at the base and collar ties would give plenty of strength for a structure this size.

Also, 2x8 rafters at 12/12 pitch would allow a 2x12 ridgepole, which would have enough strength at that span to carry the load pretty much by itself. But belt & suspenders never hurt...

I would go with the 2x8 rafters and use plenty of insulation. It's not that much more weight or expense at this size, but will make a permanent difference in comfort.