Joists over beams??

[optionguru]

Forgive me if the terminology is incorrect.  Having previously lived in Florida I have built a few decks low to the ground and I  would put in shallow 4x4's with concrete poured around them.  I would then put 2x10's around the perimeter and internally to hang 2x8 joists from.  

I noticed in a few posts that people frame their decks much the same way as the 14x24 cabin.  2 built up beams on posts and piers with the joists going across and on top of the beams.  I was looking to build a 16 x 16 or 20 x 20 deck and I now don't know if I should do 3 large beams with joists on top or should I do a square box with a grid of 9 posts all 10 feet apart with joist hangers holding 10ft joists?

I feel like an idiot now if I've been doing it wrong all this time?

[Jens]

Smaller lumber can cover the same area if the beams are within the perimeter.  The rule of thumb is, cantilever 1/3 of the supported span.  In otherwords, if you have 9' from the ledger on the wall to the beam, another 3' can overhang the beam without support at the far end.  Doing it this way, a 16' wide cabin can be framed on two beams, using 2x8 floor joists (min), 10 ft supported between beams, 3 feet to either side.  I don't know if this was the answer you were searching for, but no I don't think you are stupid for framing decks the other way.  In the future though, you can frame a 9' long deck with 2x6, having no bounce, simply by pulling the beam in 3'.  I would run it with this cantilever, but that doesn't mean you have to.

[MikeT]

I do not know if it considered bad form or not to ask a related question within this thread, so if I am out of bounds here, I am sorry.  

I understand the cantilevering strategies, but I was wondering if one has a positive connection on two sides of a deck (the X and Y axis, if you will), how does that affect the beam and where it is placed?  Or does it simply affect the number and location of support posts?  This is for a small, 8 x 12 deck.


Thanks,
mt

[optionguru]

Since 20 feet is longer then dimensional lumber how would everyone make the joists if I use the beam system?  Should I make the deck smaller or larger  to take advantage of traditional sizes of lumber.  This deck will be free standing, ie not attached to a house.  This will be a platform for a tent and porch that will possibly later be used to build a cabin on.

[John_C]

Have you checked the smaller lumber yards - not the big box stores.  Here in GA 2x, 3x and 4x in lengths to 24'  are not too hard to find.

[optionguru]

John are you saying to use those as joists or for the main beams?

[John_C]

My bad.  I wasn't really following the thread, just commenting on the length issue.

Since 20 feet is longer then dimensional lumber

But.  Visually graded #2 yellow pine 2x10's @ 16 " oc can span 16' 1" (40 lb. Live Load, 1/360 deflection)  So for a 20x20 deck you could run the beams in one direction 16' oc   and set the joists on top of them cantilevered 3' on each side.
Source  http://www.southernpine.com/spantables.shtml


If you planned to build a cabin on that platform at a later date it would probably be better to get the beams out to the edges where they would support the load bearing walls.  It would probably be best in that case to put a third beam down the center.  With the reduced span 2x8's 16" oc  would be more than adequate. You could hang these between the beams, set two 10's on top joined in the middle, or use 20' lumber in a continuous span across the tops of the  three beams.  This is a stiffer, stronger arrangement than the simple span.

There are a lot of  what if's, it depends, YMMV  in all this.  If you are going to put a cabin up at some point how much insulation do you want in the floor?  Will 2x8's be enough?

The beams are another issue.  Sizing them for the deck is largely determined by the pier spacing.  Wider pier spacing requires larger beams.  Figuring the beam sizes correctly for an undetermined cabin is much harder.

Hope it helps.  JC

[MountainDon]

Optionguru, IF there is any possibility that down the road you'd want to build a cabin on the deck platform, build the deck support strong enough to support a cabin so you don't have to go back later and re-do stuff.

Also with this thought in mind, if you were to build a cabin on this foundation at a future date, does your local zoning and building standards dept get involved? If so they will not like you building on top of something they didn't permit (you buy a permit from them, etc.) I don't know where you are, what you plans are... just a thought that crossed my mind.

As far as joists on top of beams (girders as called by some), that is usually easier and less expensive than using a lot of hangers and enriching Mr. Simpson. Unless of course there's a need for a lower profile, and then you have to watch out for the min grade to joist distance rules that the inspectors may want to enforce, especially if you go the cabin conversion route.

One last thing, IMHO, if I were to do a deck with a 3 foot cantilever as described above I'd use 2 x 10 irregardless of what the span charts say. Springy decks are strange to walk on. Just my opinion, mind you. I don't believe in pushing to the max allowable; I prefer to be on the other end of the stats.

If you're going for a larger than 16 ft width you'd be better of with a central girder. Then you can lap the joists over the center beam.

I am going to be building a modified 14 x 24 little house. Modified in that I plan on fewer piers at 8 ft spacing, with built up 6x12 girders supporting 2x10 joists (better insulation).

[optionguru]

John and Mountain Don, you have both been very helpful.  I was trying to avoid the three beams to reduce digging in my root and rock filled ground but I think three's the only way to do it right.  It's so hard to settle on one design.  I bought the plans for the 14x24 and would like to build it over the next two years while using the site to camp on and enjoy on weekends with the kids in the meantime.  I'm in an area with minimal if any building inspection so I should be ok.

