Post and beam sizes for deck roof

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Old 08-02-15, 03:25 PM
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Post and beam sizes for deck roof

Hello, I'm planning to build a roof over my deck, and I need to know what size the posts and beams should be. I live in Olympia,WA, so I have to consider snow (25 psf), wind (85 mph), and earthquake (?) loads. At 16 feet wide, my house is a long, narrow rectangle on a slab, with a single peak that's 12 feet high. The roof rises 4 feet over it's 8 foot run (there are no eaves). I built the deck to match the level of the slab, and the 16 foot width of the house, it extends 12 feet out from the house, but I may extend it another 4' to 16 feet. I have been covering it with a tarp that mimics the roof line, while collecting wood to make a permanent roof that will also mimic the house's roof. I want it to have as few posts as possible, to give it a more open feel, preferably just 2, one at each corner with a beam/truss between them to hold up the ridge beam. I'm planning to use 2x6 rafters, on 24 inc centers (is that sufficient?), and 1/2 inch OSB to cover it. I don't have roofing material yet, but it will be either asphalt shingles, or metal. I plan on creating eaves of approximately 18 inches. I'm planning on gluing/screwing together the various 2x4's, 2x6's, 2x8's, and 2x10's I have to make the beams and/or posts. Specific questions: What sizes do the 16' ridge beam, 16' cross beam/truss, the 2-16' lower side beams, and the 2 corner posts (9-10 feet tall), need to be? Also, it seems best to attach it to the house, for stability, and to create a continuous roof. In that case, should I attach posts against the exterior wall to increase the strength of the wall, or will the existing framing suffice? That wall is 16' wide, with a 12' peak, has one 5' sliding glass door (with a 4x6" header), and 2x4 framing on 16" centers with T1-11 siding and no additional sheathing. I'm just a layman, so the beam spec websites I've scoped get into engineering lingo that I can't follow pretty quickly, so any help would be greatly appreciated. Thanks, Bob
 
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Old 08-02-15, 03:41 PM
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It sure would help to have pictures so we could see what you see.http://www.doityourself.com/forum/el...your-post.html
 
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Old 08-02-15, 04:15 PM
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Your local building department will require a permit for the roof, so you would do well to contact them for names of qualified engineers who can design the deck's roof members for you. Stamped engineered plans and calculations are usually required to be submitted with the permit application. And designs taken from an internet website aren't likely to be accepted.
 
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Old 08-02-15, 06:34 PM
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Yeah, this is a tall order. I would advise hiring an architect and not rely on any tables even if they are understood. Think about this aspect:

You basically want to extend your house out 16 feet. This is normally done with exterior bearing walls and trusses that match existing roof pitch.

You want to extend it with no bearing walls or trusses. Everything will be bearing on beams and a few posts.

An architect doesn't necessarily cost a fortune for simple projects. We can definitely help with the framing and such once you have a set of plans.

An option is a standard patio cover roof which is much easier to construct.
 
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Old 08-02-15, 07:32 PM
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Thank you for the responses! Chandler, I tried to get good pictures, but the back yard is too small to get a good perspective. I'll draw up some diagrams.

Bridgema45, I will definitely consult an engineer or architect and get a permit. I'm just trying to get an idea of the amount and size of the lumber materials I will need for the project. This might seem like a backwards approach, but I need time to gather the materials and prepare them.

Handyone, Your description of extending my house is right on the nose! That's exactly what my intent is, because it's a small house at 736 sq ft.
I built the deck outside of the sliding glass door at the same level as the slab to give it the feeling of an extension of the house. When you say that I could build a patio cover, do you mean a single slope going away from the house, attached to a ledger? The deck goes out 12' now, and possibly 16' later, so starting at the 8' height of the gutters, I think the slope would make the roof too low to stand under. Having it open like a pavilion will allow access to the yard, more light, and a better view. Using just two corner posts is my preference, but I could add more posts, like a post for the ridge beam at 16' or even 12' but I want it as open as possible. My real first choice would be to build it with towering posts at the corners of the house anchored by guy wires, and suspend the whole roof with no posts at all.
 
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Old 08-02-15, 09:27 PM
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Here's a diagram. Please excuse the lack of accurate proportion, I just used the paint program, and I didn't want to spend hours making it perfect. My original post has more dimensions.

-BobName:  Deck roof diagram.jpg
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Old 08-02-15, 09:47 PM
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Actually that drawing is great!!

I'm planning on gluing/screwing together the various 2x4's, 2x6's, 2x8's, and 2x10's I have to make the beams and/or posts
The cover needs engineered. I wouldn't plan on using any home made beams or posts (built up).
You're probably looking at solid 6x6 posts minimum set on a proper footing and that's to be determined by the architect.
 
