Will be building a 14′ X 31′ square-deck in a month or so. Have a design in my head and was wondering how far off I am. Here’s what I’ve got:
– 2X8 ledgers already attached to house by framers and flashed
– putting in 7 4 X 4 posts along the 31′ side
– using 2 2X10’s, one lagged into each side of 7 posts, as a beam to cut 14 foot span of 2 X 8 joists in half (joists resting on top of 2 X 10’s)
– all posts down 31′ side and 14′ side spaced about 40″ o.c.
Do you all think this design will be structurally sound? All posts will be buried below frost-line with limestone gravel packed under and around posts. Major question is the “beam” used to cut down the span of the joists. Was wondering if it may not be better to make a built-up beam out of 2 or possibly three 2 X 10s and rest them on top of the posts running down the middle of the deck and rest the 7-foot 2X8 joists on the beam or with joist hangers.
Thanks in advance for any input and items I’m sure I’m overlooking.
Derek
Replies
Our specs call for twice the footings. 14 for that size atop undisturbed soil, 12" holes, atop concrete-backfill with gravel is fine.. Normally we use sonos for decks, mount the posts in brackets-easier to straighten the beam.
We do a lot of things differently... but that is your tip for the day. Posts sitting on gravel are only bearing on 7 square inches of soil... it will sink quickly.
My calculations have your footings 5' apart. Two beams required for that deck.
L
GardenStructure.com~Build for the Art of it!
I find the posts 5' OC too, but a 4x4 at 3.5" has 12.25 sq inches of landing. Not that that will always carry it... but it's more than seven
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Ahhh, but on crushed stone... follow me here. It doesn't bear on the entire size of the post... merely on about 70% of the area. (not exact-an estimation only-depends on the gravel size- Over 1" stone, bearing +30%, 3/8" -35%.) IMHO
LGardenStructure.com~Build for the Art of it!
Duhtanks fo do co-rrection;)I use 6x6s
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No worry Piff, takes a twisted mind like mine to even think about that. I like the idea of no concrete... likely take me another 10 years to convince the building department to go for it though. They finally accept my footings.
Undisturbed soil, 1" crushed stone... maybe give a concrete base under the gravel, but really there is no need for concrete to be in contact with the post so long as you backfill with gravel. (of course it depends on the soil conditions-Don't do it in the mississippi delta or north florida or California this year!)-but in dry climates should work well.
L
GardenStructure.com~Build for the Art of it!
Am I missing something here?
Take your 22 oz. framing hammer and try to drive a 1X2 into the ground. Now take that hammer and try to do the same with a 6X6. Which post sank the furthest? I'ts all displacement. Thats why the codes want a concrete pad or solid footing. If you want to bury the post put it on a concrete pad. A 12" diameter pad has amost 4X the displacement as a 6X6 post. This is minimum area for a footing in Minnesota.
Pardon, but it is not displacement. that is for floating boats.Bearing posts is about loaded surface and bearing capacity combined with friction on the sides.Also, I checked math again. the footing you mention haas nearly ten times the load bearing surface to distribute the weight as a 6x6 post does
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1" stone won't settle and it enlarges the footprint. Don't know how best to explain this to you Hammer. They pile up like a pyramid is the best way to describe the principal.
Soil conditions vary greatly across 5 miles of this earth, so blanket specs like those of any building department seem deficient to me.
L
GardenStructure.com~Build for the Art of it!
Lawerence,
Re-read my posting. Sorry for my attitude.
Thanks for the clarification. I wasn't aware that horizontal friction could support a vertical load.
How is your approach used when working with a 42" frost depth? How much of the post do you bury beneth grade? I have to admit that a buried post sure steadies a deck platform.
Piffin,
6X6 post 5.5sq. =30"sq. 12" circle 3.14 X 6"sq. = 113" sq.
Edited 2/23/2005 11:59 am ET by hammer-n
Don't worry Hammer, it wasn't taken as attitude. Good Question relating to footings and frost.
