11mm osb3

[Sean foley]

I am putting osb on the underside of a garden shed/room I am building to hold the insulation in place , it will be fixed to the  underside of the floor joists , the osb3 is 11mm thick and I am wondering will it crush under the weight of the shed ? It will be in direct contact with concrete blocks which are the foundations 

[Temp]

I think it will be fine but I'd put a DPC between the blocks and osb. 

[Sean foley]

4 hours ago, Temp said:

I think it will be fine but I'd put a DPC between the blocks and osb. 

Would it be necessary to use doc or will I be fine with out it , it’s osb3 it says it’s weather resistant 

[Mr Punter]

11 hours ago, Sean foley said:

the osb3 is 11mm thick and I am wondering will it crush under the weight of the shed

 

No it will not crush.

 

As long as the block are dry you will be fine.  If not, run DPC over them.

[Temp]

11 hours ago, Sean foley said:

Would it be necessary to use doc or will I be fine with out it , it’s osb3 it says it’s weather resistant 

 

You can probably get enough for £5.

[Nickfromwales]

11 hours ago, Sean foley said:

it’s osb3 it says it’s weather resistant 

It'll withstand a bit more exposure than regular OSB, nothing else, and will degrade quite happily if left exposed to very high humidity / water ingress etc so take that statement with a pinch of salt  

You need a DPC, go buy some ?

[Onoff]

If water can get on top of the dpc/dpm at the edges and sit between that and the underside of the osb it'll rot. Make sure the dpc doesn't stick out too much and form a "catch tray". Your dpc needs to be minimum 150mm above the surrounding ground. 

 

You can even think about covering the edge of the dpc with something like upvc strip and bringing the shed cladding down over it a bit. 

 

It certainly won't crush under the weight though.

[Sean foley]

Could I just staple the dpc directly onto the osb ?  And have it in tight so no catch tray is formed , I will also have cladding around the sides so it won’t be exposed , should be fine then

[scottishjohn]

might it be easier to just paint it before fitting ,as its not in direct contact with the weather?

or maybe use MGO board (cement board)

myabe if you heat the shed then you could get condensation between the osb and dpc ?

[redtop]

I have painted OSB board with bitumen paint successfully, its cheap and tough as.  Wouldn't hurt as belt and braces

[Carrerahill]

On 12/08/2020 at 00:29, Sean foley said:

I am putting osb on the underside of a garden shed/room I am building to hold the insulation in place , it will be fixed to the  underside of the floor joists , the osb3 is 11mm thick and I am wondering will it crush under the weight of the shed ? It will be in direct contact with concrete blocks which are the foundations 

Can you not use PIR insulation and do away with the OSB - this sounds like you are creating a cassette floor which will be prone to moisture issues and the OSB crushing will be the least of your worries in a few years.

 

Another option would be to use breathable membrane or mesh, sling it over the joists, insulation sits in the "hammock" and then fix it to the top of the next and so on and so forth, I'd not let it touch the ground though.

 

If it was me I would be knocking in PIR between them, if I was not confident in my cutting skills I would be fixing 25mm treated battens to the bottom of the faces of the joists then knocking PIR down onto them to seat.