[Kakosh]

Hello everyone- Got a question about the thickness of the plywood for a shed 8x12 feet. First time builder.

I used pressured-treated wood and 1/2" thick plywood for the foundation. It does not feel very sturdy, and I realized I should have used 3/4" thick plywood.

a few options I have:

1) buy three more sheets 4x8 with 1/2" and to make a second layer
2) remove the existing 1/2" and get three new sheets 3/4" thick
3) I have lots of leftover hardwood flooring that's 4" wide, 3/8" thick. I can keep the current 1/2" thick foundation and put hardwood floors once the shed is done.

What should I do?

 

[Nealtw]

Hello everyone- Got a question about the thickness of the plywood for a shed 8x12 feet. First time builder.

I used pressured-treated wood and 1/2" thick plywood for the foundation. It does not feel very sturdy, and I realized I should have used 3/4" thick plywood.

a few options I have:

1) buy three more sheets 4x8 with 1/2" and to make a second layer
2) remove the existing 1/2" and get three new sheets 3/4" thick
3) I have lots of leftover hardwood flooring that's 4" wide, 3/8" thick. I can keep the current 1/2" thick foundation and put hardwood floors once the shed is done.

What should I do?

By foundation. do you mean to say floor?

 

[Kakosh]

I placed the foundation on concrete blocks, with 4x4 sides and 4x4 runners. Then I put the plywood as a floor. Let me try to attach a picture, hopefully you can see

.

https://www.diychatroom.com/members/kakosh-577841/albums/foundation/

 

[Nealtw]

I placed the foundation on concrete blocks, with 4x4 sides and 4x4 runners. Then I put the plywood as a floor. Let me try to attach a picture, hopefully you can see

.

https://www.diychatroom.com/members/kakosh-577841/albums/foundation/

I would say that you build a floor frame on concrete blocks that you used as a foundation.

Yes if the floor is too flimsy adding another 1/2" would not hurt.

 

[Bondo]

I would say that you build a floor frame on concrete blocks that you used as a foundation.

Yes if the floor is too flimsy adding another 1/2" would not hurt.

Ayuh,..... Or flip it over, 'n add 2x4 stringers under the floor, every 16",......

 

[Nealtw]

Ayuh,..... Or flip it over, 'n add 2x4 stringers under the floor, every 16",......

Hmm,that too. :wink2:

 

[XSleeper]

Your "runners" would be called floor joists. You concrete blocks are the foundation.

If your shed is 8x12, the joists would either be 12' long and you plywood would run the 8 ft dimension. If your joists are 8 ft long, the plywood would run perpendicular to that, and would run the 12 ft dimension. Plywood has a strength axis, so it always runs perpendicular to the joists. If you run it the wrong way it will feel springy and soft. So which way do the joists run, and which way does the plywood run?

Spacing the joists too far apart will also make it feel springy and soft. So what was the distance between your joists?

 

[huesmann]

i.e. what do you have under the plywood—is there anything between the 4x4 perimeter?

 

[HotRodx10]

I'm not sure on the framing system, so I won't comment on what you need to do there.

I will tell you that the stiffness of the panels is related to the thickness by a cubic term (relative stiffness is thickness to the power of 3). IOW, one thickness of 3/4" plywood is 70% stiffer than 2 thickness of 1/2" plywood. Also, 1-1/8" plywood is almost 3 1/2 times stiffer than 3/4" and 4 1/2 times stiffer than 2 layers of 1/2".

 

[Kakosh]

Your "runners" would be called floor joists. You concrete blocks are the foundation.

If your shed is 8x12, the joists would either be 12' long and you plywood would run the 8 ft dimension. If your joists are 8 ft long, the plywood would run perpendicular to that, and would run the 12 ft dimension. Plywood has a strength axis, so it always runs perpendicular to the joists. If you run it the wrong way it will feel springy and soft. So which way do the joists run, and which way does the plywood run?

Spacing the joists too far apart will also make it feel springy and soft. So what was the distance between your joists?

The shed is 8x12. Joists (2x4s) are 8 ft long and 16" apart. Plywood is not perpendicular, they run in the same direction. That explains why it is feels springy. 3 plywood boards (4x8) covered the shed floor, I did not realize the direction of the plywood was an issue. Thank you, I think I will need to undo the plywood.

 

[Kakosh]

i.e. what do you have under the plywood—is there anything between the 4x4 perimeter?

Yes, I have 2x4's running every 16". I will need to take the plywood out and buy thicker plywood and install perpendicular to joists.

