hi folks simple question how does one go about stick framing a gambrel roof i am building a 24X32 workshop that i would like to clear span if possible so how do i start plaes advise thanks westy
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traditional gambrel is NOT clear span... it has kneewalls
you might be looking for an engineered gambrel, which is basically a fancy truss with metal connector plates
Short answer is
you don't.
You would need walls or posts and beams to support at the break and a structural ridge beam ( for 32' that would be massively expensive.)
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I suppose you're talking about how to clear span the attic space under the gambrel... As already mentioned by Pif, put structural ridges at the break/changes in the roof pitches and the center ridge will be non-structural as long as you have ceiling joists to counteract the spread.
The struct ridges will need a continuous vertical load path of 2xs beneath them to carry the load down to the foundation.
It's not that difficult, just need a yard to size the ridges for you.
The AHJ may require more than the yard opinion though.
I must say your on top of things! you gave the best answer so far!
thanks mike, but I've never liked being in 1st place, never there long enough. The next poster did a good job pof bumping me off. :)
a little hobby or pass time of mine is figuring how to build the old barns with todays building standards & codes. Home made trusses are good if that will work for the user but I don't see many BIs passing them without engineering paperwork to suppor ttheir use . I think the mag did a dis-servce to it's readers posting that one article that had home made trusses in them.
I built a barn with a gambrel roof in 1973. 24x36 almost the same size as yours. This was simple and had a clear span of 24'-0". No kneewalls but a kingstud in the center of home built trusses.When I presented this to the building inspector he took it to an engineer. Because the truss was self designed ( I am not an engineer or architect) I was asked to build one truss,set it up on two blocks and temporarily braced. I had to pull a line on the bottom chord ( 2x6 ) and measure the deflection with 500 lbs at the center. I borrowed weight plates from the local gym and called the inspector. There was no deflection that could be measured with ordinary tools. I then went ahead and cut everything for the trusses. My teenage son built the trusses on a staging I built so all parts fit and he could not go wrong. Each day during the summer he finished three trusses. My wife and neighbors helped him move the truss off the staging as he finished each one. This roof has stood the test of time with snows up to 30" on the flatter top of the roof.
The formula for a gambrel is quite simple, an example of your 24'-0" span.
The span is divided into 1/6 and 1/3 dimensions. The distance from the wall plate to the point where the two roofs meet is 1/6 of 24 or simply put 4'-0".The height is 1/3 of 24'-0 or 8'-0".Picture a triangle with a 4'-0" base and an 8'-0" leg. The height above the triangle is 1/6 again so the total rise to the ridge is 8'-0"+4'-0"=12'-0".You wind up with two triangles for each half of the roof, each 4'-0"x 8'-0".Bottom triangle has vertical leg of 8'-0". The top triangle has a vertical height of 4'-0".So you therectically have 4 triangles and a bottom chord for each truss.If each leg of the triangle is labled leg A,B,C only C, the rafter is cut . A and B are too get the correct cut angle and length, but not actually used.
I used two 12-0 2x6's for the ceiling joists and they were spliced with 12'-0 2x6 nailed and thru bolted on 24" centers. I spliced the 2x6's on one side only.You can get 24'-0" 2x6's as a special order in some locations. I did not know this at the time or I wouldn't have spliced them. Today you can get engineered joist material for practically any span.
The truss was made of 2x4's for the rafters and a king stud down the center. I cut 3/4" plywood gussets for each joint,each side had a gusset and was glued and nailed with 8d screwnails.
The easiest way to get the cut angles and the lengths of each piece is too snap lines on the floor. I did this ,used one piece of each dimension for a pattern.I cut the gussets on a table saw with an over hanging straight edge.This made a tedious job a lot faster.
When the roof trusses were set ,I added another short tail that nailed to the truss and created an eave of 20" on an slight upward angle,same as a regular gambrel barn roof.
I imagine today you will need a qualified person to draw this up. You will have a nice shop 24x32. By the way I put 1/2 " sheathing across the joists so I could use it as storage for light stuff. Worked well ,not a heavy duty loft but fine for odd ball stuff that clutters the shop.
