I'm building a shed using untreated wood. I have the floor built and walls framed but before I put them up I want to waterproof the bottom of the floor. The floor rests on stone blocks so it is off the ground. I'm wondering if I can use something like flex seal to coat the bottom of the floor to protect against nighttime moisture and rain splash. OR I know I could use something like thompsons water seal however I've heard you have to use that once a year and once the shed is built I will not be able to get underneath the shed to do it again. Just wondering what is best to do between both aforementioned ideas or if anyone has any ideas that might be better and more permanent. I don't forsee a lot of water getting under there but I don't know what is best. Thank you for any help you can give.

 

Welcome to the forums.

I'm not the pro here but.................

I would never allow untreated lumber to sit on damp ground.
Pressure treated lumber is available for just this purpose.

The carpenter pros will be by to comment.

 

I have it raised off the ground using concrete blocks as a foundation. I leveled the ground first, put down crushed gravel and seated the blocks in before laying the floor over them.

 

I don't understand the hindsight. You didn't use pressure treated wood for the floor framing, and now you want to treat the wood. How will you get to the underside to treat it? You could lay down 6 mil plastic under the shed to prevent as much migration of moisture as possible, but it won't stop it all.

 

What size are the floor joists? That raises the plywood more. An 8" block and 2x6 joists would put the plywood a foot off of the ground. Do you expect a lot of rain to run under the shed? or for the underside to stay damp for an extended period? The joists that come in contact with the block piers should be pressure treated!

I built a shed for my grandson 12 yrs ago. The floor is approximately 1' off the ground. I used regular plywood and it's still in great shape.

FWIW - TWS has a short life when exposed to the elements [6-12 months] but will last a lot longer in unexposed areas.

 

Floor joists are 2x6 blocks are approx 6in. This is my first shed building and I goofed up by purchasing untreated wood and didn't realize until after the floor was built. I'm thinking I might dig down underneath for some more clearance as well as lay some plastic down and waterproof the underside with thompsons. The extra clearance would give me the height needed to slide under there to waterproof it again next year and keep it intact. Also currently the walls are not attached so I can flip the whole thing over to treat it this first time. I'm not sure about how much rain would splash under just know that when it drops off the roof there will be splash. I'm thinking this is going to be my best option for right now other then scraping the floor and buying pressure treated which I don't have the cash to do.

 

Would it be feasible to lift the floor framing and insert a PT 2x sideways to bridge the gap between the block and the white wood? How do you plan to secure the framing to the piers?

I'd be more concerned with rain water splashing on the bottom portion of the exterior walls than the underside of the floor. It's not a great idea to dig the ground lower as that will allow water to pool under the shed. It's better for any moisture that gets under there to drain/evaporate quickly. Ventilation normally takes care of that.

 

That is actually exactly what I did. I lifted the floor off the blocks, water proofed the underside of the floor and then attached doubled up 2x6 pressure treated to each side that rests on the blocks. It has lifted the shed up enough that I can get under there to waterproof again should it need it. I think this should be good now? I also removed the grass under the shed and laid down crushed gravel for water drainage. I figure this will also help with splash back from rain dripping off the roof as I did it 6in around the sides too.

 

Once the shed is finished I plan on pouring concrete into a 90 degree mold covering the blocks in each corner. This will stop the shed from moving in any direction but still allow me to get underneath when necessary. I'll bring it up 6 inches higher then the floor for strength.

 

Post #9 has me confused, the concrete footer for the blocks gets poured before the blocks are laid.

 

The way I picture it post # 9 sounds like rot waiting to happen.

 

This whole thing sounds like a disaster