Shed Roof support on a raised Deck

[MJD]

I am constructing a 18' 6" (wide) x 14' (deep) deck. The deck is roughly 32" above grade. The deck posts are 6x6's on 15" deep x 18" dia concrete footers, that are also encased with an additional 6-8" of concrete.

The deck beams are doubled up 2x12's with 2x12 joists.

I am planing to put a shed roof over the deck. The placement of the load bearing columns are not on perfectly top of the deck posts AND beams. Though they are within 12" of the posts and beam.

I am bracing up the joists areas where the columns are to be positioned with 3, 2x12 joists spaced with 2" blocks between them.

Is this adequate for the load support?

Thanks, MJD

[Richard A Hetzel]

We don't know what region you're in, so we don't know the design live load that's required for the roof, and therefore we don't know the loads on the columns, nor do we know how many of them there are. We also, therefore, don't know whether the structure below is capable of supporting the roof. So, it's impossible to answer your question, but the seat of my pants says the columns should be directly above the columns below, and then the only problem is whether the footing size is adequate for all the new and existing loads coming down on the columns, which I doubt.

[MJD]

I'm in Jefferson County, WV. The columns are TurnCraft 8" fiberglass composite material with a 10,000lb load. http://www.turncraft.com/pdf/cat_poly_classic.pdf According to the Trex Designer package I used, it is spec at 90 PSF per live load. (see attached Trex Specs)

Attached is a jpg depicting the posts, beams, columns and relevant joists.

The shed roof will be constructed of 2x10's, 16" oc.

Also, isn't the weight load of a shed roof only on the board fastened to the house and on the front beams? If so, then concern of columns on the side are not much of a concern?

Thanks for the feedback.

[Richard A Hetzel]

Well, you didn't say that the new columns are landing at the end of a cantilever...see why I tell people that it's dangerous to try to perform structural design in a forum? I suggest that you have the roof sturcture deisned by a qualified professional. I'm not going to attempt it here. It seems to be pretty heavily overdesigned, so it might work. No way to tell without doing the calculations. which should be done by someone local who can visit the premises and discover all factors that may be relevant in the design, from the new roof, though the existing deck, right down to the footings (which I notice grew 2 inches in diameter over the last few hours).

[Don_P]

Let's get you as far as we can,
Half the roof is bearing on the house, half is on the porch carry beam above your fiberglass posts. That is 7', but you also need to add the roof overhang, I'm going to assume 1' there so 8'. The posts are carrying the load from midway between posts on each side, you've drawn that as a 5' span but earlier it was 18'6" total length of deck, I'll assume its a 6' span. If that was correct then the tributary load area bearing on the most heavily loaded posts is 48 square feet. Your county lists a 35 psf snow load. Typical framing and roofing is usually given 10 psf dead load additional so 45 psf X 48 sf=2160 pounds coming down the post. We have not sized the carrying beam.

Now check the deck's load on the front girder. Trib area looks like 7'x9' x 50 psf=3150 lbs

So the front girder is carrying a uniformly distributed load of 3150 lbs along its length AND a point load of 2160 pounds at some point in its span. Checking it really crudely by calling it all a point load in midspan it failed. The cantilevers are really irrelevent, they are offset by less than joist depth. The front girder is the weak link. Redesign it with center posts mimicking your middle beam and it'll be alot better. If the deck is already built put in 2 more posts under your built up joists.

A rough check on the footings sounds like the soil needs to be capable of supporting better than 2500 psf. My understanding of the soils up there is they are all over the road strengthwise.

And as Richard said this is an engineer's job, this was just a run through of the logic.