determining size of rafters/joost for 30 psf snow load for gambrel roof?

[omare]

I am trying to figure out, before I get a contractor involved, whether the specifications for a modular building would meet the snow load requirements or would if “upgraded”. It is only 14 feet x 20 feet. I know I have to talk to a real builder/contractor on this–but wanted to get an idea. Just thought DIY might guide me as the roof is used a lot on horse farms

Because of the form of the structure (gambrel roof) I am not understanding what part of the structure is used to calculate the snow loads assuming you have a loft floor to counter the outward thrust (?). The loft floor joosts are 2 x 6 but the framing/rafters are just 2x4 I think every 16 inches, which looks like a non-go to me? The first floor was to be framed with w 2 x 6s to allow for more insulation so I am guessing could carry up upgrade to 2x6 rafters? (

Thanks

[gumtree]

I have a pocket reference book that gives these details and just about everything else mankind has come up with. I call it my “doomsday” must have to put the world back together. When I have time I will look it up. Should be able to find the information on the net.

This size of rafter aka trusses can be had ‘off the shelf’ pre-made in most areas. They are priced very reasonably around here. I would think it would be a lot cheaper to buy them pre-made then the cost of ‘home-made’.

You can rent a small crane to set them. They are usually delivered by a truck that has a crane on it. For a few extra dollars they may lift them into place off the truck. Depends on the type of crane on the truck and site access around the building.

[Bluey]

When we built a large overhang to our Quonset barn, we ran the drawings we made thru one of our engineering firms that checks that out.

They were fine, but if not, they would have corrected our dimensions on the columns and span.
We them overbuilt a bit on that anyway and added braces to the roof also.

They charge by the hour and it didn’t take them long, the cost of checking our plans was $90.-, very reasonable for what we were building.

If you can’t find the information online, try giving an engineering firm a call and see what they tell you.
You can find those by asking the court house who they get inspected plans from.

You can also order trusses ready made and those come already certified by the engineer the one making them uses.

We didn’t want trusses, but standard framing, because being an open barn, birds are much worse with trusses to roost on.
That is not a problem in a closed building.

[clanter]

[QUOTE=gumtree;8519202]
I have a pocket reference book that gives these details and just about everything else mankind has come up with. .[/QUOTE]

I have one somewhere also, mine is for wooden beams and will denote grades also

but good ole internet has one:

http://www.awc.org/codes-standards/calculators-software/spancalc

30 pound load may not be enough really depends on just where you are:

The water content of snow may range from 3% for very dry snow to 33% for a wet, heavy snow, to nearly 100% for ice. An inch of water depth weighs 5.2 lbs. per square foot. Thus, a roof designed to carry a snow load of 20 lbs. per horizontal square foot is expected to support nearly 12 inches of wet, heavy snow.

http://www.trumbull-ct.gov/filestorage/7112/7114/snow_load_information.pdf

[ReSomething]

A contractor will take it to an engineer anyway. We had an open gambrel roof in a shed we built with a loft at our old house, and it was the gussets that were more important, because they hold the angle of the gambrel. We had to size the plate and put in the right number of fasteners as well as size the rafters.

If it’s a modular building they already have the snow rating, this stupid double wide has the snow rating on a sticker in the kitchen in the cabinet above the stove, “design roof load zone map” 30 psf X Middle, that’s us. We have a heavier roof than called for.
Your modular company knows how much the roof is rated for, you just need to twist their collective arms.

[shakeytails]

Joists.

The building’s manufacturer will have the snow and wind ratings for it’s buildings. Simply ask them. If you choose the DIY route and order trusses, you can specify the wind and load rating you want if different from the standard for the area.

ETA: Floor joists have to be a minimum of 2x6 to carry any load, most are 2x8 or 2x10 SYP in a larger structure. While I wouldn’t build anything larger than a shed with 2x4 framing, probably the majority of houses, at least around here, are 2x4 framed and plenty of them are two stories.

[hosspuller]

If you are going to store hay in the upper area, make sure the designer/builder knows. Ordinary floor load may not stand hay bales packed to the ceiling.

[omare]

Thanks for all the suggestions and links (and if the Amish builder does have the specs I will definitely touch base with an engineer–I do not realize they were “affordable” for little projects).

[Bluey]

[QUOTE=omare;8521489]
Thanks for all the suggestions and links (and if the Amish builder does have the specs I will definitely touch base with an engineer–I do not realize they were “affordable” for little projects).[/QUOTE]

Our Quonset barn was a rounded barn and we were adding to it a 32’ clear span extension/overhang onto the 80’ round side, with framing to add later another 28’ more, which we did.

The whole now is 80’ x 60’ long, all one long sloping roof, self standing and flashed into the Quonset roof itself.

We had already the plans made with the pylons and size of columns we thought we needed, the right jacks to brace the frame to the purlins, etc.

All the engineering had to do is to check that our figures were correct and that we didn’t make some silly mistake somewhere.

It probably didn’t take but maybe one hour at most.
They charge by the hour, so if all you want is some existing plans checked, ask up front first what it will cost, so there are no surprises, but it should not be much.

We really depend on them just to be sure, in case we ever have any blow away and the insurance questions who and how it was built.