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How many piers do I need for a shed?

tpmcnichol's picture

I'm planning on building a 10'X20' back yard shed.  My plan is to pour concrete piers and tie in 4X6X20'.  Flooring on top.  I'd like to make sure that I do it well as structurally sound.  I'm handy...but have never built anything from scratch.

1) How many piers do I need to pour and their spacing?  Can I get by w/ 2 4X6X20 layed out in parallel...say 8' apart....and piers say every 4' (Total of 10)?  Or should I go w/ 3 4X6X20 and 15 piers?

2) Should I use 2X6(s) aor 2X8(s) for the flooring? 2X4(s) or 2X6(S) for the walls?


(post #93691, reply #1 of 11)

What are you parking in there?

Unless you mow your lawn with an M-1A1 Abbrams Tank, piers are over-kill.

dig three trenches 8" deep and fill them with gravel.

Tamp well.

lay your 4x6s on the gravel.

Plenty sturdy for a shed

with three grade beams 2x6s should suffice.

Mr T

Do not try this at home!

I am an Experienced Professional!

. .

(post #93691, reply #2 of 11)

I built a shed on a leeching field near a marsh, so piers were out of the question.  12x16 foot.  I dug out a level plane, put down landscape cloth and gravel, tamped it down, then laid down 4 PT 4x4s.  Built up from there.  That was maybe 10-12 years ago, and it hasn't moved since.


I never met a tool I didn't like!

"I am the master of my fate, I am the captain of my soul."  Invictus, by Henley.

(post #93691, reply #3 of 11)

The beam-on-gravel suggestions would save a lot of work. The piers will allow you to keep the shed up in the air where it's drier. For my 12x16 shed, I have 6 piers (total)supporting 2-4x6x16' beams. The joists are 2x6 pressure treated. Studs and rafters are all 2x4. T1-11 siding. The floor has a little bounce, but is plenty strong for lawn mowers, bikes, etc.

Al Mollitor, Sharon MA

(post #93691, reply #4 of 11)

Beam on gravel will work some or perhaps most situations, but..........around here (in the wide open spaces of the country) you'd best also have the shed anchored down somehow or it's gonna move around the yard when the big bad wolf comes huffin' and pufffin' at the door. Or you may find it on its side in the neighbor's yard every couple years.

Knowledge is power, but only if applied in a timely fashion.

Knowledge is power, but only if applied in a timely fashion.

(post #93691, reply #5 of 11)

I did a 12x15 with three 4x4's, one on each edge and one in the middle; that way the joists only span 6 ft.  The ground slopes, so I set the high corner first and then blocked about every 36-48" with scraps of 4x4 turned perpendicular.  I used 2x6 PT joists at 16" topped with 3/4" PT plywood, glued and bounce.  From there up I used plain yellow pine.

The 2x6 runners mentioned earlier would work just as well, but I used 4x4 (or 4x6) so they would have less tendency to roll over and have twice as much bearing on the ground.  If you feeel better with concrete, look into the deck-blocks that are notched for joists.

If you haven't started yet, be sure to work out the actual size of the shed first.  10x20 is a nice round number, but you would be a little better off making it about 4" shorter and narrower.  That way the plywood floor and siding won't come up an inch short...not that I have ever seen that happen :)

Do it right, or do it twice.

Edited 10/19/2003 7:41:05 PM ET by ELCID72

I'm sorry, I thought you wanted it done the right way.

(post #93691, reply #6 of 11)

If you want something with a bit more permanence, pour a 5- 6 inch thick "floating slab" and then build on top of that with conventional stick framing.

Or, spread a good gravel base down, and set 4x4 x10s on the gravel, spaced 2ft OC. (You would need 11 of these to cover the 20 ft) Then, just tack your plywood floor to that and build your walls and roof accordingly. I'd use 3/4 T&G Advantech plywood for the flooring. I'd also nail a 2x4 "rim joist" into the 4X4s at each "open end" to help keep critters from crawling underneath.

