Deck post spacing

Old 06-04-03, 08:27 PM
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Deck post spacing

Looking at putting a deck off the back of the house. Deck is going to be 16' x 18' (16' parallel with house). I thought of having (3) 4x4 post in the 16' direction and (3) posts in the 18' direction plus the ledger board.

For my girders, I was going to sandwich a 4x4 post in between 2x10's. From the girders I was going to hang 2x8's.

Will the post spacing be able to handle the load of the deck ( I saw 55 lbs per sq ft) was this in the ball park????

The framing mentioned above, is that overkill of on the right track??? Look forward to someone with more design experience giving some advise!!!!!! Thanks
Old 06-04-03, 10:12 PM
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From the ground up, a 16 X 18 deck (16' across the back of the house, and an 18' projection).

First, I would make it free standing (no ledger board attached to the house), UNLESS there is some compelling reason to do otherwise. My logic is that attaching a ledger board to the house is punching holes in the wall, which is just a place for water to get into the wall and do damage. No holes, no potential for damage. Come out from the house about 2' and run your first girder parallel to the back of the house. How far apart you space the posts depends solely on what you are using for a girder. I use 4X6's for my girders, and space the posts at 5'. If you use a double 2X for the girder, reduce the post spacing (dbl 2X6's can span 4', dbl 2X8's can span 6', dbl 2X10's can span 8'). Rest the girder on top of the post, rather than sandwich the post between the 2X's -- it's stronger.

Next, how far your joists can span (distance between the girders) is determined by what you are using for joists, and how far you are spacing them apart (THAT is determined by what you are using for decking, and whether you are running it perpendicular (or square) with the joists, or diagonally to the joists. I use 2X6's for my joists, no more than 20" O.C., and space my girders no more than 5' apart. This is for 2X6 decking. If I'm using a 5/4 decking, joists are set at 16" O.C. Both of these are if the decking is being run parallel to the back of the house, or square to the joists. If the decking is going to run diagonally, reduce the joist spacing by at least 4".

Based on that, you need 4 girders with 2X6 joists. (2', 7', 12' and 17' from the house). You need 3 posts under each girder if you use dbl 2X10's or 4 posts under cbl 2X8's. Using anything larger than a 2X6 for joists usually means that you will spend more for the joists than you will save by eliminating a girder, so why do it. As long as the joists and girders are not overspanned, a larger joist is not going to make the deck any stronger.

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