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Old 03-19-2013, 10:00 PM   #1
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Foundation for 12 x 16 Shed


I am helping a friend build a 12 x 16 shed in the next month or so and need some guidance on the foundation. It will be a wood frame floor sitting on concrete pillars with buried straps to keep it from lifting during high winds. It will have 8' walls and a 6on12 gable roof. It will be used mostly for storage but I will also add a work bench or two as well as electric lighting and outlets for small projects.

Below is my tentative plan for the foundation (thanks Google Sketchup). It is made up of single 2x8s on the perimeter and double 2x8s on the middle runner with 2x6 lateral joists spaced 12" on center. I will use 3/4" tongue and groove plywood as the subfloor. As you can see there are only 9 concrete pillars and that is where my biggest question is: Should I use 12 instead? Is an 8' unsupported span on a single 2x8 too much?

Any other comments and/or recommendations?
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Old 03-20-2013, 08:39 AM   #2
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Foundation for 12 x 16 Shed


Most definetly beef up your foundation. I would also use cross bracing on the floor. It will not only make it stronger but gives a better feel once all is said and done. I would also suggest some sort of skirting around the base to discourage unwanted guests.

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Old 03-20-2013, 04:22 PM   #3
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Foundation for 12 x 16 Shed


Thanks. I will definitely add another row of blocks to bring the spacing on the runners down from 8' to 5'-4" as well as add a row of crossbracing to each set of 2x6s. Skirting is a given as the wildlife down here tend to be quite numerous especially during the summer months.


A few more quesitons:
  1. Can I use a single 2x8 for the middle runner instead of doubling up? I put two in the model to avoid nail interference from individual joist hangers on each side but I found a one peice hanger that sits on the main runner and has hangers on each side for the laterals.
  2. Are 2x8s sufficient for the runners based on the additional blocks or do I need to beef them up to 2x10 or 2x12?
  3. Are 2x6's sufficient for the laterals as long as I add the cross braces?
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Old 03-20-2013, 04:35 PM   #4
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Foundation for 12 x 16 Shed


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Originally Posted by GeauxCajuns82 View Post
Thanks. I will definitely add another row of blocks to bring the spacing on the runners down from 8' to 5'-4" as well as add a row of crossbracing to each set of 2x6s. Skirting is a given as the wildlife down here tend to be quite numerous especially during the summer months.


A few more quesitons:
  1. Can I use a single 2x8 for the middle runner instead of doubling up? I put two in the model to avoid nail interference from individual joist hangers on each side but I found a one peice hanger that sits on the main runner and has hangers on each side for the laterals.
  2. Are 2x8s sufficient for the runners based on the additional blocks or do I need to beef them up to 2x10 or 2x12?
  3. Are 2x6's sufficient for the laterals as long as I add the cross braces?
For a shed that size what you first suggested if the bare minumum.
Always better to go a size bigger then what you think you need.
Alway double up that center beam.
(There called joist not laterals.)
Under built it and the floor will bounce and sag and the doors will never stay plumb or level.
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Old 03-20-2013, 04:44 PM   #5
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Foundation for 12 x 16 Shed


you need to double up both bands where the joists land ( front and back) just like the middle is doubled up. use 2x8's for the doubled bands and girder and 2x6 6' 12" center might not have bounce in it but if that is a concern use 2x8 throughout. if using 2x6 consider 10 1/2" blocks placed mid span of the 6' span between all joists. moisture from the ground underneath will need to be addressed also as the building looks low to the ground. maybe plan the entire floor system being treated (plywood,joists) and put together with acq approved fasteners or if you figure out the moisture issue use advantech 3/4" tongue and groove for the floor, that will be the strongest floor available.
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Old 03-20-2013, 04:56 PM   #6
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Foundation for 12 x 16 Shed


I have not used plywood for subfloors once since Advantec came out.
Far more stable and moisture resistant..
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Old 03-20-2013, 05:20 PM   #7
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Foundation for 12 x 16 Shed


Quote:
Originally Posted by GeauxCajuns82 View Post
A few more quesitons:
  1. Can I use a single 2x8 for the middle runner instead of doubling up? I put two in the model to avoid nail interference from individual joist hangers on each side but I found a one peice hanger that sits on the main runner and has hangers on each side for the laterals.
  2. Are 2x8s sufficient for the runners based on the additional blocks or do I need to beef them up to 2x10 or 2x12?
  3. Are 2x6's sufficient for the laterals as long as I add the cross braces?
your questions all depend on the load you'll be applying to floor framing system. My answers are based upon a live load of 50 pounds per square foot and a dead load of 10 pounds per square foot. For reference your living room should be designed for 40 pounds per square foot live load. I do not know what you'll be putting in your shed so I'd rather go heavy than light.


If your loading would be greater than 50 psf then these member sizes would not apply.

My answers are also based upon the current physical properties of preservative treated southern pine #2 solid sawn lumber.

#1 - at minimum the center beam should be (2)2x12.