 

[Kakosh]

I'm not sure on the framing system, so I won't comment on what you need to do there.

I will tell you that the stiffness of the panels is related to the thickness by a cubic term (relative stiffness is thickness to the power of 3). IOW, one thickness of 3/4" plywood is 70% stiffer than 2 thickness of 1/2" plywood. Also, 1-1/8" plywood is almost 3 1/2 times stiffer than 3/4" and 4 1/2 times stiffer than 2 layers of 1/2".

thanks, I will switch to 3/4" plywood.

 

[HotRodx10]

Now that you've provided more information, the bounce that you feel is likely more because the the 2x4s are spanning 8'. 1/2" plywood shouldn't give much with supports at 16" centers, but the 2x4s will flex quite a bit at that span.

 

[rdy2go]

thanks, I will switch to 3/4" plywood.

When you remove the plywood, you could run at least two lines of 2x4 stringers and place a few more concrete blocks on center of selected 2x4x8. That would help some with the 2x4 flexing.

 

[jeffs55]

Unless you glue the two pieces of 1/2 inch plywood together, you do not have the same strength as one piece of 1 inch plywood. It sounds like you do, but you don't. You have two pieces of wood sliding over each other. Use thicker plywood, glue the thinner pieces together very very well, add floor joists and use minimum 2 x 6 for floor joists. 2 x 8 better and 2 x 10 more than adequate for this small shop.

 

[HotRodx10]

In case you were wondering, the stiffness increase being cubed with increases in thickness also applies to the depth of the joists. So, 2x6 joists will be nearly 4 times stiffer than the 2x4 joists (and nearly 2.5 times stronger).

 

[Kakosh]

Unless you glue the two pieces of 1/2 inch plywood together, you do not have the same strength as one piece of 1 inch plywood. It sounds like you do, but you don't. You have two pieces of wood sliding over each other. Use thicker plywood, glue the thinner pieces together very very well, add floor joists and use minimum 2 x 6 for floor joists. 2 x 8 better and 2 x 10 more than adequate for this small shop.

I will also add two more concrete bases. As for the joists, do you think I should replace 2x4s with 2x6s (or 2x8)s given the size of shed 8x12?

 

[HotRodx10]

I will also add two more concrete bases. As for the joists, do you think I should replace 2x4s with 2x6s (or 2x8)s given the size of shed 8x12?

Whether you should add deeper joists depends on several factors. If you are going to add support in the middle of the 2x4 joists, replacing them is not necessary. If not adding supports, if you anticipate having heavier floor loads than what you've had on it so far, or you want it to feel stiffer, you should consider adding 2x6 joists. You can just 'sister' them onto the existing 2x4s, by laying them along side and nailing or screwing the 2 pieces together. The 2x6 doesn't need to go the full length, just so the it spans across the middle and most of the way to the end (6'-7') if there are interference problems with other framing.


I should add, you should screw the flooring down to the sistered 2x6s, also, at about every 6" near the ends of the 2x6s, and every foot or so in the middle.

 

[Kakosh]

Thanks everyone for your comments. I rebuilt the foundation with these changes and added pictures to the album.

- Mains are 4X6, joists are (2x6) (instead of 4x4 and 2x4s)
- Plywood is thicker (3/4" (instead of 1/2" first time)
- added 6 more concrete bases (there was 4 before, now joists have support in between). Now there are 10 concrete bases.
- I laid the plywood vertically (except one to fit in 8x12).
- added two more joists (total of 12, there was 10 before)

Foundation feels very sturdy now, thanks for your help, it made a big difference.

I have a new question. I have 10 treated 2x4s and three plywood pieces (4x8) 1/2" thick. Can I use this material as studs when building the shed? I think treated wood has chemicals injected so is it safe to use this material for the shed?

 

[HotRodx10]

I have 10 treated 2x4s and three plywood pieces (4x8) 1/2" thick. Can I use this material as studs when building the shed? I think treated wood has chemicals injected so is it safe to use this material for the shed?

The main cautions with the older CCA treated lumber is that you should wash your hands after handling it (or wear gloves), and don't leave it exposed where it will be handled or come in contact with food. The only limitation with the newer ACQ treated lumber is that you shouldn't let it come into direct contact with food.



Installation as wall studs is a perfectly fine use for treated lumber. It's commonly used for sill plates in walls on concrete. The only reason it's not commonly used for studs is that it's unnecessary and more expensive.