Shed Foundations

[obeaston]

Erecting a 7x8 foot shed for the boss (wife).
Want to have a concrete base 8x9ft to allow access for wood treatment.
Don't have a clue - so far I think:- dig out size, level ground, shutter with wood, add bags of premixed shed base and water and finish surface.
polythene first for dpc coarse to stop wood rotting.
How deep
Finish with only cement
How to allow a run off for rainwater.

Cheers guys - any help welcome the sheds coming on 14/01/08!!

 

[noseall]

polythene won't make a difference in an external location as rainwater will still cause the top of the slab to get wet.

include a 1:100 slope on the slab. this you can do with the shutter boards. it will hardly be noticed and will assist in surface water run off. slope it one way only, and towards the elevation that will allow for the best drainage.

you will need a 1.25 tonnes of ballast and 8 x 25 kg bags of cement. mix at a ratio of 6:1.

the surface can be trowel finished when the muck has firmed up.

dig out enough soil to accommodate the depth of the slab only, as the thickness of the slab will be strong enough. the subsoil would have to be particularly poor to have to need hardcore in this situation.

 

[Richard C]

It’s only a shed & a fairly small one at that; this is how I’ve always done mine. Dig out & level the area for the size base you want, lay down 50mm of compacted sharp sand & bed 1 ¼” slabs over that; I’ve also used reinforced fence barge boards before. Make the size of the base the nearest multiple slab width/length to give, say, a min. 6” protruding surround. I always cover the bottom of the shed floor & 2” up the sides with roofing felt before I build it up on the base. This will be perfectly adequate unless your going to put something seriously heavy in there, the shed will be dry & the base will outlast it - 20 years or more.

 

[joboatbuilder]

What if I've bought a metal shed, with no integral floor? I like the idea of a sand/slab base which would also double up as the floor, or will it let lots of damp thru, in which case would I have to put down a dpm sheet first?
It's only a 7'x8' for keeping the lawnmower and other garden tools in, so ideally I don't want it too wet and every thing rusting away prematurely!

 

[Richard C]

If the slabs protrude beyond the shed sides, rainwater will run off the shed, onto the slabs & seep underneath making it quiet damp inside. My main timber shed has a timber floor & laid on slabs as I explained in the previous post but I have built a large extension on one end which is used for storing a lawn mowers, wheelbarrows, garden tools & chairs etc. The sides & roof were built up on a strip base & I then laid a separate slab base inside, leaving a 4 inch gap all around the inside edge of the shed walls. It’s not as dry as the main shed with an integral floor but it keeps surprisingly dry inside, certainly good enough for storing garden items. The inside slab level is only just above the lawned area there are 2 large, barn doors for access so it’s ideal for wheeling things in & out. A way of making it even dryer would be to lay the internal slabs on a DPC membrane as you said.

 

[joboatbuilder]

top suggestion!
I'll leave a gap between slabs around the edge, ie under the walls, and then lay a slab floor on a dpm slightly raised from the grond level.
If it ever stops raining that is...