Forgot DPM

[asimov]

Hi,
I have made a concrete base (5ft x 7ft ) which is my shed floor. The problem is, when i had finished laying the concrete i realised that i had forgotten to put the plastic DPM sheet under it.

Other than rip it all up and start again, what are my options for damp proofing it.

Thanks
Dave.

 

[plugwash]

hmm

I suspect you can lay the sheet and then lay more concrete on top. What I don't know is how thick that more concrete would have to be.

 

[asimov]

hmm

I suspect you can lay the sheet and then lay more concrete on top. What I don't know is how thick that more concrete would have to be.

Not that easy. I first laid a course of concrete blocks. I then filled the center with hard core, then my layer of grano to form the finished floor. I am then going to lay a further three courses of concrete blocks. Then finishing off on top of the blocks with marine ply walls.

Would it be possible to use one of the liquid damp proofing. Would this do the job??

 

[freddiemercurystwin]

Just stick some timber bearers down and then sit your shed on those, with a dpm between. This is all thousands of people do when they stick their shed on an existing slab or patio. You're thinking too much.

 

[ivixor]

Who cares, it's a shed! Any moisture will just waft away through the gaps in the structure.

 

[asimov]

Who cares, it's a shed! Any moisture will just waft away through the gaps in the structure.

No, it`s much more than a shed. This is an extension to the main 7ft X 20ft brick built workshop. It has footings, then a course of concrete blocks in which i have poured the concrete floor. Then another three courses of concrete blocks, before building the wooden framework on top of the blocks. Which is then covered with marine ply.

So can anyone recommend a good remedy to the problem please.

Thanks
Dave.

 

[maltaron]

I am confused with your floor construction, first you say you laid concrete on hardcore, then later grano on hardcore. It is normal to lay concrete first then around 30-40mm grano finish. If this is what you have done I would suggest that for the want of a bit of extra work and grano, you could lift the grano layer, lay a liquid DPC (or sheet DPC) and re grano.

 

[asimov]

I am confused with your floor construction, first you say you laid concrete on hardcore, then later grano on hardcore. It is normal to lay concrete first then around 30-40mm grano finish. If this is what you have done I would suggest that for the want of a bit of extra work and grano, you could lift the grano layer, lay a liquid DPC (or sheet DPC) and re grano.

Hi,
I put three inches of grano on top of the hardcore (no concrete), but forgot to install a DPM. Sorry for the confusion.

 

[SimonH2]

How critical is the floor height ? And in particular, relative to the walls ?

Stick a wooden batten round the perimeter (inside edges where the inside edge of your blocks will be), lay in a DPM, pour in another couple of inches, level etc. You now have a solid floor on top of the DPM, and it's not likely to move unless the original surface was really smooth.

Remove the battens and put the blocks down as originally planned - the first block sill be partly hidden by the end of the extra layer. You'll lose headroom to the thickness of the extra layer.

Or using more material, put the first layer of blocks down, lay in the DPM and pour an extra layer. Then build everything one block higher than planned.

 

[asimov]

How critical is the floor height ? And in particular, relative to the walls ?

Stick a wooden batten round the perimeter (inside edges where the inside edge of your blocks will be), lay in a DPM, pour in another couple of inches, level etc. You now have a solid floor on top of the DPM, and it's not likely to move unless the original surface was really smooth.

Remove the battens and put the blocks down as originally planned - the first block sill be partly hidden by the end of the extra layer. You'll lose headroom to the thickness of the extra layer.

Or using more material, put the first layer of blocks down, lay in the DPM and pour an extra layer. Then build everything one block higher than planned.

Hi,

I have now completed laying the blocks, before i read your post. What do you think about the liquid damp proofing, that i can just paint onto the floor. Any one better than another??

Thanks
Dave.

 

[SimonH2]

If this it the floor that will be taking traffic etc then I'd be wary of any paint-on option as the surface wears away with use.

You could still lay an extra layer of concrete/whatever, it'll just be harder to do. Put battens down two sides, cut a plank that'll fit between the walls (not too short, you can use it on a bit of a diagonal if it's on the long side), lay in a DPM, pour in the extra layer, and level it with the board.
Be careful not to damage the exposed DPM while levelling the fill. Once you've done and it's set, remove the battens, lay the DPM down into the sides, and fill in the narrow trench.
Or use vacuum impregnated wood for the bettens and just leave it there.

 

[asimov]

If this it the floor that will be taking traffic etc then I'd be wary of any paint-on option as the surface wears away with use.

You could still lay an extra layer of concrete/whatever, it'll just be harder to do. Put battens down two sides, cut a plank that'll fit between the walls (not too short, you can use it on a bit of a diagonal if it's on the long side), lay in a DPM, pour in the extra layer, and level it with the board.
Be careful not to damage the exposed DPM while levelling the fill. Once you've done and it's set, remove the battens, lay the DPM down into the sides, and fill in the narrow trench.
Or use vacuum impregnated wood for the bettens and just leave it there.

As the floor size is only 4ft X 7ft, do you think i am likely to have much problems with damp anyway.

 

[SimonH2]

Dunno - there's a lot of variables, and it's outside my knowledge/experience.

It might be that if the water table is low and/or the ground is well drained, that the layer of packed hardcore will keep enough of the ground moisture away for it to not be a problem.

And then if it's a draughty shed with loads of holes then it will "breath", but if it's sealed up then that would trap moisture in the air inside.

And then what's it going to be used for ? If all it's going to have is the lawnmower and a few tools (which are hung on the wall or put on shelves rather than on the floor) then it might be OK.

One of the biggest problems is temperature changes. When it's warm, that might induce moisture to come out of the floor into the air. If it turns to a cold night, then that moisture will condense out.

 

[asimov]

Dunno - there's a lot of variables, and it's outside my knowledge/experience.

It might be that if the water table is low and/or the ground is well drained, that the layer of packed hardcore will keep enough of the ground moisture away for it to not be a problem.

And then if it's a draughty shed with loads of holes then it will "breath", but if it's sealed up then that would trap moisture in the air inside.

And then what's it going to be used for ? If all it's going to have is the lawnmower and a few tools (which are hung on the wall or put on shelves rather than on the floor) then it might be OK.

One of the biggest problems is temperature changes. When it's warm, that might induce moisture to come out of the floor into the air. If it turns to a cold night, then that moisture will condense out.

Hi,
I take your point. I think i will wait awhile and see what happens, before i do anything.
Thanks

 

[michaelbailey]

get in touch with an asphalting firm