Started making my shed / workshop – it’s 2.4 x 4.8m in size, and built on the somewhat wonky concrete pad of an old garage. I’ve used adjustable decking joist thingys to level it out – at the moment it looks like this.

The next step was to place tounge and groove OSB3 2.4m long 59cm wide 18mm sheets onto it to act as the floor. I was then going to build the walls up from that point, and put the roof on.
However, as it’s autumn in the UK it’s very wet indeed, and realistically, once I put the floor down, it’ll take several weeks before I get the roof on (I can only work on it over weekends). So I’m a bit worried the OSB3 will not be happy with this situation – even once I’ve treated with decking stain.

Having pondered this I think it’d be better if I put the floor down only after I’ve got a roof on. This then presents a problem as the plan was that the end of the boards would be supported by the outer frame. Hard to explain, so hopefully a rubbish visio picture speaks a thousand words!

My plan B is to screw in some additional bits of treated timber into the outer edge and cut the boards down a bit. This would then support the timber ends, but have a couple of downsides – the ends would effectively only be held up by the screws (I’d probably glue it with some strong PU glue too) that attach the additional bits of timber and secondly I’d need to reduce the size of the insulation. Like this picture.

I’m inclined to go for my plan B – any thoughts on whether this is a terrible idea?

Get a big tarp to cover it when you aren’t working on it and until you get a roof on.

Just grab a big visqueen / damp proof membrane sheet. £35 from screwfix for enough to easily cover the floor and enough over to cover the roof while you get on with it.

Some of it is keeping my garage dry at the moment while I redo the flat roof.

I did consider the tarp idea, but I expect there will be a period where i have walls up (or maybe two walls) but no roof, meaning draping a big tarp over it will be pretty hard.

The DPM idea is interesting – do you mean put the floor down, then put the DPM over the entire thing, staple it to the outer frame. Leave it in place until the roof and walls are on, then cut out the inner portion to reveal the pristine floor? Can’t see any obvious downsides to this idea….

Plan B sounds fine to me. Just use plenty of long screws on the additional supports.

I’d put the floor in after – for no other reason that if any of it ever rots/gets damaged you won’t have to dismantle the walls to replace the affected bits.

Also, I’d make the floor from wood treated to the same standard as the outside walls so they’ll take a soaking.

Finally, put a dpm over the insulation before fittign the floor and run it up the walls before you fit the floor then trim the DPM flush with the floor – it’ll help keep the floorin dry from water/moisture coming up from underneath.

The plan for the walls is a frame of 2X4 pressure treated timber, clad with OSB3 and then treated featheredge on top of that.

For the floor Vapour barrier I was going to just tape the insulation up using foil tape – i’d understood that this was sufficent when using PIR board.

Not massively worried about replacing the floor at any point. Not planning to be in the house for more than a year or two, so it’ll be someone elses problem.

Now i’m in a ponder between DPM over the lot and cut it out later, or my plan B. DPM over the lot is certainly less work!

Bump for the evening crew – anyone want to settle my umming and ahhing? At the moment the DPM idea is winning – easier and doesn’t reduce insulation.

Option C: Cover the base with sheeting as it is to protect from the weather, then pre fab the walls and roof over a few weekends but don’t lift them into place (cover with the sheet each Sunday night).

Once you’ve got all the bits done, lift into place and fit the roof over a weekend (some mates to help one saturday morning at the cost of a bacon sandwich maybe), job jobbed.

I think the dpm idea is genius, but I also think you don’t want your timber wall frame sat on top of osb3 which is prone to get moist at its edge and fail a lot quicker than the timber walls will! I’d prefer to have the osb floor not part of the structure. I’ve no concerns with plan B, screwing extra timber battens will be fine to support the floor. Most of the strength is in the friction between the two timbers, so the screws are just in compression rather than shear load.

Be sure to use the green, impregnated, OSB designed for damp environments. It costs very little more than the plain stuff. And as above, make sure the edges are not exposed. Have you considered using decking boards instead of OSB for the floor. Not sure how much more expensive it might be, but would certainly be tougher and longer lasting in that environment. I have just floored out an outbuilding this way that is 2m x 4.8m in size, and the decking boards (38mm x 125mm) cost £150 +vat. That was enough cover the floor without the gaps of a normal decking.

Right, decided on definitely doing plan b (screwing extra timber in). Figured it would probably get damp under the dpm if I did that and then trap in the moisture.

Welshfarmer – using decent OSB3 which is ok for humid environments. The floor is purchased, so decision is already made 🙂