More concrete does not mean protection against frost. Frost as you know settles in from the surface and moves down towards the base of the footings in layers. As it expands it heaves. If the bottom of the footing is smaller than the top of the footing it will heave the footing out of the ground a little at a time, year after year until you get the situation in the photo below.
When it comes to a deck the critical part is bearing. Support upwards and connection to the house. The weight of the deck is more than sufficient to keep the footing in place, resistant to frost so long as the base is solid enough and the post is in gravel rather than soil. Soil will freeze to the post and the upward action will cause the post to move year after year.
Fences are not as heavy and thus we reccomend a plug at the base of the post and backfill with fine crused stone (screenings) ideally. With the plug keeping the post in place, and the gravel offering drainage and lubrication to resist frost we have never ever had a failure. Furthermore, the gravel gives a bit and actually allows the fence post to move slightly to resist occasional high winds. Next time it rains, the gravel washes back in offering a secure post again.
We are still using a sono for decks due to building department being ill equipped to understand the gravel angle-but the theory is sound when it comes to decks bearing on concrete, then gravel and backfilled with gravel.
There are other things we do to increase durability... but we have to keep some secrets... Just business!
L
GardenStructure.com~Build for the Art of it!
Here's those photos...GardenStructure.com~Build for the Art of it!
Looks like the frost had quite a bit of rough concrete to grap.
Had a neighbor that added a midspan beam and post on his deck to sturdy it up.. Did the same, dig a rough hole and filled with concrete. Ever winter the end of his deck would lift off the original footings when this midspan was lifted by frost.
Funny how some people can ask for help and despite advice and even drawing a picture, they choose it do it their way.
Main problem is people trust books... and most of them teach you to set the posts in concrete-without knowing why posts pop out of the ground.
Shame really... they certainly aren't beating down my door to write a proper book.
L
GardenStructure.com~Build for the Art of it!
Yah, and I was refering to the previously mentioned 4x4 post which at 3.5" is 12.25 sq in
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Actually, posts bearing on full-fractured crushed rock are bearing on an area whose diameter is approximately equal to the depth of the rock. The nature of crushed rock is that it spreads the load out with depth.Of course, posts bearing on other forms of gravel are bearing on less.
using 2 2X10's, one lagged into each side of 7 posts, as a beam to cut 14 foot span of 2 X 8 joists in half (joists resting on top of 2 X 10's)
I don't get this. The only way you can cut the joist span "in half" is to put two rows of posts in (one @ 7', the other out at the 14' edge), and thereby, have two beams. Or are you thinking of cantilevering 7'? <grin>. I'm probably just missing something here.
My fault, I was not clear. Planned on doing the same sort of thing as the center beam (sandwiching a post with 2 X 10's) on the outside of the deck, with the most outside 2 X 10 (or possibly 2 X 12) acting as the ribbon.
Thanks for all the input guys. Definitely going to research this further. As a quick thank-you to everyone, I'm a 22-year old construction engineering student who has worked for a couple of custom home builders beginning when I was 16. Was involved with framing and trimming and roofing (and have become sort-of kind-of good at roof-cutting, by far my favorite activity) and as a kid trying to learn all he can all of you have been a great resource. Thanks.
Derek
A wooden girder is 2 or more 2x8s, 2x10s, etc, laminated together to form a solid member. Laminated means nailed, in this case. By bolting 2xs to either side of a post, what you end up with is a post with a board on either side. Not a beam or a girder.
See attached pic. It's a bit hard to see since this deck is quite low to the ground, but the girder is sitting on top of 4x4 posts, and then 2x4 scabs are nailed to the side of the 4x4 and up beside the girder to stabilize the girder on top of the posts. The bolts you see are for 2x6 diagonals that add further lateral stability. The 4x4 post bottoms are sitting on brackets which are set in concrete that extends to below the frost line - which in my state/local, is only 12" min.