Thanks Again,
Peter

[optionguru]

One last idea (I think).  If I go 24 x 24 with three beams (girders) could I have each joist be 2 12 foot 2x10's butted end to end with the butted end on top of a built up 4x8 beam.  The outer ends would have a beam support at 10 feet and a two foot cantilever.  This way I could eventualy build a 14 x 24 cabin with a nice 10 x 24 deck.

[MountainDon]

...was trying to avoid the three beams to reduce digging in my root and rock filled ground

I hear you loud and clear on that Peter.   That's the reason I went from 4 to 8 foot spacing and increased the girder size to two 6 x 12 built up.  

...could each joist be 2 - 12 foot 2x10's butted end to end with the butted end on top of a built up 4x8 beam.  The outer ends would have a beam support at 10 feet and a two foot cantilever.  This way I could eventually build a 14 x 24 cabin with a nice 10 x 24 deck.

Hmmm. I'm not certain but somehow that doesn't seem good. If there was no cantilever overhang you might be able to get away with that.  :-/  If there was a gathering of say 3 of today's average size people gathered at one edge, leaning on the railing or whatever it would be easy to have 500 pounds there 2 feet over. 2 x 500 = 1000 ft-lbs. At the other end, 10 feet from the outside girder (pivot) point you would have a 1000 divided by 10 ft = 100 up force. With the joist end toe-nailed to the beam with about 3 nails over time it could be problematic. I've always seen an overlapping of the joists of at least the beam width to a couple feet or so with those ends face nailed together as well.

Myself, I'd be more comfy with an overlap. You might be able to find 14 footers for joists. I know some of the big box stores don't stock much in the way of 14's though. If you had 14's you'd be able to cut one blocking piece off an end and then have a nice overlap. Ditto on a 16 footer; maybe cut 2 blocking pieces and overlap the remainder??

Maybe Glenn, PEG or John will pass thru this thread and have a better answer.

Would your future cabin subfloor be laid on top of the decking or were you thinking of pulling the decking up to use for something else?

[John_C]

I can't get real excited about that idea.  It would put one of the load bearing walls of  the 14x24 cabin 2' from the beam, running across the joists of the deck.

If the plan was to build the 14x24 at a later dater you could make the beam spacing 14' and 10'.  That would give you a beam under each of the load bearing walls of the cabin. There would still be a few other problems.  An outside deck is usually a bit lower than the subfloor of the house.  Since the decking material is usually a bit thicker than the subloor material -usually something like 5/4 vs. 3/4-  there would be some ht issues.


One way around that would be to set the beams so that the ~14' spacing was from outside to outside of the beams.  Use the exact spacing from the 14x24 plans.  Then set the other beam in the appropriate location to get the 24' width.   The center beam would probably need to be a bit heavier than on the plans since it is now also supporting what will be the inboard end of the deck.  The joists for the 14' span can be whatever is specified in the plans...  2x10  I presume. The deck joists could be 2x8 or perhaps 2x6 since the span is shorter.  Set them so that the tops are 1-1/2" lower than the tops of the other joists and put a 1-1/2" piece on top of them before laying the deck (See note below).  When it is time to build the house you can remove the deck (a good place to have put the deck down with screws, and coat the screws with something like anhydrous lanolin so they can be easily removed.), remove the 1-1/2" spacer and put the deck back down.  In the area of the house you can remove the deck and lay the subfloor right over those joists.  

I'm thinking through this on the fly at work so I may not have the numbers quite right.  You can draw it out and make it work but in any case it will be extra work.  If the decking sits out in the New Hampshire weather for a year or three before you get around to building the house it probably won't be in good enough shape that you'll want it under the floor of your house.

Later Note: When I got home I measured & my porch deck is about 4" lower than the house subfloor. You could still work it out with a temp. 2x4 spacer.


I can see lots of problems popping up later.  You decide to build a 16' wide house  or a longer house.....  As always YMMV

BTW Does anybody know what the design loading for a deck should be.  I am assuming 40 lb live load.   I often hear people talking about building a deck fairly lightly and casually attaching it to the house,  hey, it's just a deck.  Then they have a party and a herd of folks start dancing up there. Deck and party folk come crashing to the ground... never a good thing.  Here in GA it happens on houses costing upwards of $1 mil.  Seems to me the designer/architect/engineer/building dept/builder/building inspector were all asleep at the switch.  It's on my mind right now as one like it is going up a quarter mile or so from my house. I pass it every night on my bike ride.  Expensive house, weak deck

[Jens]

usually, decking is about 2" lower than the threashold of the ext doors.   The reason joists get lapped over a central beam, is because it is faster than cutting them to length to butt them, no cutting at all, just lap and nail.  cantilevers are perfectly kosher, as long as guidlines are followed.  2x10's would be better for the floor framing of a cabin.  Each 16d nail has a shear strength of 1600lbs if I remember correctly, and when toenailed I believe it is about 2/3 that per nail.  Houses aren't usually built on a cantilever, because continuous perimeter foundations are the norm.  The 1/3 to 2/3 rule is for a load bearing situation.  You shouldn't have any problems getting lumber in 20 ft lengths, I just had a bunch of 2x12 rafters 26' long.  Again, this is all IMO, and feel free to consult your friendly local engineer, or building inspector.  Correct me if I am wrong John, but isn't this the way your post and pier plans are designed?

BTW, I wouldn't use 2x6 for a deck of this span, but I have seen it done in a house.  10 ft spans with 2x6 are not uncommon in older houses, especially with attic additions, but yes, it can get springy.