Hi everybody,
I have a 24x18 structure in northern New Mexico and I need to design the roof. What is minimum slope for mountain homes? What is the snow load? Do I need to use trusses; how do I design the trusses? Once the trusses are lifted to the top of the structure how are the attached? How are they attached to each other? Is the ridge beam segmented? Its obvious I'm at a loss here and need advice. A thanks to all.
Zim
It's unlikely that anyone here will know the snow load for your specific location. There are maps available on-line that should get you close, although there can be huge variations in the mountains. Once you know that, you can either calculate conventional prescriptive construction using span tables or calculators (again, available on-line), or you go to a truss manufacturer. They will design and engineer your trusses as part of the cost of making them. The programs they have will calculate everything in a matter of seconds. When I ordered my trusses, they designed and quoted them as we talked on the phone. If you want to do something outside of either prescriptive design, or having trusses built, you will have to hire an engineer.
Trusses are attached to the top plate either with several toe-nails, or using metal connectors (hurricane clips). You need to find out what your local jurisdiction requires, as it varies depending upon designed wind and seismic loads. The trusses are temporarily tied together with bracing. Once the roof sheathing is fastened, it pretty much locks everything together, and the temporary bracing is removed. If you buy trusses, the drawings will show what kind of permanent bracing required, if any. In many cases, you do not need a ridge beam. It depends upon whether you want a cathedral ceiling or not. Most trusses do not use a ridge beam. If the ridge beam is required, and it is segmented, you would have to have vertical support where each section begins/ends.
You said you 'have' a structure- I'm a little concerned that you have apparently started your project without basic knowledge of construction. If you haven't started yet (or even if you have) I recommend getting a comprehensive book or three to learn about house construction, and study up before you proceed. There are a lot of things you need to know (including the roof load) before you even start the foundation. The strongest roof in the world won't do you any good if the structure under it can't hold it up. This is a great site, and you will get a lot of advice and support here!
Much of northern NM is a CS, case study, area. Snow and terrain varies too much to make accurate maps. Call the local building official office, town or county. Or try the state CID office. Those are your best bets. If you are in the boonies though, don't be surprised, if they aren't certain. We were told "maybe 50 psf". We built for 75 psf ground snow just to be sure
We did 5/12 pitch because I don't like working on steeper pitch. Steeper pitch sheds snow easier though. But then plan ahead for what the snow is going to slide off onto. Sliding snow tears gutters off too. So sometimes it is not desirable to have the snow slide off and people install devices to prevent the sliding.
http://publicecodes.cyberregs.com/icod/ibc/2012/icod_ibc_2012_16_par089.htm
scroll down to western map
Rafters vs factory built trusses. Either will work.
There is no ridge board with trusses.
Can a big truck access the property to deliver the trusses? Is there a truss source who will deliver there? Will you have helpers to raise them, or is there a crane truck that can come and lift them into place? Those are questions for which you need to find the local answers. The truss supplier will design the trusses for local conditions.
If there an upstairs? That affects truss choice? Also: Simple gable roof? Dormers? Attic usable for storage or living? Cathedral ceiling? More details about the structure, the walls, etc are needed so the roof designer can do a proper job. Simplest roof would be rafters with a flat ceiling below the roof, no rooms upstairs.