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guyod 02-20-2008 08:15 PM

Building Deck high off ground
The guy I subcontract for wants me to build a 20x15 on a sloping back yard. The back will be 15 feet high. I built decks before but they where under 4 feet high.

I feel confident on the deck part but I'm not sure about the pilliars. I would like to stick with wood if possible. Im thinking 6x6 posts every 6 feet to 8 feet bracketed to cement pilliars. Then x braces 2 high.

Is pilliars needed on the sides if my floor joists can span the whole distanceand does it have to be x braced? Is 2x4 strong enough for x bracing? what else can be done to stop wobbling? How high can make my cement pilliars if i cant get long enough 6x6's. Will 4x4's work? Is this project to big for a diy deck builder?

glennjanie 02-20-2008 09:32 PM

Hey Justin:
You are eactly on course. Pillars on the sides are not needed, you will need X Bracing, 2 X 4s are sufficient, a couple of cable ties from outer deck level to inner ground level will secure it, the height of the concrete pillars depends on how much is undergroung. You could consider Sono-tube forms, 2' into the ground and a maximum of 4' above ground. No 4 X 4s will not work and no, it is not too big for you. I would recommend 2 X 10 floor joists with a doubled 2 X 12 for the outer band. Hand rails are crucial at this height and make them uncomfortable to sit on. I know a man who turned his beer up and himself off the handrail on his head; now a quadraplegic.

triple D 02-20-2008 11:50 PM

You guys sound like you're on track. Just don't forget, with a deck this size you want lag bolts 5" with adequate washers a max.16" apart, usually 2, one on top of the other, between every joist. And joists should be 16" on center. If your bolting to siding, you will probably want to cut siding out 10"x length of deck. Then tuck new building paper in behind, and put flashing above and below to prevent water from getting behind siding. If your bolting to a belly board you've got it made. Just check wall inside for danger of power conduits or plumbing before sending lags in. You should run 3 posts, 6x6 and use pressure treated 4x10 for beam, maybe two twelve footers to leave a little over hang on ends or just two tens if you want it clean. Standard x brace screwed to underside of joists from house to beam should stabilize fine for wobble, plus your two x's between three posts. Maybe cut some plywood gussets to attach post and beam, one on each side, this is cheaper than steel brackets. But hey, that's all just one man's opinion, good luck....

guyod 02-21-2008 07:27 PM

Thanks for the replies.

I wanted to try out tube forms for this project. What diameter where you thinking of when you said 2' in and 4' out of the ground?

Im having trouble visualizing both of your ways for additional stability though. Are you saying to have a cable angled from the foundation of the house to the top of the far 6x6 posts. If so are they perpedicular to the house or criss crossed. I have never seen that done before. Triple d are you saying to make a big x one the bottom of the deck corner to corner?

Triple D you said i could use a 3 posts which is 10 foot spacing. That would make the 2x4 x-braces 18+ feet. Will there be too much flex. I would make my x braces on each side of the 6x6 so should there be a 6'' spacer where the 2x4's cross to connect them together?

How does everyone attach thier railing posts. I have been lagging 4x4's to the outside of joists or main beam. They still have a little wobble to them. The 2 decks i built before were not very wide and had the joists going parrell with the house. I think it was the end joist that was wobbling. I had cross beams between the joists but maybe i didnt have enough. What do you think?

triple D 02-21-2008 08:15 PM

Yes, the x braces will be long. You only need one on the inside going one way and the other on the out crossing the first. They will be 6 " apart where they cross and no need to connect at that point. However, where they meet in center, one inside, one outside, one at top, one at bottom, they should be angle cut to face each other at center of post. I guess what I'm trying to say is put a giant v on one side right side up, and the other on oposite side upside down. Your sona tubes should sit 2/3 in and 1/3 out max. height 3-4' out. But if soil is hard you might just dig a hole about 30"x30"x16" deep and pour redi mix 5000psi reinforced crack resistant from the old wheelbarrow. Then place a simpson 6x6 post bracket in, pour and give it about three days above 40 F, and if colder cover with straw or paper or something. And another x under joists one solid corner to corner, the other cut in two at angle to meet with first. Two screws at every joist. Just remember ample lags to house. This is where most decks give when there are too many people at the party. A stronger rail post will come from 4 3/8" through bolts with washer and nut at bottom through rim as opposed to the old way of 2 1/2". Good Luck....

guyod 02-21-2008 08:40 PM

I think i will take the job if he gets me a crane for an hour or two.

What do yous think about connecting the pilliars, x braces and main beam all on the ground then have a crane lift it in place all at the same time.

How do could i brace the pilliars and main beam in place while the the floor joists are put in place? Will the bracket be strong enough to hold it up

Can i take it one step farther and frame the floor joists on the ground in like 10x10 sections and have it lifted into place. Will this be too much weight? If it is can i frame out every other joist.

triple D 02-21-2008 08:54 PM

Did you say deck at house was 4'-5' high and 15' at other end? In any case, I would bolt to house my first board then set my joist hangers on this board. Then measure, level, and cut my three posts. Bolt them to bracket on footing blocks, then brace with wood you will later use for x bracing. Now it will withstand a ladder leaning on it. Now you and a helper can hoist up beams and secure to post tops. Then you can lean your joists up to beam then take other end up to house side and drop in hangers and fasten one at a time, no help needed here. Remember your beam will run about 18"-24" inside of deck end on width, and go flush or beyond on length. Good luck again...

guyod 02-21-2008 09:26 PM

Your forgeting something.. How does anything less than a football team lift , balance and place a 15' 6x6 on 4 bolts 2+ feet in the air.

I have a 20 foot aluimum pole jack set up that probably weight 1/4 the weight of a 6x6. That is a bitch to upright with 2 people.

Hack 02-22-2008 11:10 AM

I know that in my area, to get a building permit for a deck that high requires a Structural Engineer to do the calcs and spec out footings, posts, and bracing. It only cost me about $150 for that...

morrijon 03-02-2012 03:21 PM

An easy way to measure your post height for cutting is by using a long clear 1/4 to 1/2 inch tube filled with water. Once you set the ledger board, one man holds one end of the clear hose next to the top of the ledger board and watches the height of the water. Two other men prop up the 15' 6/6 on each footing and someone on a ladder holds the other end of the hose next to the post and watches the water level. Water will always find it's level so the man with the hose at the 6/6 should raise and lower his end of the hose very slowly until the man at the ledger board tells him the water is exactly at the top of the ledger board. The water mark at both the ledger board and the 6x6 will be the exact same height (level). Mark the spot on the 6x6 and take it back down. Now account for the beam and joist widths and cut your 6x6. When you put it back up, toss on the beam and joist (assuming the joist is setting on the beam), the joist will be level with the top of the ledger board, which is where I assume you want to fasten it using the standard hanging brackets.

You can also run out a string with a level or run out a joist and level it before dropping a plumb bob or measuring tape down to get the height, but the I find the water meathod to be full proof. It's also great if you are measuring heights around corners in any other difficult instances.

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