Gambrel Truss Design

[Dennis Putnam]

My next project is to build a storage shed (20'x17'). I want to minimize the footprint but maximize storage so I plan to use gambrel trusses for a usable attic. My span is 21' and that is where the problem lies. I understand the concept of gambrel trusses but not the engineering. I think I need to use 2X16 stock for that span but I cannot get stock that long so obviously I need to splice 12 footers. The problem is that I don't want any supports in the center so I don't know how to build it. I also can't run a perpendicular span down the middle to use as a hanger as there will be a double door at one end and thus no support. I'm guessing I need to splice the center joint with plates of some kind but I don't know what to use , how big they should be or how to attach them.

I also need help with the support members (2x4) as well. The goal is to maximize the usable space between the break points.

Can someone point me to a source for designing these trusses and getting the details right? Thanks.

[Bob Lloyd]

Dennis

This may help you get started.

http://www.ag.ndsu.nodak.edu/abeng/plans/Construction1.htm

Bob

[John P Clark]

Dennis;

I admire you is that you want to build this roof and be a part of the design. However, there are numerous items that you need to be aware of when designing a truss, some members are in tension and some in compression, moments, shear, etc. You did not state where you live, but you have dead loads to consider, snow loads, what are you going to store in the attic, wind loads, etc. Any combination of these loads may be the controling load for the design of the truss. A 21-ft span is not something to be taken lightly. Please seek the advice/ design from a professional engineer in your area. Based on the design of trusses and gang nail plates, you might not have the necessary items for the construction of the trusses on site. My two cents as a professional engineer

[Dennis Putnam]

Thanks. That was the information I was looking for. While snow is not an issue here, wind may be. As I studied gambrel trusses I was becoming concerned that I was getting in over my head (I'm an EE not a CE or ME). As much as I dislike it, I think I am going to change the design to a standard roof and forgo the attic concept. I originally thought I was going to be making something smaller but after measuring the equipment I needed to store in it the size grew. I supposed I could make it narrower and longer thus reducing the span to something I can buy pre-made but I don't think I want a bowling alley.

[Brian Effinger]

Dennis, instead of making the trusses yourself (which is difficult engineering & construction-wise), why not buy pre-made trusses. When you go right to the manufacturer, they will do all of the engineering and will even supply stamped drawings, if your building department needs them.

[Dennis Putnam]

I'm afraid that is becoming cost prohibitive. This is for my back yard for storing all my outdoor equipment. While additional attic storage would be nice, I can't cost justify it. Buying pre-made trusses of that type and size will be very expensive.

[Chuck Werts]

I went to a local truss manufacturer and talked to the engineer. Paid him $100.00 to draw them up as a side job.

[Lee Schierer]

My next project is to build a storage shed (20'x17'). I want to minimize the footprint but maximize storage so I plan to use gambrel trusses for a usable attic. My span is 21' and that is where the problem lies. I understand the concept of gambrel trusses but not the engineering. I think I need to use 2X16 stock for that span but I cannot get stock that long so obviously I need to splice 12 footers. The problem is that I don't want any supports in the center so I don't know how to build it. I also can't run a perpendicular span down the middle to use as a hanger as there will be a double door at one end and thus no support. I'm guessing I need to splice the center joint with plates of some kind but I don't know what to use , how big they should be or how to attach them.

I also need help with the support members (2x4) as well. The goal is to maximize the usable space between the break points.

Can someone point me to a source for designing these trusses and getting the details right? Thanks.
My storage building has a gambrel style roof and has storage space.
http://www.home.earthlink.net/~us71na/barn.jpg I worked with a local truss manufacturer to design the trusses for my barn to give me the storage I wanted in the attic area. He did the design work and I built the trusses. My barn is only a 16' span so you may need specialized design and proper wood type selection to get a 21 foot clear span with load carrying ability. Take a look at this span calculator (http://www.awc.org/calculators/span/calc/timbercalcstyle.asp) to see what you are up against with that much clear span.

[Brian Effinger]

I'm afraid that is becoming cost prohibitive. This is for my back yard for storing all my outdoor equipment. While additional attic storage would be nice, I can't cost justify it. Buying pre-made trusses of that type and size will be very expensive.
Another option might be a simple gable roof. If you span the rafters across the 20' dimension, and have a 12 : 12 (or 45*) roof pitch, you would have 9'-8" under the ridge board if you set the rafters on top of the attic floor joists.

