Page 1 of 1
Posted: Thu Oct 14, 2010 2:58 pm
by kiddo
guys,
I'm building a heavy-duty shed (5x 3.5m). Constructed from 100mm Softwood, it will be clad in 32mm Featheredge and have 12mm ply lining inside.
I am clearing an area for the base, but not sure how to go about the slab. This was what I thought.....
Dig 150mm down to remove loose topsoil.
Compact MOT1 to approx 75mm
Then compact 25mm sand so the DMP doesn't rip.
Then construct shuttering to pour a 150mm Slab, with a layer of A451 REBAR inside.
Does that sound right ?
I know the DPM is important to stop water soaking into the base from capillary action, but even if I put a DMP down, what is to stop water socking into the sides of the slab ?
Many thanks for any words of advice you may have.
Joe.
Posted: Thu Oct 14, 2010 7:24 pm
by lutonlagerlout
the dpm really just makes the concrete a better job
very little capillary action through a concrete slab.just make sure water cannot lay on it
your design looks very good
LLL
Posted: Thu Oct 14, 2010 8:10 pm
by Mikey_C
surely the dpm will come up the side?
i'd personally go a bit lighter on the sand 10-15mm.
Posted: Thu Oct 14, 2010 10:23 pm
by GB_Groundworks
a451 to heany duty a142 will be fine and cheaper
Posted: Fri Oct 15, 2010 6:34 am
by lutonlagerlout
if you are really worried you could paint centaprufe around the edges joe when its done
you need about 2.6 M3 of concrete i would go for one of the we mix you lay companies and get a real strong mix c30-35
i say this because we only pay about a tenner a cube more for the strong stuff than we do for leanmix
most of the cost of it is labour and haulage
I like your plan,its well thought out
no need to compact the sand though,only the MOT needs compacting,the sand can just be screeded off to level
LLL 
Posted: Fri Oct 15, 2010 10:50 am
by kiddo
Cool, thanks guys. Glad I am on the right track. I'll screed using a little less sand, use A142 rebar and will DEF get a mix company in, lol.
One thing I am still unsure off:
I need to build the shuttering, and to stop it giving way under the weigth of the concrete, I will need to stake round the edges, so what do I do with the edges of the DPM ?
If I allow it to raise up over the shuttering, I'll end up with rumples in the base, and a foot of DMP showing round the outside of the slab ?
If I cut the DPM to the exact size of the slab and then pour the slab, won't water just seep inbewteen the DMP and the SLAB down the line?
Not sure if I have explained that very welll ? lol
Posted: Sat Oct 16, 2010 8:23 am
by seanandruby
Personally i would'nt go for the sand, id just place the visquinn on the DTP1. Nor' would i bring the dpm up the sides, it will look ugly, it is only a shed base. once you've laid concrete and it's gone off, you can nail dpm to bottom of timber
Posted: Sat Oct 16, 2010 9:25 am
by mickg
to take it one step further you could lay a course of engineering bricks to the size required on top of the new concrete base, lay DPM on top of the course of bricks and build the shed off the bricks, I would do it this way so the timber is not sat on concrete and long term will not rot
the sand is only required to stop the stone from piercing the visqueen, if the MOT is well compacted and has no loose stones all you need is a very thin layer of sand
assuming that you have allowed additional concrete base size to the shed size another way is to give the area of concrete outside of the size of your shed a slight slope outwards so any water will fall away from the timber shed but this can be difficult if you are not experienced in concreting