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Shed Construction

Good Morning!


I am constructing a 12x16 shed.  I have 2x6 pressure treated joists on 16" center.  These joists are resting on concrete blocks.


I am trying to determine a flooring choice. I am looking at 3 options:

Pressue Treated 1/2" plywood

Unreated 3/4" plywood

7/16" OSB ( This option will be doubled up, so 2 layers)


Longevity is important.  I want to do it right the first time. If there is another option, let me know.


I this floor will be exposed to weather for about 60 days.  I am buliding this in my spare time.



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Posted 2012-04-07T11:26:34+0000  by 06fusion4us 06fusion4us

you can always plank it with PT 2x or deck boards and if desired put the 1/2" PT ply on top of that to give you a smooth floor. as for longevity, 1/2" alone wont be strong enough. you could also double up the 1/2" ply, making sure to run the sheets per layer at 90° to the other layer.





obviously youd need more block under your framing, but this should give you something to start with.


instead of block, you may want to consider using dek-block, theyre found in building materials at your local home depot. they look something like this




then your planks alone or planks and 1/2" ply on top.


but the best way to do it would be to sink piers, using some anchored simpson strong tie post anchors embedded in the concrete piers and build your frame on that if you want longevity.


whatever way you decide to go, id suggest clearing out all the grass and weeds under the shed area and spreading gravel. go out at least a foot beyond the size of the shed. this allows for good drainage and helps to keep water from pooling leading to wood rot and foundation issues from erosion.

Posted 2012-04-07T14:00:55+0000  by Paul

Hello 06fusion4us,


Thanks for your question and welcome to the community!


Sounds like you have quite the project going on in your spare time, lets make sure we get all the materials right so you'll have a long-lasting and sturdy floor.


If you already have the floor joists 16" o.c., you are ready to grab the materials needed to finish out your floor. 


The plywood choice you use is just as important as the fasteners and how you'll protect it over the next 60 days as well.


Of the 3 choices you listed, I'd never recommend any OSB being put outside that long, as it is easily prone to warping and scaling out. Even if you covered or coated it, moisture can and will find a way to get in-between the strands (the S in OSB) and break apart the glue that holds it together. 


That leaves you with 2 choices, the untreated 3/4" plywood or pressure treated 1/2" plywood.


If it were my shed, and I'd be using it frequently, doubling up the 1/2" treated plywood would be the best option. 

Pressure Treated Plywood 15/32


But it doesn't end with simply that recommendation. As stated before, getting the other items will guarantee you'll have a durable and long-lasting floor. 


The right fasteners and adhesive will make all the difference for ensuring your floors won't creak or have problems down the road. 


For starters, I'd recommend using galvanized deck screws that can hold up to whatever time and the elements dish out on it. 

Deck Mate #9 2 1/2 Inch Coarse Interior/Exterior Screws


To go a step further to fasten the plywood to the joists, use a strong construction adhesive. One that is very water-proof and extremely strong is shown below. Use it between the 2 layers of plywood as well as the top sides of the joists to further bond the floor down.

PL Premium Advanced Construction Adhesive 

Now that you've got the items for your floor installation, its time to protect your new floor for the next 60 days. Contrary to popular belief, having a pressure treated surface does not make it 100 % weather-proof. The materials used in it prevents rot and decay, but it can warp after prolonged exposure to rain and the elements. The easiest solution to this for the next 60 days is by covering your new floor with .3 mil or thicker sheets of plastic. Allow for overhang so the 2 x 6's can be covered, stapling them underneath the joists to allow the best protection. After putting the plastic down, I'd recommend to occasionally check the plastic over the floor for any leaks or tears, especially after a good rain.


Lastly, if this shed is going to having electricity running to it, I'd strongly suggest anchoring it into the ground. Most local codes (and common sense) states to anchor the structure whenever you are permanently wiring the structure. While I won't get into wiring details since I don't know how you'll be using the shed once its built, using eye anchorscan easily be installed to ensure you'll have a secure shed. And as always, if you do plan on running a sub-feed out to your shed to power up your lights, receptacles, and any larger circuits (generator, compressor) consult with a licensed local electrican  who can tell you more about local codes and more in-depth information.


I hope this information has helped you out, and please let us know if you have any further information.