You are correct though by not trying to span 14' with 2x8 joists. They would be much too bouncy, and wouldn't satisfy any span chart I've seen. 2x8s spanning 7' will yield a nice firm floor.
Matt
Edited 2/21/2005 8:51 pm ET by DIRISHINME
I will agree with the others about support issues. Looking at the footing a poured cement (sonos tube) is the best approach. The diameter of the tube plus increasing the pour down below the frost is best. This resembles a "bell" shape. The post spacing is adequate up to 5' on center.
Is the choice of using a 2X8 made since that is what is attached to the house? Unless this is a ground level deck I usually use 2X12's so I have support for attaching railings. With your dimensions using 2X12's (16" oc) you can get a 14' deck using a 20" cantilever past your beam assembly. Don't forget a wind shere support. Diagonal support below the joists.
Lets talk about the beams and post. The 4X4 post are strong enough but usually not used for above ground decks. They simply look undersized. I use 6X6 and I notch the tops for insetting the 2X10's. One on each side. and lag bolt to the post. mke sure that over the 31' span that the 2X10's don't seam on the same post. Also cut you notches so you can fill between the 2X10's. I use a 2X10 and 1X10 sandwiched and all bolted together. Use carriage bolts here and go all the way through. If I'm not clear,You can also bye plates for this if you choose.
Make sure you make a clear plan and submit to your City Hall. All minisipalities have special codes. I would also bye a deck building book, if for any other reasons to get ideas of what can be done.
Good luck.
Edited 2/21/2005 6:33 pm ET by hammer-n
Draw a rectangle 31'x14'
on the 31' side come in 2' from each end this makes it 27' pier to pier with a 2' canterlever on each end of the beam. Divide that distance by 4 and you get 6'7" center to center for your piers for a total of 5-piers
from the house go out 12' instead of 14' this gives you a 12' span with a 2' canterlever. Too much span for 4x8 so divide that in half and you will need two beam runs 6' apart. Pier lay-out would be the same as above.
We would use 4- 16' 4x8 beams on top of 4x or 6x posts depending on how high of the ground this deck is.
Sono tubes for footings with Simpson stand-off brackets work best . I would not recommend burying your deck posts.
Hope this helps, now go make some sawdust...............
"Rather be a hammer than a nail"
Bob
Derek , you never mentioned how far off the ground the deck will be, given the discussion on the pro's and con's of 4x4 vs. 6x6 posts, bearing areas , and joist spans, etc. one thing that was never mentioned was the attachment of the ledger to the house. It should be checked to be sure it is, at a minimum, lagged to the rim joist (or wall studs , if applicable ) and preferably thru bolted. Second, you don't mention how you plan to attach the posts for the handrail. Your posts should be attached on the inside of the rim (band) joist and next to a joist, and the post should be the full depth(height) of the joist. The connection should be a minimum of 2 1/2" dia. carriage bolts in a staggered pattern (one top ,one bottom, diagonal from each other) thru the floor joist ( not the rim joist) and thru the post. I prefer to use three bolts, 2 up and 1 down ( an "L" pattern ) See the current isssue of JLC for more info. that an engineering student such as yourself might enjoy reading. Good Luck!!
Geoff
The deck is only about two feet off the ground. The ledger is through bolted to the rim board. I read the article in JLC on handrail attachment and had planned on using their advice by using the Simpson ties. Just as a side-note, the design has since changed. Going to be upgrading the size of the joists to 2 X 12's, doing away with the middle beam, setting a built-up beam on top of 4 X 4 posts that are resting in post braces on top of concrete footings that extend below the frost line.
Drew up a plan that a builder I once worked for looked over, thought it looked good, and passed it onto a building inspector friend of his who is going to make any sort of necessary changes if need-be so I can pass it onto the necessary township people.
I appreciate all of your comments and suggestions and will probably be asking more questions in the future.
Derek