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Old 08-03-15, 11:06 AM
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Thanks Handyone! I'm on a pretty low budget right now, so hiring an architect isn't in the cards for a while. I designed and built a shed using lumber reclaimed from a fireworks stand and a deck. It's a lot more work, but I also like the idea of keeping materials out of the landfill. Buying big beams and posts isn't really an option either, so I was hoping to get some advice on sizing, so I would be sure to build a solid, safe structure. I've been looking for an affordable planer, so I can resurface the lumber I've salvaged (and a metal detector, so i don't ruin the knives on a hidden nail). Have a great day!

-Bob
 
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Old 08-03-15, 11:40 AM
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OK, I'll work on that as time allows. Others will have some ideas. There are ways to build it and it will stand.
 
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Old 08-03-15, 02:02 PM
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You should definitely check with your city/county building department. I don't know about WA, but in CA something on this scale would require 2-3 inspections before getting to the point where the first post could be erected, and the plans would probably require a signature of someone with a state-issued license (maybe a GC, put more likely either an architect or structural engineer), and the building code would specifically define the sizing requirements for beams/posts, may not allow for the use of OSB on covering something that's open on the underside; code might also require the use of engineered trusses or that any laminated beams must come from certain sources. Also, there may be very different requirements for something that will be attached to the primary building vs something that is technically detached.

Also, code will have specific requirements for the footer under the post bases, may require the posts to be set into the ground to provide shear strength for seismic issues; solid walls can contain triangulation by bracing and will have shear strength in the sheathing, where using just vertical posts eliminates most options which could provide bracing for lateral loading. This may result in you being required to either alter the width of the planned supports to have the posts wider than the existing deck, or would require you to tear up part of the deck to accommodate the support requirements for the roof.
 
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Old 08-03-15, 06:00 PM
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Thanks for your reply bmgreene! That's good information. I chose OSB because I have a bunch of it. I will look into the code requirements. In my research, I read that collar ties can sometimes be used to increase lateral stability, in the absence of full trusses. I would be willing to go that route, as it would maintain the open feel I'm seeking. I have thought about building it free standing, instead of attached, but it seems like that would just open up another can of worms regarding structural integrity. I've also thought about building a lighter structure with more posts along the perimeter, and roofing it with poly-carbonate. I decided I would rather make it solid, because the heat really builds up on the deck in the summer. Maybe I could use a light weight metal that would block the heat? Currently, I have a 2x4 ridge "beam" held up by a 2x4 "post" holding up a 16x20 foot tarp that's tucked in to the trim along the roof line. It's all held in place by ropes tied the the side fences to make it conform to the slope of the roof. I have to replace the tarp every two years, and it tends to leak. I just want to have a permanent cover that is safe, low maintenance, meets code, and I can build myself. From the feedback I'm getting here so far, it seems like my modified post and beam arrangement may be more than I can swing. Any alternative suggestions would be welcome!

-Bob
 
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Old 08-03-15, 06:53 PM
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A lot of it will depend on your local codes. If you're OK with the cover being non-rigid, and high winds aren't a significant concern in your area, you might want to look into options for something like pavilion tents or awnings with some amount of framework; commercial versions of that would be made of material more resilient to the elements than most tarps, and would be much more affordable than a permanent wooden structure big enough to cover that much area.

If one side is going to be against a fence, you might consider building a wall on that side to get better shear/seismic integrity. One wall shouldn't do too much to hurt the open feeling, especially if that side would be visually closed anyway. If it's too close, it could interfere with maintenance of the fence, but if it's conformal to the side of the house, it'd have the same setback from the property line which should allow decent space.

If you could afford a third large beam, putting posts under the ridge beam might eliminate the need for trusses, but the location of the exit from the house might not make that a practical option; also just adding one additional post on each edge might dramatically reduce the scale of beams you'd need at the outer edges, under the same loading, a beam with twice the length will have four times the peak bending at the midpoint, which doubles the height of the beam required to handle the load (assuming a rectangular cross section); so if spanning 8 feet would require a 4x6, then spanning sixteen feet would require at least a 4x12, you couldn't just double up two 4x6 side by side, it'd take four of them. Adding more posts might also reduce the size of posts needed, although you might also look into using galvanized pipe for the verticals rather than wood, those could have better strength for less width, can be found in greater lengths more easily and cheaper, and might be more durable when set into the ground with concrete piers.
 
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Old 08-05-15, 12:21 AM
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Thanks for the information! I've definitely thought about an awning type of cover. I like the galvanized post idea too. I need to have a conversation with an architect who know city of Olympia, and Thurston County building codes.
 
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Old 08-05-15, 11:34 AM
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Thank you all for the informative responses! It really looks like a modified post and beam isn't the way to go for me right now. I'm going to post a new thread, with fewer parameters, to get fresh ideas of how to cover my deck using the salvaged materials that I have, that is safe, and meets local building codes. Please feel free to post any ideas that you have here, or in the new thread!

Thanks,

Bob
 
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