I used my barn mostly as a packing house and tractor shed. I did have tools in there too.It would have made a great shop if I had stayed on the farm.I hope I did not confuse you, did the best I could with the description.
mike
Edited 6/23/2009 7:33 pm ET by mike4244
those are some interesting layout numbers. Do you remember where you found them?
Interesting in the way that the points of the angles fall on a radius that is centered below and to the opposite side of the roof that is being laid out, that offset being 1/12 of the span (1/2 of 1/6). Below and to the right figured as a 24ft total span of the roof = center of radius 24 inches below and away. 30' wall to wall would be 30" down and 30" away to get the center or the radius.
The roof pitches come out at a 24:12 (aka 12:6) for the lower and the upper is the inverse of the lower at 6:12. Actually not exactly right on the whole number but close enough for practical purposes.
The radius of the circle is just about 3-1/2 times (actually 3.533) the 1/6 span.
Mike Smith mentioned in a different thread that all points of the house and roof section (sill plate, eaves points & roof peak) should fall on a circle got me to thinking on this. In this case the roof lines fall out on that circle.
I guess the carpenters long ago didn't need tape measures all that much. A length of string could do the trick.
So how about that for some worthless trivia?
Edited 6/24/2009 11:36 am ET by john7g
John I have a book on carpentry somewhere in the house that I got in 1962 when I started my apprenticeship. I 'll try and find it as it was bible for four years. Very simple formula and does not mention a radiuss or anything more than I described. Of course it's been a long time since I read this book and I may have forgotten if the radiuss was mentioned.I had not heard or read that all points should fall on a circle. Sounds logical, my roof looks correct to me regardless.
mike
The thing about the circle placement is more of a design/taste thing as a way to get decent looking proportions more than a structural deal.
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I think you're right, Pif. That layout is fairly pleasing to my eye (although gambrels are not my favorite overall).I've seen some weird pointy ones, where the upper slope seems like a tiny afterthought.And some where the upper slope has been stretched out to accommodate an over-wide floor plan, and now looks like a candidate for membrane roofing.But as far as engineering goes, there's plenty of ways to skin a cat:With one of those small, pointy roofs, you might be able to get away without purlin beams or support walls. Likewise if the lower slope is fairly vertical, and supports a broad upper truss.Us peons are better off planning on carrying the load right down to footings, though.AitchKay
1962... 5yrs before I was born. Did they have tape measure back then?
gdr
I am curious to the name of the book. GoogleBooks has a lot of them on-line now.
i don't remember tape measures in '62.....just 6" & 8' folding rulesMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Actually we used 6'-0" folding rules. 50'-0" tapes kept in the tool box for long layouts. I do not recall when tapes became popular, probably when they were made stiff enough to extend several feet. Work has become much different since I began.I do not recall seeing pneumatic tools in the 60's and 70's. No such thing as a slidesaw or even a mitersaw. Handsaws were the norm, you carried an 8 pt for rough work, 10 or 12 pt and a 5 1/2 pt ripsaw for finish work. My cordless drill was also known as a brace.My light duty cordless drill was a Yankee screwdriver, I had three of different sizes.
I could go on but don't want to bore you. Before you ask, yes ,there were horseless carriages in my day.
mike
That's not boring to me. Pretty amazing how things change over the span of a career. Dad still lives by his folding rule and I hardly have ever used one even though I do own one and I even think I know where it is.
In actuallity some of the hand tools are still faster for some things like single cuts and a brace with a phillips bit hardly ever wrecks the screw head.
Mike,
Mine is 24x36 also. Same basic construction for the upper floor - I found some 16" high used steel beams pretty cheap, so I've got plenty of stiffness. I went with a kneewall with 3 wide (5' & 6') openings on each side rather than the more open method you used. Makes a good area for storage.
The break is 4' in from each side. Lower roof is approx. 24/12 pitch, upper is about 5.5/12. Rafter ties crossing a bit above the roofline break give 8-1/2' ceiling upstairs.
Don
Don.... that is almost a perfect example of a classic gambrel
notice that the 5 points all fall fairly close to the circumference line of a circle
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Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Edited 6/25/2009 5:02 pm ET by MikeSmith
>that is almost a perfect example of a classic gambrel. Notice that the 5 points all fall fairly close to the circumference line of a circle
It should - I finalized that design after a similar thread where I think it was you who mentioned that geometry. Someone also described it in terms of proportions - both of which work out about the same for this size building.
