Truss design

[Bob Cooper]

I would like to build a simple carport/car cover. The roof will be metal. The look I'm after is what you see in the rest areas along the interstate in the South. It's kind of a scissor truss with metal brackets. I want to span 24' to allow for two cars.

This is needs to look beefy. So I'm thinking the posts will likely be 8x8 or 6x8.

Im wondering if there is anything I can reference for the scissor truss design. I'm thinking of using pressure treated pine and staining it. Overall the footprint is 24' wide by 20' deep so I'm thinking three trusses sitting on 6 posts.

[scott vroom]

Bob, building trusses on the scale you're talking about involves fairly advanced structural engineering expertise to ensure the trusses will handle the various external and internal loading forces. Have you considered having the trusses built? If you supply a truss builder with your carport design specs, their engineers will design and build a code-compliant truss that will safely handle the loading. If you prefer building yourself, then at a minimum I'd pay to have the design done by a qualified structural engineer experienced in truss design. 24' is a long span, and those trusses are going to be very heavy. Do you have access to crane or hoist equipment to place them? Will you be installing the purlins and building the load bearing wall structure yourself?

[Brian W Smith]

Bob,here we'd be pretty hardpressed to buy the materials for what our truss co's can supply the finished unit for.At least on the more simple straightfowrd designs.When they start to become "super trusses"(marketing term that used to apply to trusses that had 2x10/12 bttm cords,for upstairs living qrts).or very complicated designs then we could start to compete with their pricing.Which,explains why you don't see more sophisticated truss designs,at the home builder level.Further,even going lite commercial,we see simple designs that get stacked(piggy backs),or turned at right angles and sometimes even worse,considering traditional framing methods.

Not trying to dissuade you.We built a set of rather complicated design 10 or so years ago.It was for a long wing for a big barn.Long story short,we gave the guy a lifetime warranty on them.Cpl years later his farm got hit with a Tornado that busted things up pretty well.Our trusses survived,purlins and metal being completely ripped off/destroyed.Along with 6x6's being "uprooted".So,you can make a case for DIY,but it usually means higher rated parts with maybe a little better design criteria.....but thats gonna be tuff.Good luck.

Oh,and to anyone with scizzor trusses........be extremely careful with how you store them.Do NOT store them in a "bowed" condition.They need to stored and erected in a flat,single plane.Theres been a metric ton of accidents over the years installing warped or bent scizzor trusses.Gets into some pretty heavy physics explaining the reason.

[Michelle Rich]

I too vote for buying those trusses..they really are not expensive for what they are. I built my house with a hammer & a handsaw, but when it came to the roof, I trusted the engineers.

[julian abram]

I agree with what has been said. Depending on your location, you can have factory engineered trusses delivered to your site for little more than the cost of the materials you will purchase to build your own. You will definitely need some type of boom truck or crane to stand trusses with the design you mentioned. With some help you might be able to stand them if you change your design to lighter trusses on 2 or 4 foot centers.

[Dan Hintz]

This sounds like a job needing a PE/engineer to design and validate...

[Jason Roehl]

This sounds like a job needing a PE/engineer to design and validate...

Easy to engineer something that will work and last (way over-built). Where the professional comes in is to keep the cost and safety factor down. That's truly how engineers are used--to design things within specifications, including cost.

There are millions of old structures still standing, built by "rules of thumb", with nary an engineer involved.

[roger wiegand]

If your inspector is like mine, he will want an engineer's stamp on any site-built truss design (no matter how over-built it might be). That will immediately eat up any possible savings vs having them made -- the manufacturer will give you the stamped drawing you need.

[Dan Hintz]

Easy to engineer something that will work and last (way over-built). Where the professional comes in is to keep the cost and safety factor down. That's truly how engineers are used--to design things within specifications, including cost.

There are millions of old structures still standing, built by "rules of thumb", with nary an engineer involved.

If your inspector is like mine, he will want an engineer's stamp on any site-built truss design (no matter how over-built it might be). That will immediately eat up any possible savings vs having them made -- the manufacturer will give you the stamped drawing you need.

This is where I was heading. Most here could overbuild the snot out of something... but when it comes to safety and such, I would expect the local authority will want a PE stamp on anything not "pre-cast", so to speak.

[Phil Thien]

Menards sells 24' 4/12 trusses for $47 and change:

http://www.menards.com/main/building...791-c-5658.htm

I think they use about $35 in materials.

I don't think building them would be cost-effective, is what I'm getting at. You can do it following published guidelines, I just don't think it makes sense.

And at least around here so few do it, that tossing up DIY trusses may cause an inspector's head to explode.

[Peter Quinn]

I see the address.....SC....I'm thinking hurricanes.....you need an engineered truss to meet your local codes. It not only has to not fall down, it has to not lift up.

[Jason Roehl]

This is where I was heading. Most here could overbuild the snot out of something... but when it comes to safety and such, I would expect the local authority will want a PE stamp on anything not "pre-cast", so to speak.

I wasn't really thinking along the lines of permits/inspection since the OP didn't mention it. Indiana has a "Log Cabin Law", which means if you DIY, no permit truly needed (though your insurance company may take exception and not insure you). And, the better inspectors (I know a retired one) will say that they are there to help you do things safely for yourself.

[Andrew Hughes]

I used to work at a lumber yard that built trusses,I ran metra cut centerline saw oh boy those were the days.Loved it We were put out of business by a machine with computer.Mid 80s
Even if one could figure out all the webs and cord angles,You would fall short the nail plates are sized for the loads and need to be pressed in with a big roller table or hand held press.

[Kent A Bathurst]

You would fall short the nail plates are sized for the loads and need to be pressed in with a big roller table or hand held press.

The truss engrg software designs the plates to be the minimum required, given a specific plate orientation.

YOu can easily oversize them wildly, and have no concerns.

O the press - there is a third: a 6" x 6" x 1/2" steel plate on the bottom of a heavy bar, or tube. Wood on floor; plate on wood; pound away - lift the steel , and bring it crashing down.

Donbe for repairs in the truss plants, or on job sites.

Having said all that - - I would buy them. If they are straight-forward 24' garage trusses [the most common truss there is] you stand fair odds of the lumber yard's truss supplier keeping them n stock, even. They are too cheap to try to compete with.

[Brian W Smith]

Major tangent alert..........got a problem with it?Please stop reading now.


30 years ago or so,I had a guy who worked for us that was the grandson of the inventor of the nail-on....or bang-on T-hangor.IOW's that plate,and it's attendant co. that made what we consider the connection on trusses.He was really cool.......very nice to talk to.