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Posted: Thu Sep 20, 2007 11:51 am
by arthur daley
Hi
I am about to put down a concrete base for a 10x8 shed in my garden. I have laid and compacted hardcore and put down some shuttering.
Next i need to put in some blinding sand and then put in a sheet of Visqueen dpm. So far so good.
When the plastic sheet goes in and lines the shuttering and pokes up above the shutter - do I cut the plastic so that it only comes half way up the shutter? The edges of the slab will be covered by a bed of gravel surrounding the shed. How do I finish the dpm - does it just 'flap' around loose ? On the web site it says the dpm is just to aid the drying/curing process but i always thought it was supposed to stop damp from rising through the slab?
Alternatively do I lay the dpm and then put the shuttering on top of the dpm and then cutt off the excess plastic when the shutter is removed?
regards
Arthur
Posted: Thu Sep 20, 2007 11:59 am
by Ted
The DPM is basically a vapour barrier to prevent the concrete fromn drying out too quickly. I just put it under the shuttering. If you were trying to install a traditional DPM you would need to incorporate a DPC and marry the two together. Sheds don't generally have DPCs, particularly timber sheds.
Posted: Thu Sep 20, 2007 12:18 pm
by arthur daley
thanks for that Ted
The question first came up when I had a slab cast to use as a base for a sectional concrete garage - where the slab would be the floor of the garage and not just a base as withthe shed.
So how should it have been done with the garage?
regards
Arthur
Posted: Thu Sep 20, 2007 1:07 pm
by Rich H
Can't answer your second question (not my field) but sheds definitely don't need a dpm.
Posted: Thu Sep 20, 2007 2:53 pm
by Tony McC
I *always* recommend a vapour barrier beneath *any* cast in situ concrete slab. It's sometimes referred to as a DPM or a slip membrane, but the key function for these lightweight projects is preventing early loss of mix water and the effect this has on the design strength because of the reduction of the water:cement ratio.
Posted: Thu Sep 20, 2007 4:15 pm
by very simple simon
listen to the boss...membrane is the way to go. You need a bit of water to get a finish, esp. for a shed base which will be the finished floor. If it bleeds out too much, squeegee it off gently, but better too much water comes to the top than it all goes into the stone underneath...result is a floor with more cracks than a brothel
go easy with steel float too...or you will regret it when the floor dusts up when you walk on it
best of luck arfur
Posted: Thu Sep 20, 2007 5:08 pm
by lutonlagerlout
go easy with steel float too...or you will regret it when the floor dusts up when you walk on it
eh? the idea of floating concrete simon is to push the larger aggregate slightly away from the surface ,leaving the fines which can be troweled up,same as plastering
if the floor dusts up then the cement content must have been wrong,we always use cemex paving mix (strong mix with fibres used by pic mobs) for any finished oversite or base
it works well for us
cheers LLL 
Posted: Thu Sep 20, 2007 5:28 pm
by very simple simon
fibres & dusting ...ok I get the theory... but personally have had lots of probs with surface finish, fibres coming up etc.. bagged fibres have been bad for this, have seen floors looking like a plucked turkey.
as for floating...yes do float it, but don't get carried away...can end up with not only dusting but problems like delaminating as well. what I meant was don't float it more than you need to, finish it, smooth it, write your name in it if you want but don't just keep floating for the sake of it or you will get problems.
Posted: Thu Sep 20, 2007 9:51 pm
by lutonlagerlout
why would you keep floating once its done?
sometimes we get a batch of crete that lacks fines and you have to give it the extra tamp to get the fat up,but once its troweled ,walk away and dont look back
LLL
Posted: Fri Sep 21, 2007 7:01 pm
by Tony McC
How to get rid of those pesky fibres at the surface? Blow torch! Just threaten 'em with a bottle of calor gas and they shrivel up to nothing. 