[Brendan Plavis]

Being a current CAD student, I can tell you that I would recommend using a Span Chart, such as http://www.ci.medford.or.us/Files/SPAN%20CHART.pdf (douglas fir.)

The you would examine the ceiling/rafters sections(dependant on if you have above storage.) I didnt see where you are located, but, if you are in the north, chances are you have a New England pitch(I believe its 5/12, could be mistaken), or in the South you have a lower pitch.


Below is an unscaled look at how I believe a Gambriel truss system works.

If you care to, I can draft you up a blueprint for your garage, and my software will automatically insert the needed beams and colums.

[John Bush]

Hi Dennis,
I built a shop using old barn beams and lumber, and initially designed a gambrel, but I didn't have enough of the old 2' x 10's to frame the the roof rafters. I ended up building a 45 deg. gable and adding shed dormers on both sides. The span is 20' and the dormers are 8'. Still plenty of space and it looks good from then ouside. I would strick with the gambrel if you can--you will have lots of extra room. Good luck, JCB.

[Brendan Plavis]

I did a basic rendering of what the proper frame work would look like using the pitches of 6 and 8(upper and lower respectively)

Attached it is.

[Bill Leonard]

Take a look at "Barnplans.com"

[Dennis Putnam]

Thanks all for your advice. You have at least convinced me to reexamine gambrel trusses. Using your calculator, it appears 2X12 will span 21' 5". However, some of the terms I did not understand so I guessed but it appears a 20' span with 2X12 is safe. Unfortunately, as I stated earlier, I don't know where to find lengths like that around here so I am back to my original problem of how to properly splice a couple of 12 footers to make the 20' span. I know I will need to sister the splice and I assume the sister will be 2X12 also. What I don't know is if I need 1 or 2 sisters per joist and how long they need to be (bolt? nail? glue? all the above?).

While I am not yet convinced gambrel trusses are within my capabilities and I still have questions about the details, I think they would be fun to do.

[Glen Butler]

Having 16 years framing experience, I can tell you that 20' is really pushing it for a 2x12, though tables may say it can span properly. It won't fail but it will sag. But I do live in Utah where we get snow loads, and you said you don't have to deal with snow. In my area 20' spans are called out using an 11 7/8 - TJI 360 series, again this factors in a 40 lbs/ft snow load. I say this just to make sure you have thought everything through.

If you do want 2x12 they are available in a 20' length. For cost you are better off buying TJI 210's. Additionally if you have to sister the 2x12's it will require enough extra material that you might as well use TJI.

[Dennis Putnam]

Thanks for the info but could you explain how snow loads matter to gambrel roof joists (although that is not a concern here)? Perhaps I am missing something with the concept. All the loading on a gambrel roof is transferred to the outside walls or at least within a few feet from them is it not? It seems to me that the loading on the joists would come from whatever is stored on the floor. Also, what would I estimate as the loading for something that will be used for general storage? I doubt it would be more than what a person carrying something would add per sq. ft., right?

I've seen TJI but have never used them. Looking around, I don't see any local retailers that carry it. That implies ordering it from somewhere and having it delivered. Does anyone have a ballpark price for 21' TJI 210?

That brings up a curiosity question. I know how to hang perpendicular joists (like for stair openings) on normal lumber using standard hanger hardware, but how do I do it with TJI?

[Brian Effinger]

I think some of the terminology is getting mixed up here, in regards to the rafters. The rafter span of Dennis' building is not 20 feet (from wall to wall), but it is only 10 feet. The rafter span is from the bearing wall to the ridge. Therefore, he wouldn't need 2x12 rafters, even if he did have a snow load. 2x8's would be sufficient. Maybe even 2x6's.

I have also attached a section through a gambrel style pole barn I drew up a few years ago for a client. The gusset that holds the two roof pitches together was designed by my engineer. While this building was 20 feet wide, the roof wasn't that tall - there was only 4 feet between the floor and the joint in the roof. Of course this is all you may need, because I was able to stand up in the middle.

[Dennis Putnam]

Probably true as I am just learning this stuff. You can tell me where I am going wrong. In my mind a "gambrel truss" (or "truss" in general) refers to the assembly that sits on the load bearing walls. That assembly is made up of a joist (spanning the 20' width), the roof rafters and the supports. My focus so far has really been on the joists because of the 20' span. I don't think the rafters and supports are that big a deal, 2X4 construction should be sufficient.

