Replacing OSB shed floor with concrete.

Deregos.

We had concrete patio and paths layed around the back of the house four years ago, but I didn't cop that they got the angle of the fall wrong around one side of the shed, so unbeknownst to me the water was seeping in and rotting the OSB floor below the lino that was covering it.

The shed is 12x18 and my first thought is to rip out all the OSB, take out the blocks, level off the stone, put down some polythene and pour in five inches of concrete.

Is there any alternative suggestions or any other tips I should consider before getting stuck in?

10-10-20

I'm initially wondering about the long-term viability of doing any work in there. Is the ledger in the frame of the shed sitting on the OSB there or the blocks directly, and is there a DPC to prevent wicking up into the ledger/frame?

Given that you won't be doing the foundations, a concrete pad might be a waste if the shed needs to be replaced - what about using binding grit and a whacker?

If you do install a DPC you'll need some blinding sand over that 804, otherwise the membrane will be punctured.

Is that a wooden riser below the framing, or is it block. If block then you should be good to go - if wood it's a bodge that won't last.

The Continental Op

I'd jack the shed up and put in a concrete, block or wooden sill plus DPC strip all the way around then concrete the floor. Might even put in a small foundation for the sill.

Very boring to jack up but it can be done working around the shed from about 7-8 locations 5-10mm at a time. Just put jack under a block attached to the additional uprights. Add some cross bracing while your at it.

Not aimed particularly at the OP but I don't understand why people put sheds with wooden floors up on single blocks with next to no ventilation under them. I've two sheds here one over 20years old and the wooden floor is still 100% but its up at least two blocks above ground level with loads of ventilation.

Deregos.

https://www.boards.ie/discussion/comment/122573463#Comment_122573463

It's a 9 X 2 wooden riser we put on top of the OSB flooring which was laid on blocks to give some extra head height in the shed. I pulled out one of the blocks no problem, so I'm thinking I could insert a DPC all the way around the perimeter and push the blocks back in again.

Lumen

How are you going to shutter the concrete pour?

https://www.boards.ie/discussion/comment/122574089#Comment_122574089

Your not laying concrete up to that and winning. It would just amplify the problem and make that 9" beam rot far more quickly.

As the continental op suggested, jack it up and get at least two blocks in with air gaps between the blocks, then build another suspended floor off the 9" timber. If the 9" timber won't take the joints hangers then you are fairly screwed anyway.

Deregos.

https://www.boards.ie/discussion/comment/122574095#Comment_122574095

See, this is why im seeking advice. Are you saying that the weight of the concrete could push the sides of the shed out? I hadn't thought of that.

Deregos.

https://www.boards.ie/discussion/comment/122574121#Comment_122574121

What if i eliminate the 9x2 timber riser al ltogether and use all those blocks I have to underpin the shed with block footings with DPC on top? Could that work?

Think about something like this for retrofitting to the 9". It's what I was planning to use on my recently built shed, but I went with screw pipes in the end.

https://shedbasekits.com/product/design-your-own-kit-with-quickjacks-for-hard-surfaces/

Lumen

https://www.boards.ie/discussion/comment/122574126#Comment_122574126

An important consideration here is what sort of threshold you want. A suspended timber floor will have a high threshold, which makes it unsuitable (or at least less suitable) for certain uses, like rolling heavy equipment in any out (which would also add load to the suspended floor).

A poured concrete base can give a nice low threshold and enough strength for anything you put on it. That's not to say it's better, just better for certain uses.

As others have suggested, if you do want concrete, you want the shed on the pad, not around it. The best way to achieve that depends on tools, skills, access, shed finish etc.

Deregos.

https://www.boards.ie/discussion/comment/122573413#Comment_122573413

Hadn't heard of binding grit, looked it up and not sure it will be tough enough as a shed floor. Apart from keeping bikes, gardening stuff and lawnmowers in there, I do a lot of repair work and tinkering of all sorts. I'd hate to be constantly worried about doing anything that'd make the floor unravel.

Deregos.

https://www.boards.ie/discussion/comment/122573763#Comment_122573763

I'd jack the shed up and put in a concrete, block or wooden sill plus DPC strip all the way around then concrete the floor. Might even put in a small foundation for the sill.

Sorry, I didn't understand what you meant at first but I get that now. This seems like a good suggestion.

10-10-20

https://www.boards.ie/discussion/comment/122574331#Comment_122574331

Yep, sure. It only supports some use-cases.

GreeBo

You could make a picture frame/strip foundation around the base by pulling out a couple of blocks at a time and making the strip in multiple 1M pours. As it goes off move onto the next one.

You can then choose to pour some pads or use more blocks to make interim supports for a new suspended timber floor, lay some joists and put down new OSB?

It might be fun getting a DPC in between the new foundation and the shed floor, but if you leave some gaps in the strip there should be enough ventilation, but I would use something to keep the mice/rats out.

If you do choose to use grit, putting down some rubber "gym" mats would make for a nicer floor to work on and would also protect the grit.

Deregos.

https://www.boards.ie/discussion/comment/122575160#Comment_122575160

Thanks for that Greebo, do you know would there be any point lobbing down some insulation before pouring the concrete or would that be an unnecessary extra expence?