I’m looking for a gambrel section if anyone has one they can post. I googled and had no luck.
In particular I am looking for one with no vertical support beneath the purlins. Or if that is incorrect terminology, I mean beneath the point where the two different rafters meet.
Thanks
“When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking.” — Sherlock Holmes, 1896
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Custom Gambrels
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The custom gambrel option allows complete flexibility of design.
Like this, only the one I am looking at has "ceiling joists" at the connection point of the two rafters. The lower rafter has a birdsmouth cut at the top that is basically upside down if you were to compare it to a normal single pitch rafter.
Seat cut is horizontal and the plumb cut goes up, not down.
A 2x4 sits in this birds mouth with the upper rafter's lower plumb (no seat) cut nailed to the other face of the 2x4. There are 2x4 ceiling beams nailed along side the upper rafters also to the face of the 2x4 purlin.
If I can get a few of you understanding me I have a question. It's about the physical dynamics of this geometry.
It seems to me that the ceiling joists would be in compression. Am I right or wrong?
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
Forgot I had Holiday's book...........nuttin much in there on Ganbles.
The roof I am looking at someone apparently inserted a structural ridge.
Took out the ceiling joists which originated as described in above post. Then they rebuilt one wall plumb and built a single pitch roof from the new wall to the new ridge. If you look at the roof line from the gable end, one side is a gambrel and the other is a shallow single pitch.
With no vertical wall or posts beneath the rafter joint; I'm trying to figure out what keeps the thing together.
I don't understand the dynamics.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
I did one for my barn that looks very similar to your pic. I put ceiling joists in on every rafter. I just face nailed them into the rafters.
The ceiling ended up being 8' 1 1/8" in the rough on mine. I used 92 5/8" studs with the three plats to achieve that. Essentially, the two top plates were my "purlin".
I left studs in at 4' oc because I didn't need the floor completely cleared.
My top rafters were 2x6 and my sidewall rafters were 2x4 if memory serves me correctly. It's possible that I used a 2x4 top rafter because I was building the barn for myself and I'm not immune from "under building" things for myself.
The ceiling joists were 2x 10s and they spanned 18'....I think. I remember them being more than 16' and since I usually build from my stockpile, they were the only things available. I would have much rather used 2x 6 because basically they were in tension....I think.
I believe that I could have taken all the vertical studs out and the structure would 've been fine. I don't understand how it would collapse without blowing out the vertical studs.
I haven't taken the time or effort to run the numbers but I think the ceiling joists would be in tension. The lower rafters are more vertical making a vertical force component moreso than a horiz one that lower pitched rafters tend to do. There is a slight compression load from the lower rafters at the perlins (lower ridge?) but my guess it's minimal and easily overcome by the tension loading form the upper rafters.
just a guess though.
Thats how I see it too John.
Eric,
I had some time to play around using some stuff I learned from an engineering book a while back. I will say that I am NOT and engineer but I think the attached pics have the load vectors correct. If not someone please correct me.
I made a model as close to the drawing you posted here and then put in 10 lbs force as a referece load. The lower rafter only exerts about 1.28 lbs to compress the ceiling joists whereas the upper rafters create 32.31 lbs of tension in the ceiling rafters. Teh balance of the 2 makes it about 31 lbs of tension for the 10 lbs load.
But thinking in reality, the lower rafters will hardly every carry a live load at the steep angle they are set at and the upper rafters will carry a live load (I'm thinking snow, leaves, rain, people) quite well so the tension the ceiling joists will probably be even higher.
But I'm not an engineer.
7g
Edited 5/5/2009 2:36 pm ET by john7g
Thanks john7g. It was extremely thoughtful of you to go to that length for me. I truly appreciate it.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
wasn't only for you... :)
I got a lot out of it too. Now if someone would just come along and tell me if it's right or wrong that'd be helpful too.
It sounds about right to me. But given that there is no real standard formula for the actual shape of a gambrel there would seem to be a lot of unknowns lurking.
Changing the lengths or angle of the rafters would entail differing formulas.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
no formula, but tradition has the 5 points of a gambrel fall on a portion of a circleMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
that's good to know Mike.
That is a traditional gambrel; at least according to info I have pulled up.
BT'rs, perhaps myself included seem to have an issue with reading comprehension as only one person was able to understand what I was looking for.
I'm working on a house that was a gambrel and was abortionized to the max. I was looking for some info to help me decipher what I was seeing and what I needed to plan to do with it.
Nice shot of the house. Next time I'm there I get the full tour. I haven't seen the tower, nor the room over the garage.
Say hi to Helen for me. And sorry about your buddy the excavator. We're all getting older.
Except for Dino. He wears the magic speedo.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
Here is one from a recent posting over at the JLC forum.