[Brian Effinger]

2x4 walls are ok, and I'd use 2x10's for the floor, preferably doug fir. By code, they can span that if it is just attic storage (30 pounds per sq. ft.). Technically, what I had wasn't a truss, even though all of the members are connected together in the same plane. A truss is a pre-made assembly that acts as a single member, and would include the rafters, floor joists and any web members, like this:
http://www.dutchcrafttruss.com/images/trusspics/gambrel.GIF

I just went back and re-read your first post, and saw that you can't get long lengths of 2x material. In that case, yes, you would need a truss design, but it is something that should be engineered by a licensed professional or a truss manufacturer. Trusses are not easy to design. There are a lot of different forces at work in them, and when the design gets funky, like a gambrel, it gets complicated very fast.

[Dennis Putnam]

That doesn't look like gambrel trusses to me.

[Greg Cuetara]

As a Professional Engineer I will second John and Brian. GET pre-made trusses and or get them engineered. IMO by the time you buy 2x12 material 20' long etc and all the rest of the pieces you might be close or cheaper to just buy trusses. I know around here I can go to the local lumber yard or even Home Deposit and they can order any style truss for me.

Before you say that it is too expensive you might want to price out the options. You might also be able to get the long span TJI's or another manufactured material.

Please seek the counsel of an Engineer who does this all the time and / or talk to the contractors desk at the lumber yard. Lumber yards typically have engineers they counsel for unique projects. Also, if you do not understand the inputs for the online calculators they can be dangerous and get you into trouble. It might seem like a monkey can engineer a truss or a beam from an online calculator but there are a lot of variables which can make a big impact on the design of the pieces.

Greg

[Glen Butler]

Now I am confused at what exactly a gambrel truss is. The truss above looks like a dormer truss with a duel pitch. Is that not a gambrel? Gambrel refers to a duel pitch roof there are two ways to achieve that: the design above and the one below (which I just pulled from the www). I forgot you were doing a gambrel and TJI's only work with a raftered roof. To answer your question current pricing in my area puts them at 1.27 per foot.

You are correct that dormer floor loading does not come into play. Yes weight is transferred to the walls but the roof members can still sag. Though this is a consideration to an engineer, it also depends how steep your roof is. Anecdote: A few years ago I remodeled an early 20th century home that was raftered 20 ft. with mere 2x4's on a 12/12 pitch. The steepness of the roof is likely the reseason it hadn't caved in after 90 years. If you had a truss design like the one above most likely the top cords would be 2x6 and the bottom cords would be 2x10.

Because engineered trusses are built under compression the members can be much smaller than a raftered roof. I have seen trusses with webs spaced such that a 2x4 is spanning twelve feet. The tension in the truss keeps that 2x4 exceptionally stiff for the length it is spanning.

I also second the motion to get something engineered for your situation.

[Larry Edgerton]

Having 16 years framing experience, I can tell you that 20' is really pushing it for a 2x12, though tables may say it can span properly. It won't fail but it will sag. But I do live in Utah where we get snow loads, and you said you don't have to deal with snow. In my area 20' spans are called out using an 11 7/8 - TJI 360 series, again this factors in a 40 lbs/ft snow load. I say this just to make sure you have thought everything through.

If you do want 2x12 they are available in a 20' length. For cost you are better off buying TJI 210's. Additionally if you have to sister the 2x12's it will require enough extra material that you might as well use TJI.

I'll second the TJI's. I was a holdout on engineered floor systems, until the quality of 2x material became too hard to work with.

If it was mine I would build a deck out of TJI's and frame in the gambrel roof out of conventional 2X material, stick built. Its not that hard and with care will be a lot more accurate. Trusses, especially Gambrel/attic trusses never seem to be very precise. With 360's on a 21' span you can build your kneewall in 2'6" from the outside wall to carry your upper roof, use smaller trusses on the upper section if you wish, or frame conventionally, and fill in your 36/12 lower roof sections with 2X material.

Do some studying on what makes a barn roof look proportionally correct. Too often the upper roof is all too flat, often dictated by truss companys having to drive down the road with them on the back of the truck. I like to see 36/12 sides with a 7-8/12 upper section. Anything flatter just looks like a compromise.

Drive around and study the barns you like the look of and copy their proportion. Or go to the library, there are lots of books on barns in most librarys.