Need advice on how to build a concrete base for a shed?

turkystick asks: Need advice on how to build a concrete base for a shed?
I want to lay a concrete base for a 7×5 shed over grass but im unsure what is the best way to do it as my garden is on a slope, now i don’t know what angle the slope is but if i put a board at the highest point of the slope and level the board there is a 6 inch gap at the lowest point of the slope. If you had to make the base how would you go about it as i don’t know what boards to get and how much concrete will be needed.

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Answer by Bazza
There are concrete calculators on the ‘net. Invest in a longish spirit level and dig out to give a roughly level base. Then set in 1″ x 4″ sawn planks levelled to contain the concrete. Brush oil on the inside of the planks to stop the concrete sticking and drive pegs down on the outside to combat bowing. You can mix the concrete by hand if you are fit, but a small hired electric mixer is much easier as wet concrete is extremely heavy. Use a mix of six parts 20mm ballast to one part ordinary Portland cement by volume. Use a bucket to measure, not the shovel. Do not add too much water as this weakens the concrete and you will be swimming in it when tamping the mix down. Add just enough water to make the mix workable. This is the secret of strong concrete. Cover to protect from frost and keep it damp. It is the presence of water that cures concrete. Rapid drying will weaken it.

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  1. If you want to use premixed concrete in 80 pound bags you will need 18 bags for a slab 7 x 5 x 4 inches thick. Dig out the high side 9 inches deep and deposit the dirt on the low side. Make the space for the slab at least one foot larger in area then the slab. The dirt on the low side needs tapered edges and tamping and soaking to make it sound. Cover the area with 3/4″minus gravel 3 inches deep and roll & tamp it solid. Make a form for the slab out of treated 2 x 4″s ( screw the corners together with 3″deck screws), lay it in place, drive in wood stakes longer than needed in each corner ( that is on the inside of the form ). Use a level on top of the form and screw the stakes to the form ( use 2″ deck screws and keep them low ) Now the form is level and secure. Now drive in more stakes, 2 on each short side and 3 on each long side. Screw them to the inside of the form. Now saw the stake tops off about an inch below the top of the form. Mix your concrete in a wheelbarrow with a hoe. Put the wheelbarrow inside the form for awhile to reduce shoveling in the mid part of your job. To level the concrete to the form lay a long 2 x 4 on top the form pull in a zig zag motion. As you pour your segments shore them up with a 2 x 4 fit inside the form ( a 5′ length with cleats screwed to the top that will rest on the top of the 7′ form sides) just move it back as needed. Just leave the form in place when finished, it is treated. You can screw the treated bottom plate 2 x 4 of your walls to the form to secure the shed to the ground.

  2. You want to level it up at its lowest point and at the place you going to erect the shed.Once you have that sorted it time to shutter(frame the base)you want it about 2 to 3 inches bigger than the shed more if you want it.If you make the aver all frame by 7ft 6 inches by 5 ft 6 inches.The board to use is 3inches wide by 1in thick by 7 ft 6 long you need 2 lengths and 2 lengths 3inches wide by 1 inches thick by 5ft 6 inches. long.Once you have the frame together you also need wooden stakes3 to every side about 12 long.
    When you order the concrete let them know the size of the base they will then let you know how much concrete you will need………..good luck.

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