Concrete shed base

[Craig Graham]

We're planning to put a little log cabin/shed type thing in the back.

The ground isn't particularly nice so I was planning to dig it roughly level, place level ballast, position a sheet of concrete reinforcing fabric/latticework on some bricks or somesuch so the lattice is 5cm above the ballast and then pour 10cm depth of concrete to make a 3m by 3m slab that's not going to break into a couple of pieces sinking at different rates over time.

By my reckoning, this will be around 800 litres of concrete and, since a cement mixer holds around 100l of mix, around 9 cement mixers full. Neighbour has one I can borrow. However, manufacturer spec sheets say a 25kg bag of concrete mix (the largest in keyline's web site) will cover 0.1 suare metres at 10cm depth, meaning I need around 80 bags at a cost of around 600 quid. I can get that much concrete delivered for £118. Am I right in thinking delivery is the cheapest option?

Whatever way I get it, I will have to work on the reinforcing fabric; moving a wheelbarrow over it and standing on it. I'm considering some boards to spread the load and stop me tripping over the bars. Is this the way it's usually done or will this end up knackering the latticework? How widely spaced will the supports have to be to stop the lattice buckling under both my weight and the weight of a wheelbarrow full of concrete?

Am I going over the top to be thinking of reinforcing the concrete? The fabric sheet only costs around £40 and I'm not sure what the ground is going to do over time and under load; there's a lot of rubble under there, since I gather some buildings were once there (used to be railway sidings and yards).

-- Dr. Craig Graham, Software Engineer Advanced Analysis and Integration Limited, UK.

[Rick Dipper]

I guess your "latice" will need to be reasonably thick to be any good, I was using 2 layers of a393 sheet (10mm bars, 200mm squares) in 300mm of concrete, its enouth to hold a house up. a393 sheet is 32 quid a sheet (4.8x2.4m). You support the mesh on briks, it MUST be 40mm or more from the edge of the concrete in all directions.

It easier to work with less than a full mixer full, it gives some room for manover. The concret will me MUCH cheeper from wicked, 6 bags of ballast (1.09) to 1 bag of cement (2.60) will give you 150kg of the stuff.

The a393 mesh will hold your weight. space the supports so thay seem right.

I guess you will end up with something that will take one heck of a bit of getting rid of again, so be sure you want this shed to stay forever.

The mesh will allow "voids" to appear under the concrete, and uless the void id huge, it won't move.

Rick

[Terry]

Three by three metres; roughly 100 square feet. Concrete pad seems like an awful lot of work and a very heavy installation? More like something one would install for a small very robust building in which to house a diesel generator at a radio site? I'm just thinking that one can buy, just for example, an 8 by 12 foot shed kit that requires anywhere from six to sixteen treated wooden or concrete posts poured into those cardboard tubes, depending on what load one needs to put on it's floor! Such sheds, starting at around $1800 Canadian (say 800 quid) or less can be simple storage units or can be dressed up with rustic siding, extra windows, half (stable style) doors, skylights etc. Also available up to 14 by 20 or longer (I was considering one 14 by 24 as a second garage. Substantial posts every four feet, vehicle weighing about

3500 pounds plus tools work bench etc. Sheds mounted on short posts above ground allow ventilation below, and raise the floor a little above a small amount of snow. With typical maintenance they last 30 to 40 years. They also can be given the same exterior finish and/or painted to match the main house. Because of frost heaving we have to go at least 30 inches down. With a manual post hole digger, and a digging bar in our stony/rocky ground one can dig a hole in about twenty minutes, with suitable tea breaks! We have two such home built sheds in which we store lawnmower, snowblower, extra sets of winter wheels and tyres for several vehicles, garden tools, a 2000 watt standby generator, spare flower pots, spare gasoline (away from the house) and other useful junk! One 8 by 8 foot shed is on four posts and is 20+ years old and the other about 4 years is on six posts. I may move the 8 by 8 to another location by cutting off the 4 x 4 inch treated posts and towing it within the garden with our pickup truck; to make space for the proposed 14 by 24 unit.

[Craig Graham]

[Craig Graham]

That sounds excessive for this- it's a 3 metre single storey shed, not a house! The specifications say a 100mm concrete base; since I'm unsure of the ground I was planning a single layer of a393 sheet.

Do you mean Wickes?

I need 80x25kg bags giving 2000kg.

150kg your way costs £9.14, and I'd need 13 of these making the total cost around £122, excluding delivery since more than two trips to get the stuff is a bit too much hassle. Given I can get a mixer to come and deliver that amount ready to place for £118, it still seems the way to go even though your figures are much better than the £600 that I reckoned from Keyline's web site.

That's one thing that put me off originally, but since I only need 100mm I could always hire a breaker if I needed to get rid of it. Or use the SDS drill and patience.