#2 - by runners I'm not sure if you mean floor joists or outer perimeter beams.

#3 - laterals, I'm not sure what those are

floor joists should be a minimum 2x6 at 16" o.c., I'd recommend 2x8 at 16" o.c. for a stronger floor. The deeper the joists the less bounce in the floor.

outer beam should be a minimum 2x12, but I'd double that to make it easier to attach your walls to the floor framing below.

Hand Drive is correct about using preservative treated flooring and framing member since you'll be less than 18" from the ground. Moisture will have bad effects on framing. As joe and drive said Avantec is a good product.

with all that said I'm a fan of having a concrete slab with a row of concrete block laid on the slab and then building my walls on top of the block. Stronger floor with less issues, keeps the wood in walls above the ground so less moisture issues.
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Last edited by GBrackins; 03-20-2013 at 05:23 PM.
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Old 03-20-2013, 05:48 PM   #8
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Foundation for 12 x 16 Shed


What do you mean with "on concrete pillars" ?

The sketch looks like precast piers. Are you setting those on grade ? Or, are you pouring concrete in ground and setting the precasts on top ?
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Old 03-20-2013, 06:40 PM   #9
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Foundation for 12 x 16 Shed


A couple of considerations.

1. I would use sonotube and pour the pillars right to level with each other, and embed your tie downs right in the pillars. Im sure you dont need to go down too far since you are in LA.

2. Your span of 6 foot to the girder is very short. I would check span tables and open up your joist spacing to 16 inches on center, unless you plan on a lot of weight. It gets tight working between 12 inches on center. Forget cross bracing, you wont be able to get under there. Use solid blocking staggered. Frankly I dont know what you are putting in there, but it would take a lot of weight to twist a 6 foot long joist.

3. Consider using longer studs and building a Gambrel roof with storage above. With a door on both ends that you can open, you will get a nice convection going to cool the shed.

Good Luck to you.
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Old 03-20-2013, 09:22 PM   #10
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Foundation for 12 x 16 Shed


Find out what the prices of wood are, find out how much you will need. Then start increase the size and see how much more things will cost. You may be surprised to find using something beefer is not much more. I would only build on a slab as well.
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Old 03-21-2013, 07:33 AM   #11
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Foundation for 12 x 16 Shed


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Originally Posted by joecaption View Post
I have not used plywood for subfloors once since Advantec came out.
Far more stable and moisture resistant..
Amen to that. Costs more than plywood, but well worth it. Especially in a shed, where the floor will likely get wet once in a while.
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Old 03-21-2013, 09:22 AM   #12
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Foundation for 12 x 16 Shed


Let me clear a few things up:
  • By "runner" I mean the four 2x8x16's
  • By "lateral" I mean the 2x6x6's
  • By "crossbrace" I mean wooden blocks the same dimensions as the joists they are connecting.
  • All of the 2by lumber in the foundation will be pressure treated.
  • The concrete "pillars" are precast piers and will either be 12" or 16" tall. I plan to pour a small concrete pad below each of them to make it easier to level out and have less of a chance of settling.
  • I would love to pour a concrete slab but unfortunately there is only one access route to the backyard and it is barely 2' wide so even wheelbarrowing concrete back there would be difficult. And using a pumping truck w/ crane would definitely blow the budget. However this project has been pushed to the fall so we'll see what the budget looks like then.
  • The main purpose of the building is for outdoor equipment storage as well as bikes and some patio furniture. So the heaviest thing in there will be a pushmower. But you never know what it may be used for in the future so I'd rather err on the strong side.
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Old 03-21-2013, 11:14 AM   #13
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Foundation for 12 x 16 Shed


Quote:
Originally Posted by GeauxCajuns82 View Post
Let me clear a few things up:
  • By "runner" I mean the four 2x8x16's
  • By "lateral" I mean the 2x6x6's
  • The main purpose of the building is for outdoor equipment storage as well as bikes and some patio furniture. So the heaviest thing in there will be a pushmower. But you never know what it may be used for in the future so I'd rather err on the strong side.
I would recommend (2)2x12 for your runners, the center runner support 1/2" of the floor load of your shed.

the outer runners (under the load bearing walls) support the weight of the wall plus 1/2 the weight of the roof, and 1/4 the floor load

your laterals can be 2x6 at 16" o.c., but I'd recommend 2x8 at 16" o.c., the taller the better as that will provide you with less bounce or vibration

The live load I used is more than what would be required for your living room.

Good luck!
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Old 03-21-2013, 11:37 AM   #14
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Foundation for 12 x 16 Shed


funny thing about sheds & garages,,, IF you have the space, you'll fill it the corrolary is whatever you use to fill it will always weigh more than you thought MUCH easier to build it stout now rather'n later when you notice sagging floors
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Old 03-21-2013, 11:44 AM   #15
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Foundation for 12 x 16 Shed


How about renting a couple of motorized Georgia Buggy's? You could pour a monolithic slab down where you are.

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