Gambrel Attic Truss

[Nathaniel Wilkerson]

I'm trying to design a gambrel roof for my 3 car garage and I was wondering if anyone had any particular thoughts on the correct proportions of gambrel roofs.

What I have so far is below but don't pay too much attention to my truss its a layman's attempt to engineer a complicated structure.

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You can see I went with a perfect octagon in laying out the top chords, somewhere I read that this is the true form of a gambrel roof, but to me its looking a little boxy.

[Rick Alexander]

That's nearly identical to the trusses on my shop roof except for the width. Mine are for a 24 foot span rather than a 28. I will say mine didn't have nearly that much support structure so I'm thinking you have plenty of overbuild there to be more than adequate. I was dead set on building mine as well but when I got to looking at what was available just to get design hints I found I really couldn't build them nearly as cheaply as I could buy them. Plus, then I had the assurance that the tolerances were designed by an engineer and knew the thing wouldn't fall down the first wind storm. I also don't think you can put plates on nearly as strong as the ones the companies use and I just didn't trust wood plates at the joints long term. I have a feeling you're going to need a crane to put those up because they are going to be a bit too heavy to lift by hand. We were able to lift ours up by hand and install them without a crane which saved quite a bit of money. I got a little worried about the apparent skimpiness of the trusses they supplied so I decided to get a few extra and make it 16 inch centers and still think I came out cheaper than if I had built what I had planned in the beginning. I tend to overbuild a lot.

Putting them up wasn't nearly as hard as I thought it would be. We muscled the back one up and braced it hard to the side walls on the inside of the shop. Then we ran a pulley system over the top and down the back wall to a man on the end of the rope. Took them in upside down (prior to building the front wall of the shop) and layed them across the wall and used the pulley tied to the peak of the truss to stand them up straight and against a cheater board on each side at the base of the truss on top of the wall to get the 16 inch center. Nailed the bottom and nailed a few long boards right on top of the truss hanging past the first 2 trusses. We used that board to hold the trusses in alignment until the support braces could be nailed in place to keep them vertical and at 16 inches up top. Just continued that until all of them were in place and then removed the top braces off the surface of the trusses. Also we made sure to put one row of decking on the lower steeper pitch as we went to keep things strong. We also put temporary walk boards in that center room upstairs to make it easier to work on the bracing as we went with full sheet of ply down the middle and a couple of 8 inch 2x stuff on the sides. This made it easier to work from a ladder underneith the roof line so the bracing was easier to reach. Me and 5 other guys had my trusses up in about 6 hours ready for decking and we worked pretty slowly making sure nobody got hurt.

[Steve Milito]

I'm trying to design a gambrel roof for my 3 car garage and I was wondering if anyone had any particular thoughts on the correct proportions of gambrel roofs.

What I have so far is below but don't pay too much attention to my truss its a layman's attempt to engineer a complicated structure.

257601

You can see I went with a perfect octagon in laying out the top chords, somewhere I read that this is the true form of a gambrel roof, but to me its looking a little boxy.

I'm in the process of having my shop built. It uses attic trusses to gain space, although I went with a gable roof. I used an engineer to design the structure. He didn't design the trusses himself but instead worked with the truss company for the design of the trusses. He provided them with the engineering parameters and they designed the truss system. You may want to do the same, truss systems can be complicated to design by hand, and the truss companies have design software that does the math.

[Steve Milito]

Here's a pic of the roof trusses. Note the truss system for floor support.

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[Steve Rozmiarek]

After admiring the old barns around here for years, I've noticed that the ones with a bit more slope on the top roof are more visually appealing to me. I suspect that the old timers did a steeper pitch on the top one to help the roof last longer. They were mostly shingled with ceder shingles, and IIRK, they last better with more slope.

[Jim Andrew]

You don't need to get an engineer involved, just make a scaled drawing and give it to the truss company, or the salesman at your local lumberyard. They deal with trusses every day. The truss company does the design. There are drawings sent out with the trusses advising how to install the trusses. Be sure to follow the instructions. And get the measurements right on your drawings, as trusses can NOT be modified.

[Steve Milito]

You don't need to get an engineer involved, just make a scaled drawing and give it to the truss company, or the salesman at your local lumberyard. They deal with trusses every day. The truss company does the design. There are drawings sent out with the trusses advising how to install the trusses. Be sure to follow the instructions. And get the measurements right on your drawings, as trusses can NOT be modified.

Just to clarify my post. I meant that you should have a truss company design the trusses, not hire an engineer.
I agree with everything Jim says.

[Tom Ewell]

Back in the day, gambrel roof design was based on a semi circle with a radius pegged at the half span of the structure.(can also add overhangs)
The points of the sides and peak were made tangent to the semi circle, one could vary the lengths of the sides to accomodate esthetics as long as the peak remained tangent.
Going outside of the semi circle leads into the territory of mansard, which is OK if that is where one wants to go.

[Jim Andrew]

The gambrel that applies to modern homes, is designed for 8' walls. You frame a wall 4' in from the lower outside wall 8' tall. The slope is 24/12. The reason for the 5/12 pitch of the upper roof is safety. On a 2 story house, anything over a 5/12 is slippery. When you roof the gambrel, you use a ladder on the steep part, and when you get to the upper part, you can walk on it without worrying about sliding off.