The house - small ranch which is just to the right of the garage - has a 5/12 pitch roof, so I figured I'd make the upper roof close to that. Minor tweaks to get the ceiling height I wanted & match the available lumber dimensions, and there it is.
4 years in the making - just need to finish milling, staining & installing the shiplap on the back end & I'll have the exterior finished. Swap the wood stove for the old wood boiler I picked up recently, and I'll be able to take advantage of the radiant tubing I put in the floor.
Don
Nice looking building Don. Looks like mine almost except for the windows,which I really like.
mike
>Looks like mine almost except for the windows,which I really like.
I lucked out on those windows - they're decent double pane builders grade vinyl windows that I got for $20 apiece - a nice lady in a big development of 3 year old McMansions was replacing all her windows with triple pane Pellas.
She was tired of seeing her curtains moving whenever there was wind. When these were being removed, they found that the Tyvek was cut back several inches from the windows, with very little sealing around the jambs. Expensive for her, good for me!
I took all 6 of the upstairs windows she had. The downstairs windows were all about 6' high - she had 9' ceilings, I think. There were several of these available, including some double & triple units. No one wanted to buy them, even at $20 - the guy who replaced the windows ended up taking them away. Good screens & everything.
Edited 6/25/2009 9:02 pm ET by DonNH
check out the article in FHB issue #141.
The responses you've received thus far are mostly true, some being completely accurate, some maybe need a disclaimer.
I built my shop in 07, 36x26. Wanted to keep the downstairs open, so had steel beams engineered for clear spans at 1/3 and 2/3. That way, I ended up with 2x10 floor joists 16" on center, spanning 12'. Blocked above beams between floor and subloor with lvl stock, at the point 3'6"(apprx) in from outside wall, at the point that the shallow pitch on the gambrel meets the mansard section. Then installed 3 beams, each 12' long, 2x12 3 piece. These sit on a triple 2x6 post, over the lvl blocking, on the steel in the floor. I did tell the engineer that I'd be doing this, so he knew that the steel would be carrying a portion of the roof. Then, I used cathedral, scissor type trusses to frame the shallow portion of the roof, from beam to beam. This method allowed me to keep the entire rec-room upstairs open with the exception of 4 posts, without knee walls, or a structural ridge. I think this is the same method shown in one of the previous posts drawings. I've been happy with the results, and have actually done a chin-up to eye the 2x12 beams for straight....Still dead on after 2 years of snow load, etc.
Good luck.
Bing
thanks for the info this project is going to be a fun bonding attempt between my fifteen yr old and myself wish me luck westy
Dozens of plans for gambrel type roofs here.
http://www.ag.ndsu.nodak.edu/abeng/constructionplans.htm
Joe H
I started out simply saying it can't happen with out some truss work.
Generally that is true for someone with no experience with these.
but as others have said, not 100% true.
I am attaching a sketch of how I've done a couple, more or less with a lot of optional details on the two sides of the drawing.
The way I do this is to do an accurate full scale layout on the floor, and cut the lower legs and upper legs, then assemble two together in the pattern blocks with a gusset at the joint - for each side of the roof - and then erect them to a ridge board which is supported on temporary king studs. you could possibly skip the ridge board and assemble all four legs together to make a C-truss. Notice there is a plate on the floor cantilevered. The lower leg has a notch to fit snap onto that.
It is the cross leg ceiling joist that is added after it is up that makes it all stable and strong. you could also make that as you create the 'truss' also, but that would make the whole thing top-heavy for erection purposes. For use with a smaller crew at the time, we elected to just erect a half - one side at one time to a supported and braced ridge board.
Once the cross leg ceiling joists were all glued and nailed in place and the sheathing on, this was rock solid. I drew at the size you mentioned. First we built was on a 28' span without the cantilever. second was on a 24' span with a 16" cantilever as drawn.
This one has a 28/12 lower pitch and 6/12 upper.
You'll have to do some drawings to scale to know what works for you re: rafter lengths, overall height, location and height of kneewall and all those variables.
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!