How do I pour a Concrete Slab

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How To Pour A Concrete Slab With


Reinforced Concrete.


How to pour a Concrete Slab for your Garden Shed, Storage Shed or Outdoor Home Office, was once the domain of the professionals, but now the DIY handyman can do the task just as well.

Now is the time to think ahead about the type of floor you are going to have.

If you have decided to build your new shed off the ground with stumps, bearers and floor joists, you will not have to worry about this, but if you have opted for a concrete slab, this is what I have done getting the slab prepared and poured.

The next thing to think about, is your shed or garage going to sit on the slab or are you going to build the structure and pour the concrete as the final job.

Before We Do Anything.

Whether you are building a small storage shed, a garden shed, you may need to think about any permits you may need from Local Building Authorities or Councils. There are many by-laws pertaining to distances from boundary fences, other structures and building near or over easements.
The Local Authorities may even require a specification of how you are going to lay the slab, with Reinforcing Mesh and Concrete Strengths etc, but highly unlikely if its only a small structure in your backyard.

Placing your Shed on the Slab.

Lets work on an example that you are building a 20 ft x 20ft (6Mt x 6Mt) shed. I would always allow an extra 6 inches (150mm) around the shed, making the actual concrete slab size 21ft x 21ft (6.3Mt x 6.3Mt) of our shed. The reason for the extra slab around the shed walls is to allow for proper fixing down. If you poured your slabs the exact size of your shed, you may only have 2 inches (50mm) between your actual fixing and the edge of the slab. Being such a short distance, this could be a recipe for disaster, because there is a better than even chance that your slab will crack, when you attempt to put the fixings in.

Setting out the Concrete Slab and the Form Work.

Preparation of Concrete SlabI am writing this section for people who would like to know how to pour a concrete slab and build their shed on top. Some of the Garden Shed Plans you will purchase, are where you will build the structure with posts / stumps etc and pour the slab inside.

Before starting with your Slabs design, check with your local authorities if they require an Engineering Design before laying concrete.
Step 1.  This may sound strange and completely unnecessary before you even lift a finger. If you don’t have one, I would go out and purchase a Laser Level, they are so cheap now, even with a small tripod, maybe a $100.00 , you will see the value of this is the whole scheme of things.

You could hire one for say $30.00 a day, but when you own your own, there are no time restraints and believe me, they are a very handy item to have around the workshop for years to come.

Now that I have spent your first $100.00 lets get on with the slabs. I am going to use an example that you are building a 20′ x 20′ (6Mt x 6Mt) garden shed, and that is going to be placed 4′ (1200mm) off the side boundary fence.

Step 2.  Work out where you want to start the slab, whether this is an optional or you have it on drawings, and knock the peg into the ground exactly 4′ off the side fence. Go down 20′ and place your other peg exactly 4′ off the fence, and pull the string line tightly around this peg. You now have your starting point.

Step 3. 

Perfectly Square CornersUsing the Pythagoras Triangle System, this is a 3-4-5′ triangle (when the measurement between the 3′ and the 4′ sides measures exactly 5′, you have a perfectly square corner).

Use this method on the other two corners, knock in your pegs, string your line tight and you will have a perfectly square concrete slab.

Step 4.  Now that you have your string lines set out, its time to start the physical stuff. Inside your lines excavate 3” (75mm) down from ground level, this is where you will be filling with Crushed Rock / Gravel.

Some people may say, why not just put the slab directly onto the ground, but I always believe a concrete slab design needs a firm base to rest on. If something happens down the track and your slabs cracks with ground movement, there is not much you can do about it then.

Step 5.  Now your excavation is finished, this is where you start to use your Laser Level. Set it up away from your work site somewhere, most of the smaller levels have a 40′ – 50′ range. Check in each corner and a couple of spots in the middle, just to see how your levels are going. At this point you don’t need to be to precise.

Step 6.  Using the standard Length x Width x Depth Formula, work out how many Cubic Yards or Cubic Metres you will need to fill your excavation. Fill your excavated hole with the Crushed Rock / Gravel and with a Plate Whacker (can be hired for a day) go around the complete area, until you have a firm, tight level surface.

Step 7.  You now have the firm 3″ (75mm) base of your concrete slab, ready for the next step. This base should have finished at ground level, now you are ready for the form work. I believe that shed  slabs should be 4″ (100mm) thick.

Using 4″ x 1” (100mm x 25mm) timber for your form work, set out your forms to the exact size of your shed, using a peg on each corner. The reason for only pegs on the corner at the moment is that you now have to check to make sure your slab is square.

With the pegs on the corners only, measure from each corner on the diagonal, this will tell you when your form work is square. When you have the form work squared up, place your pegs every 2′ (600mm) around the complete form work. DO NOT NAIL the form work to the pegs, once you pour your concrete, it will all hold in place.

You have all the form work pegged out. The final task in to check for levels, this is where you new Laser Level comes into play again. Check each of the corners first with the laser, and then do a few random checks here and there on the form work.

The reason why we do not nail the form work to the pegs, is that while you are checking levels, the form work my need to come up or down in places, you may need to scrap away, or add some crushed rock in places to lift it up.

Step 8.  The final stage, then we pour. Our form work is all in place, square and level, now we need the reinforcing mesh. This comes in different grades and strengths, depending on where you live, check with your supplier to get the grade you need for your slab.

Place this on your base, keeping about 1″ (25mm) back from the form work, this will save any unsightly steel pieces projecting out when the slab is finished.

Your supplier will have what we call mesh stools, these are made of plastic, have a base for support, and you buy them at whatever height you need for your slab. The idea of the stools is that they hold your mesh at the height you want it to stay in your slab. As our slab is going to be 4” thick, we will buy 2” stools. This will keep the mesh in the center of the slab, when you are pouring your concrete.

The Concrete Slab being pouredUsing the same Length x Width x Depth formula, order the Cubic Yard / Metre of concrete you need, and start pouring.

Make sure the concrete is vibrated in properly to get maximum strength. While the concrete is still wet, screed it out with a straight edge, and then a wooden float.

After approximately one hour, you can then do the final trowel off.

Very Important, don’t forget in the process of the screeding and final trowel work, that you must have your expansion joints in the slab. With a 20′ x 20′ slab, one either way is quite sufficient, so you will have 4 squares, of 10′ x 10′ each. If you don’t wish to do this now while the concrete is still wet, the joints can be cut in with a Concrete Saw afterwards.

Concrete Materials.

There are only three basic Concrete Materials in concrete.

Some people have different terminology, but the general Rule Of Thumb measurements are as follows.

  • Three parts 3/4 inch or 20mm Screenings or Road Base as some people call it.
  • Two parts Concrete Sand.
  • One part Cement
  • Water, usually half of the weight of the cement to the weight of water.