Or, you could pour your concrete  piers and set your 4x6s on top as you proposed, but you would need 3 parallel rows, not two. Each row ( 20 ft length direction) would need a pier every7 to 8 ft apart... But, in this situation, I would divide the 20 ft into equal distances and place piers accordingly... (you would need 4 piers per row X 3 rows = 12 piers total))

 A variation of the concrete pier theme  would be to use treated 4x4 posts ( or 6x6s. if you like)  Dig the post holes, set some gravel in the bottom, and/ or pour a "punch pad" in the bottom...then place your posts in the holes and backfill with dirt. The posts can be at any height you want. ( ie...just at ground level, whereby you can spike your 4x6s to it with a piece of 1/2 inch rebar....or you could raise the  posts off the ground, and bolt 2x10s or 2x12s to the posts a la deck framing style)

If you go with a pier/post foundation, you would still need joists to span across the 4X6 beams. ( Using 3 rows of piers, I would space the middle row approx 5 ft OC from the 2 outside rows.) With this configuration, I would use 4x4X10s as joists to span the 5 ft lengths, but 2x6s would also work equally as well. Then, nail down a plywood floor and proceed to errect your shed.

The cheapest, easiest method, would be using a good gravel base, and laying down 4x4s or 6x6s 2 ft OC ( the first described method), and nailing the actual flooring right on top. The 4x4s act as both foundation beams and floor joists...thus saving you on material costs. This type of shed should easily last you 12 to 15 years...unless you have 5ft deep snow drifts lying against it every winter. The worst thing about this type of constructed shed is it's close proximity to the ground.  But, If you are in a nice climate, build this and don't look back.


(post #93691, reply #8 of 11)

Hello -

I am setting up a similar style shed to what you describe (I think).  I have found the table for the joist span, but not for the beams that will be supporting the joists.  Is there somewhere I can find that? or is it just common knowledge? 

My shed is only 8x12, but on a fairly steep incline, so that is why I am not putting it on the ground.  And the 'lift' should help with termites too.

The layout as it stands now is in the drawing.  I only have 4 posts for the whole shed - 3 foot deep cement piers for my double 2x8 beams at the front and rear of the shed.  Then my 2x6 joists 16" OC. 


"One experiment is worth a thousand expert opinions"
"One experiment is worth a thousand expert opinions"

(post #93691, reply #9 of 11)

Your layout looks OK, but I would run the beams the other way (12') so the 2x6 joists would span only 8'. You might also consider moving your piers in a little from the corners to create a slight cantilever. This would shorten the span for joists and beam a bit, tuck the posts and concrete under the shed and make the more protected and less visible and give you room to fasten any skirt you might install.

Good luck!

Al Mollitor, Sharon MA

(post #93691, reply #7 of 11)

We had a shed out back when we moved in. It turned out to be on post bases and a lot of cement blocks. After a couple of years, the floor got spongier and the door didn't open too well. Then, one day, I stepped through the linoleum and wet particle board into a squirril hole.

A few years later, we lifted it, poured a foundation and a slab, and fixed the dryrot and termite damage. With a new roof and an added greenhouse (8 4'x8' surplus sliding door panels), it's good for another thirty years.

The builder didn't take the squirrils and skunks burrowing underneath into account, nor the plastic nature of our sily loam when in the winter (wet). The post bases and cement blocks were buried one 2X6 joist deep. The floor was on the soil. The critters had continually fluffed the soil making the shed sink. The dirt was full of walnut shells and olive pit pieces under the floor!

(post #93691, reply #10 of 11)

Have two smaller sheds (finishing the second tomorrow).  For both we have leveled the ground and used cement blocks and PT to frame the base.  2x4 framing and T1-11 siding.  Roof sheathed with 1/2" ply.  I think Piers may be over kill, but your "shed" shounds more like a single car garage size.

I would make your base as flex free as possible, just say no to the bounce.  We framed both with 14" OC and minimum 1/2" ply.

Good Luck



Know a little about alot and alot about little.
Know a little about alot and alot about little.

(post #93691, reply #11 of 11)

1.  Space 12-in square piers no more than 8-feet apart.  If you want to space them further, increase the size of the piers.  You want the piers at least 6-in above the ground and at least 6-inches into the ground (this is for areas with no frost line - if you have snow on the ground, consult your local building department).  The 4x6 floor girders will span 8-feet max.

2.  2x6s for floor joists will span up to 7'-10" for a 24-inch spacing.  9'-8" for a 16-inch spacing.  2x4's for walls at 16-inches ok up to about 7'-6" height.

All of the above for Douglas Fir No. 2 or better.

IMHO I prefer your proposed type of construction to the suggested wood foundations because of termite infestation problems associated with wood on the ground.  As a suggestion, I'd also put down some plastic sheeting under some gravel beneath your shed to keep things from growing underneath it.

Edited 10/27/2003 5:31:51 PM ET by KAORISDAD