Deck Building

I want to build a 12 x 20 freestanding deck. I noticed that some plans double the 2x lumber on the outside ends attached to the posts that hold the joists. Some plans do not. If I use one 2x10 lumber on the ends to support the joists which will be 2x8 on a 16 in. center, will it be stout enough? I plan on using 4x4 posts buried 2ft. in concrete. What should that spacing be also if the single 2x10 is sufficient. I really would appreciate any help or suggestions.
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Depends on many factors mostly the span and spacing. Try out these sites:
http://www.awc.org/calculators/span/reversecalc/reversecalc.asp http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
using 2x10s on the ends and running 2x8s perpindicular 16 OC sounds to me like a good plan for a trampoline. For a sturdy freestanding deck you need beams resting on posts (not 4x4s) or concrete. The joists should then ideally be on top of the beams. My friend built a deck using 2x8s 16 OC spanning 9' and he has a slight bounce.
2ft. also doesn't sound like its deep enough unless frost isn't a worry. Normally the posts should be on top of a footer (12"x12" concrete). Most towns in the northeast now want the posts attached to the concrete with brackets and the lunber out of the concrete. Check your local codes! The man can make you take it down.
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The frost line where I live is only 12 inches. The code around here says it is okay to bury posts 2 ft. deep with poured concrete. That I do know. Maybe I'm not clear on the beams. I was planning to attach them to the 4x4 posts with lag or carriage bolts. I also plan to let the 4x4s run continuous 3 ft. above the beam to attach railings. the joists will be attached to the 2x10 beam with metal joist hangers on 16 in. centers. Any experienced opinions on this would be appreciated.
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I don't think this is the right group for this thread, but you need to find the table similar to the one on this page labeled 'beams' (about halfway down page) for your town. They usually have them at the municipal bldg. http://www.hopkinsmn.com/inspections/building/decks / If you are going to span 12' (size of joist irrelevant) you need two 2x12s at a minimum for beams. In addition if the beams are only two 2x12s the posts need to be every 4 feet. So... to go 20' with your beam you would need 6 posts for each beam or 12 posts. Its that whole pythagorean, traingulation thing; the longer the span the more force on the beam. If you lag the beam to your posts all that will be holding up the entire deck and everything on it are 12 bolts (doesn't sound too safe). Much better to notch a 6x6 so the beam is being supported by solid lumber. You could still notch the 6x6 on either side and through bolt 4x4s between the double beam to go up through the deck for a railing. The table on this site for Joists states 2x8s can span 12' 10". If you span 12' with 2x8s it WILL bounce even with 5/4 PT deck boards. If you are having a railing I'm assuming this deck will be up in the air. If true then you get into a whole new triangulation plane with x bracing required from the posts to beams. That aside, if you are up somewhat then you could set the 2x8s on top of the two beams cantilevered 2' on each end. That would cut your span down to 8' which MAY be acceptable for 2x8s. One more thing, if your not used to swinging a hammer all day, rent a palm nailer. There are a lot of spots to put nails in a joist hanger and you need to fill every hole.
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Well, actually its more about how the deflection on a beam supported at its endpoints goes up with the cube of the span between support points.
John D'Errico
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