[jwnova99]

Hey Guys,

I'm building a 24 x 48 passive greenhouse. I need to span the 24' with the back wall being 9' and the front wall 4' high. The walls are poured 10" and 8" concrete walls respectfully. I'm having trouble finding treated trusses in the style I need. My south roof needs to be 12/12 pitch and my north roof needs to be 8.5/12.

I'm trying to design the stick built roof trusses like this but on a larger scale: http://aes.missouri.edu/bradford/education/solar-greenhouse/300/figure3.jpg with a ridge beam along the peak, trusses coming down each side, with whatever bracing I need between the two.

I welcome everyone's critique of my design, what size boards to use, if the design would hold, if I need supportive posts either in the center or where the horizontal meets the long 12/12 truss board... etc.

Thanks in advance.

 

[joecaption]

Why would you want treated trusses?
That drawing shows no over hangs, do you want water running down the wall?
Bring that drawing with all the dementions on the wall and any truss company can engineerer them for you so no one has to guess if they will work or not.

 

[jwnova99]

Thank you for your questions.

The roof itself will over hang a couple inches with a rain gutter tied directly to the header plate.

I want to use treated so they don't rot in the high humid environment of a greenhouse.

Out of 4 engineer truss companies only one does treated. However their design has a board running from the short wall 4 feet into the horizontal piece in my drawling. So it would be hard for anyone 6' tall to approach the 4 foot wall without banging their head.

I've seen what I want done on 12' building (see link). It looks like they used 2x6s and no internal bracing, I was hoping there was a way to engineer it for 24'

Thank you.

 

[joecaption]

I've been in a ton of green houses and never once have seen pressure treated use in the trusses.

 

[GBrackins]

with a ridge beam along the peak

Welcome to the Forum!

if you have a structural ridge beam then why are you building trusses? I'd install the ridge beam with rafters. I'd then install a rafter tie (tension tie) that would connect the front rafter to the rear rafter (like the bottom chord of your truss).

Install a collar tie under the ridge beam and connect to sides of rafters. This will prevent uplift from pulling out the rafters at the top. Use hurricane tie based upon uplift calculations at rafter to wall connection and toe-nails (again rafter to wall connection) for lateral and shear resistance.

Wouldn't need the other web members, or truss plates or a crane in setting them into place. Of course a 48' long structural ridge beam supporting 12' of tributary roof load would be a large beam.

Could frame with preservative treated wood.

Good luck! :thumbsup:

 

[Daniel Holzman]

The photo you attached does not appear to be a truss system, it appears to be rafters with a ridge beam. The system you drew is close to being a truss, but there are two features that mean it is not a true truss. The extended support leading down to the four foot point on the wall is not part of a triangle. By definition of a truss, all elements must be part of a triangle, else the structure is a frame, not a truss. Also, the diamond at the center is missing a horizontal cross member, which would change the frame to a truss.

That said, as Mr. Brackins has pointed out, it is not necessary to build using a truss, although it is certainly possible to do so. Truss design is generally done by specialized computer software, which any truss manufacturer would have. The truss manufacturers would be delighted to quote you a price to build anything you want, they would size the members. Unless you have pretty strong structural credentials, you would be way over your head sizing truss members, and I believe you would be foolish to rely on advice on something as critical as truss framing sizes from an internet chat forum.

If you design using a ridge beam and rafters, it may be possible for an experienced carpenter or contractor to size most of the elements, perhaps the ridge beam would need to be sized by an engineer or architect. Certainly it should be possible to find a contractor willing to put up a standard framed greenhouse, or you could do it yourself if you have the skill and the time.

 

[jwnova99]

Thank you everyone for the information. Yes I was misusing the word Truss. A ridge beam and rafters coming down the wall and a collar tie would be what I'm looking to do. I was hope to get a contractor out there can tell me the size these members need to be. I'm thinking 2x12 ridge, 2x10 rafter, and 2x6 collar tie? I have contractors locally that almost insist we go with engineer trusses, but I know there has to be contractors out there who have done this stick built. If you search Stick Built vs Trusses you'll find contractors that sit on both sides of the fence.

Thank you again.

 

[GBrackins]

I'm thinking 2x12 ridge

not even close unless you are installing multiple multiple columns under the ridge beam. You'd be looking at a large steel beam to clear span 48' for a structural ridge beam. Typically a single 2x12 is a ridge board not a ridge beam (unless the span is quite small), its purpose is to provide a surface for attaching rafters not carrying roof loads. For example depending on your required loads (snow, wind, etc.) I'd expect a (4)2x12 solid sawn lumber would span 12' based upon the 12' tributary loading of your roof.

if you could have several columns under it (in the middle of the green house) then you would have a lot smaller beam. Beam length and tributary loading (12' of roof load) together with wood species (solid sawn or engineered lumber) determines the size of the beam. The longer the span the larger the beam.

Engineer would be well worth the money!

 

[jwnova99]

GBrackins,

I'm glad you are setting me straight about the ridge beam.

I'm having issues with an engineering truss. I went to a lumber yard, gave them my building specs and told them what I needed. I told them what I was using it for and the scope of my project. What I got in return a few days later is a truss diagram and quote which in the fine print said that I was responsible for the engineering load and that it was going to take structure roof material on both roof pitches as well as on the ceiling to achieve the loads they quoted. I was planning to leave my trusses exposed on the inside, and one roof side is plastic not structural material. There seems to be a disconnect. When I questioned the lumber company, they told me that the fine print means that the trusses can handle that much material. But in my lay mans term, Bracing makes it sound like the bracing necessary to provide the structural load.

Looking at the design they have, its pieced together with much less wood then I'm talking about using here.

When you say engineering, is that working directly with a truss company? License Contractor? or are you talking about hiring an Engineering Firm?

Thank you,
Jason

 

[GBrackins]

Jason,

Glad I could help, now to your question .....

I would hire a local structural engineer. They would be familiar with your local codes and can design any structural elements needed, including trusses or beams. They work for you and look to protect you. They will prepare construction drawings that your builder will use not only in construction, but in providing you a contract price. For the money you are going to spend in building this it will be money well spent!

Yes trusses do require bracing. Bracing consists of wooden members that connect multiple trusses together so that they act as a unit (more or less). Gable end walls also need bracing to prevent wind loads from pushing them in and acting on other members. A structural engineer should be able to address all of these items.

Some lumber yards (truss manufacturers) are better than others, I work with an outstanding group locally. They have designed trusses similar to what you're looking for for a Cape Cod style home that the builder wanted to do with trusses and with a shed dormer on the rear. They designed these with the long member coming down on the short wall with structural composite lumber (engineered wood) and the remaining members being solid sawn lumber.

Your lumber yard may be apprehensive in using preservative treated lumber (don't know why) as it has typically only 85% of the strength of non-treated wood, or it could be the fact no one there has done it before. I can't say for sure.

If you do not mind having a column about every 12' or so then it is easily done with a ridge beam and stick framing. Remember the shorter the span between supports the smaller the beam becomes. You could use a preservative treated parallam beam (treated engineered wood) and increase the distance between columns. Could always build tables/stands/benches around the columns. Just some random thoughts .....

Your engineer should be able to put everything together for you for a successful project.

Hope this helps! Good luck! :thumbsup:

 

[jwnova99]

GBrackins,

I found an engineer to scope out the roof design. Better safe then sorry. While he is at it, I'm having him re scope my concrete walls.

thank you for your help.

 

[GBrackins]

sounds like you have a plan now .....

glad to be of assistance!