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/p...struction1.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.
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 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.