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# Strength of a deck

#1
06-08-04, 05:58 PM
Jeff Fojkar
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Strength of a deck

Hello, I am putting a spa and an enclosure on my deck. The spa weigh's 5,800 lbs. and the enclosure is 1,700 lbs. My deck is pressure treated and about 2 yrs. old. The posts are 4x4 every 6 to 8 ft. The headers are 2x10 and the joists are 2x8 on 16" centers. The decking is 5 quarter. The deck is 3 ft. tall. Will this support that kind of weight? I will add on if not. Thank's

#2
06-09-04, 05:46 AM
pw1972
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What's the size of the spa and enclosure? How far apart are the beams? What are the size of the footings for each post?

Just something to think about you're looking at 7500lbs, roughly guessing a spa is about 8x8(assuming the enclosure is about the same), you're looking at 117psf load.

Then you have to figure out over which posts the load is going to be distributed. Each post supports the load half way between the next post.

Given the info you have stated I would build the thing up a little in the area you want to put the spa. Then again, Im an engineer and I tend to over build things and lean to the safe side.

I built my deck to spa strength and to give you an idea, I used 6x6 posts, 57" apart with a 14" round footer. A built up beam of 2, 2x12's and 60" between the beams. And I still have to be very careful that the spa weight is distributed correctly over the posts.

#3
06-09-04, 09:01 AM
Jeff Fojkar
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The spa is 9'-4"x7'-7". The enclosure is 12'x12'. The footer's are 12" round and 3 to 4 ft. down. The posts are 4x4 with 2-2x10's as beams. The 4x4's are 6' apart and the beams are 6' apart. The deck is about 700 sq. ft. If this isn't strong enough i have room and i will just have to add on to existing deck. Thank's

#4
06-09-04, 09:32 AM
pw1972
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First thing, I'd recommend running any of this buy a structural guy before you do anything.

The biggest concern to me is that 4 posts are going to be supporting the spa load.

5800/4 = 1580

1700/9 = 189

So those 4 posts with the spa need to handle up to 1769 pounds on them.

Soil conditions are difficult to estimate, so I usually use 2000psf. I'd ask around your area to find out for sure.

Assuming soil conditions that can handle 2000psf the footers would be able to handle

I don't have a span table for the kind of loads you're looking at handy, but off the top of my head I'd say they would be pushing it a little with the joists.

What might be easiest to do would be to add an additional beam and 2 posts under the area where the spa was going, between the other two beams that would support it.

This would give you 6 posts to bear the load, and shorten up the span of the joists. That would put you somewhere around 1100 lbs per post and wouldnt have a problem with a 12" footer.

There are also a lot of things to consider... you probably would want to put some bracing between the joists to help prevent rolling. The support beam best supports the load if its resting on top of the post or notched.

#5
06-09-04, 11:56 AM
Jeff Fojkar
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I am leaning on expanding my deck. What if i used 4x6 posts every 4ft.,2-2x12 beams every three ft. and 2x8 joists every 16". The addition would be 16'x16'. Thank's a ton for your help.

#6
06-09-04, 01:36 PM
pw1972
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That sounds like it would work.. your load would go over 9 posts reducing the load on each post to about 933 lbs. You'd still want to make sure your posts had at least a 10" round footer on it, I'd go with 12".

Put those beams on top of the posts if you can and embed the posts in the ground(make sure to use .60 ACQ) for the most stable support.

As always, check those numbers out with someone else to make sure everything is up to par.

#7
06-09-04, 02:08 PM
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Join Date: Jul 2003
Location: Eastern USA
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Its not just how strong the lumber is, its also how good your cement pillars are and the strength of your mounting hardware. I would go with 12" holes 36" deep. You dont list a location so make sure you are below the frost line and whatever local code asks for. Think twice about cementing posts into the pillars since it causes them to rot faster. For decks with a roof load i get 1/2" x 3" wide steel bent into an L and put a 1/2 galv bolt through it and use a galv anchor plate to keep the wood off the cement.

#8
06-09-04, 05:36 PM
Jeff Fojkar
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Thank's alot guy's for your help. I am in the Cleveland, Ohio area. I will go at least 12" wide on the footer's and 3 to 4 ft. deep. Also if i put my 2x12 beams on top of my posts, what is the best way to attach to keep beam from rolling? Should i notch the post and then bolt them together?

Last edited by Jeff Fojkar; 06-09-04 at 06:46 PM.
#9
06-10-04, 05:23 AM
pw1972
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Wow, another Clevelander. Same here.

FYI the frost line in the area is 36", but a lot of municipalities have started to adopt 38" because of how cold and how much snow we've received the last couple years. There are even some municipalities that require you to dig down to the same depth as the house footers.

Notching the posts is a good way to attach the beams. With a large enough beam I notch on the inside so it forms sort of a "U" shape and lay the beams inside of that. I still use bracing to help prevent against the beams and joists rolling when a heavy load is expected.

There's a lot of debate about embedding posts in the ground. People will argue both ways, and while it's true that posts will rot faster in the ground, no one can really say how much faster. We recently dug out some of my friends fathers posts for a 20 year old deck and they showed no signs of rot. Just make sure that if you cut anything bigger then a 4x4, treat the cut end with cuprinol. On lumber that big sometimes the pressure treating doesn't always get to the center of it, and its better to be a little proactive there.

#10
06-10-04, 07:23 AM
Jeff Fojkar
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Thank's for all your info. Sound's like i better just build on to my existing deck and make sure i overbuild everything. I am going to use 4x6 posts every 3-4ft. and go 4ft. deep. 2-2x12 beams every 3 ft. with bracing and 2x8 joists with bracing for a 16x16 deck. Your help is much appreciated.

#11
06-10-04, 02:05 PM
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Location: Eastern USA
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If you didnt do it yet, might as well go to a 6x6 post, there are more options for fastening it to the concrete plus if you are notching the posts it will provide a larger contact point to bolt through.

#12
06-10-04, 09:08 PM
Jeff Fojkar
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Thank's guy's. I purchased 6x6's today. Your help has been much appreciated.