I clipped it and brought it into Sketchup to see how a circle might fit, tried a couple arcs, and neither worked.
Must be a bad one. ;-)
You are right about the circle hitting the five points. The authors of the excellent book, "Getting Your House Right," illustrates that for gambrel sections.
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"A stripe is just as real as a dadgummed flower."
Gene Davis 1920-1985
Wondering if the circle touches if you start where the flared eaves take off from the side rafters....
They can't get your Goat if you don't tell them where it is hidden.
Changing of the anlges will change the forces but with varying lengths the forces will stay the same. I guess since no one has objected so far maybe my figgering was close to correct? never did this stuff publicly before now.
He also mentioned that a structural ridge was added, so it's safe to assume that the structural ridge is "holding things together".
In the original post, he shows a gambrel, without ceiling joists and states that there are some in there and he wants to know if it's in compression. I think the ceiling joist are in tension. Did you answer that with your figures?
yeah, if i was figuring correctly there was ~31lbs per side of tension in the ceiling joists for that 10lbs of load so a total of ~62lbs tension in the ceiling joist.
7gnot an engineer
Your diagram looks about right from what I can tell.But with unbalanced and/or wind loads some pretty wacky things can happen. Unless a gambrel has some sort of reinforcement at the joints it probably isn't sound IMHO.
It's often a good idea to let the other fellow believe he is running things whether he is or not. [William Feather]
thanks Ron. I'm thinking the best way to build a new gambrel would be to make the lower ridges as structural ridge beams and the top ridge could just be a ridge board. The connection between the 2 rafters just seems to be too much of a weak point in the whole process.
I'm curious to see the problems Eric is seeing in the gambrel roof that prompted this thread.
I agree with the ridge beams at lower ridges. But if you only have a ridge board at the peak you need ceiling joists. I also hope that Eric will post some pics so we can see what he's dealing with. Are ya listening Eric ???
Success is a science; if you have the conditions, you get the result. [Oscar Wilde]
This is a gambrel roof under construction
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Edited 5/4/2009 7:36 pm ET by Mac
Thanks. Those look like some huge timbers!
There seems to be a HUGE amount of variations in methods of framing a gambrel.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
Hey Mac
What's the story on those steel plates at the junction of the upper & lower rafters? Off-the-shelf or custom made with engineered specs?
7g
John - the plates were custom made from architectual plans, and sign by engineer. Timbers are 6"X 12"
Edited 5/5/2009 4:25 pm ET by Mac
Eric--
There's a bunch of different ways to frame a gambrel. Easiest way I've ever found is to frame the lower roof like a slanted wall--bevel the front and back edges of the top and bottom plates so the gyprock and roof sheathing will lie fair--then lift each side and brace them in place. Then construct sets of 'rafter trusses' for the upper roof that you can just drop on top of the lower roof top plates and toe-nail into position.
Here's a few photos of one of mine under construction:
This is the lower roof. Both sides were framed and sheathed flat on the deck, as if they were walls, then raised by 6 guys and braced in place as you see here. (The helper's nickname was 'Wingnut'.)
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These are the 'rafter-trusses' which form the upper part of the gambrel. They get placed directly onto the 'top-plates' of the lower roof and toenailed down. You can see that joint clearly in the lower left of the photo. No ridge board needed; we just brace them temporarily with 1x3 until the sheathing is on.
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Here's a shot of the interior after it is all assembled. There are no vertical supports under the upper/lower roof joint, but I placed a 5'-high stub-wall 3' in from the lower roof edge to counter racking forces (that roof has a lot of sail area). You can see the 4x8 beam down the middle; at this point it is actually hanging from the trusses but when I framed the bedrooms I integrated two 4x4 posts into the partition walls that divided that 30' span into three ten-foot spans.
PS--A 'rafter-truss' is nothing more than a site-built truss constructed using the same scantlings for the truss chords you'd need to use for rafters. The advantages are that it's easy to build flat on the floor; no bird's-mouth cuts to futz around with; you don't need a ridge board; and it's stronger than dirt.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
Thanks Dino.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
I probably have a set of the original prints around here somewhere if I dig deep enough. Lemme know if you need 'em and I'll make a photocopy and mail it to ya (that job dates from before I 'digitized' the drafting room, LOL).
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
eric.... gambrels have always been my favorite
in our house we wanted 10' solar collectors , so we needed about a 12' steep pitch...
to do that we extended the upper pitch beyond the ceiling line, ie: the upper roof is a truss with the same front pitch as the lower roof and a top pitch of 4/12
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you can estimate the ceiling ht from the casement window in the gable end
Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Edited 5/4/2009 9:16 pm ET by MikeSmith