Posted 2012-04-07T16:09:17+0000  by Joseph_HD_ATL

Thanks for all the information.  I have seen 3/4" PT plywood and I am guessing that is a viable option in this case.


Before the floor goes down.

I will dig up all the excess weeds and spray weed and killer.

I will lay plastic or landscape cloth down and cover with gravel.

I am planning on laying plastic across the joists prior to the ploor going down to create a moisture barrier.

I am going to tie the joists together by inserting crosspieces.


There 15 concreate blocks supporting this structure.  Holes were dug and gravel was laid and stamped down in the bottom of the holes.  Here is a picture of where it stands now.  I have 4 30" anchors that will have cable running through the floor and into the header to hold the shed down.



Posted 2012-04-07T22:02:27+0000  by 06fusion4us



My name is Tom, also known here as HD116 and as a remodeling contractor I have built a few sheds and other structures.


I would use 3/4" pressure treated plywood as you are using 16" centers on your joists. You may have been able to use 1/2" with 12" centers, but even in that case I would have suggested the 3/4" for longevity.


Further, if you are going to leave the flooring exposed for an extended period, buy some inexpensive 2 mil plastic sheeting to cover it during the down time. Keeping it dry will make a difference in the finished product.


Hope that helps.

Posted 2012-04-10T18:59:29+0000  by HD116
Thanks for all the information. I am laying down some landscaping cloth and gravel under the shed. I am also going to use plastic under the floor as a moisture barrier. I am hoping to have floors done in the next few weeks.
Posted 2012-05-01T01:31:17+0000  by 06fusion4us


I would personally go ahead and paint the plywood flooring with a couple coats of acrylic porch and floor paint. Acrylic floor paint has the ability to flex, oil does not. Plywood with age tends to have the grain separate. The acrylic can span the grain. Glidden P& F is self-priming. Behr P&F requires a primer.

Posted 2012-05-01T06:08:31+0000  by ordjen

I'm not sure if in your area shed and outbuildings require building permits but if they do most codes call for setback of 3 feet or more from property line. Comparing concrete block (16") to the space in between framing and fence it looks to me like your setback is more like 2’ - 2-1/2 ‘feet the most .


Also in some areas, for structures that are over 120 SF or 12ft and longer they require concrete pad and anchors.


Floor framing looks good they only thing I would add is some more blocking -your rim board is bowed inward - maybe a concrete block or two in the center of the floor  would help too.


As far as the plywood goes i think you're better of using tongue and groove plywood or 3/4 T&G. 3/4 pressure treated plywood comes in square edge only and square edge will leave you with  gaps (long side)  in between sheets. Plywood sheets are not suppose to be butt tight, there should be 1/8 “expansion gap in between sheets. Otherwise with temperature changes floor can bow upward.


I would recommend using something called DRY PLY which is basically water repellant plywood. This product is not intended to be used in exterior applications but it can be left exposed in normal conditions for 30 days.

For longevity purposes you can seal bottom of the sheets with foundation tar or wood preservative.


Also don’t use screws to fasten plywood, screws will lock plywood and again will  not allow for expansion and contraction, use ring shank nails (2-3/8) in combination with subfloor adhesive.



Hope this helps,



Posted 2012-05-01T21:17:25+0000  by George_HD_CHI
@steeltoes - You are correct, the area I am in requires 3' and that is what I have. I added cross braces between the floor joists. All the braces are screwed in with exterior grade screws.

Follow Up---
I laid the floor today and I used pressure treated lumber. 6 4x8 sheets covered the entire structure floor. I added 10ml plastic between the joists and floor for protection.

I see a suggestion above and will paint the floor in some fashion.

Now, I am planning to construct the walls. I am wondering if I can use pressure treated 2x4 studs instead of kiln dried studs. Any issues with this? Pros and Cons?

Posted 2012-06-26T00:03:09+0000  by 06fusion4us
I forgot yo mention I have 4 30" anchors that will have cables running through them into the shed's headers.
Posted 2012-06-26T00:05:08+0000  by 06fusion4us

You can, but I wouldn’t.


I would only use PT lumber for the bottom plates.


When used for studding pressure treated studs tends to warp and twist and that’s mainly due to drying process of the preservative. If you really wish to used something other than the regular studs look in to Bluwood.



 Hope this helps.



Posted 2012-06-26T14:28:58+0000  by George_HD_CHI
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