Reinforcing a flat garage roof

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Old 10-09-13, 12:59 PM
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Reinforcing a flat garage roof

OK, time to pick some brains.

I've got a flat garage roof, roughly 24x24 feet. It's mostly made of ceiling joists running the same direction but not completely so. The thing has obviously been expanded at least once and not very expertly. It's got two mid support beams, neither reaching across full span. The one is just two 2x6's nailed together and nailed to a semi-central post. It doesn't even have a jack stud so it's only as good as the sheer strength of the nails.

It sags a bit. Water pools on it. Three years ago we had a new torchdown layer put on so at least it's water tight. However, being as I live in Denver, I'm worried that one day my roof will collapse under snow weight.

I'd like to add a couple beams at, roughly, the 8' and 16' points with some jack posts on either side and near the middle (actually, somewhere between 30 and 40% across the span.) Then I could tap in some cedar shakes as shims where necessary and, at least, feel better about the snow.

Is there a good way to know just how much beam I need? Tripled 2x10's? 2x8's? Do I need something from the engineered beam list?

I could just go with 2x12's, I supposed, the largest I can get from my local big box lumberyard but they're heavy and I'd rather not use them unless I have to.

FWIW, code for snow load in Denver is 30".

Any thoughts?

Thanks in advance.
-M
 
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Old 10-09-13, 02:48 PM
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There is really no way to tell what is needed without seeing what you have there. When you mention joists not running completely and support beams not going all the way it gives me the chills. For some general guidance you can search online for span tables. If you plan to install columns you also need to have a footer to support the bottom.
 
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Old 10-10-13, 09:46 AM
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Thanks for the reply. Here's a quick diagram I put together. Measurements are approximate. My minimal plan is to pull the one cross beam marked and put up a new one. I'm not sure what I need there, though. Anything would be better than what I've got but I'd like to put in whatever is most correct.

 
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Old 10-10-13, 11:37 AM
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You might want to look into LVL beams. They are available in longer lengths than standard lumber so you could span the entire length or width with one member.
 
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Old 10-10-13, 11:58 AM
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Yeah, there's where I started. I called up the local beam supplier and said I have a span of such and so and they said I had to find an architectural engineer to do the planning for me.

Way more than I wanted to do with this project.
 
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Old 10-10-13, 02:51 PM
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One of the problems with a flat roof supported by wood is that it creeps or sags over time without any extra load beyond the dead load. Adding rain (which can pool in the smaller depression) or snow adds to the more permanent deflection, causing more load.

Dick
 
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Old 10-13-13, 12:40 PM
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Would cross bracing (prevent joists from twisting) add the desired strength to the roof?

sheer strength of the nails - Ok so if I remember basic structural mechanics from way long ago you have 2 problems. The bending stress in the center of the beam and sheer stress at the ends where the beams are supported by the wall with nails. So if you are worried about the sheer on the nails then I think you need to support the roof better at the walls.
 
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Old 10-14-13, 09:40 PM
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If it was mine, I'd seriously consider using a salvage steel beam. Much stronger than wood (assuming proper end supports), and it wouldn't have to be nearly as deep as wood members if headroom is an issue. I used a 22' salvage WF beam (cost me $80) for a carport I built in Montrose, where snow loading is almost non-existent. It was just a bit of a struggle getting it up onto the end columns by myself, as the thing weighed slightly more than 350 lb.
 
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