Building a Form For a Thickened Edge Concrete Slab

A thickened edge concrete slab is meant to hold a lot of weight.  It's a footing and slab all in one.  A steel building for example pushes down and out.

If the steel building was resting on a six inch slab, the slab would probably crack. If there was lots of rebar in the slab and properly placed, it might not crack.  But do you really want to take that chance?

Besides that...you have to build the thickened edge concrete slab according to the engineered design.

A thickened edge concrete slab or a slab on grade is what we do for our steel buildings. This project isn't something you can fix easily if something goes wrong. Pouring concrete should be done by a professional or you can waste thousands of dollars.

It normally cost $6.00 per square foot to do a thickened edge concrete slab. The $6.00 covers building the forms, concrete, rebar, pour the floor, install anchor bolts, power trowel, saw cuts and all labour charges.

The building has to fit the concrete slab! This part of the project is extremely important so don't cut corners. Did you by any chance get a building permit? If not...stop right now and get one! Municipalities can make your life miserable if you don't go by their rules.

Assuming that the gravel pad is the right size...level and properly compacted...we can continue with the thickened edge concrete slab and begin forming. Building codes must be followed when it comes to structures...and you must build the thickened edge concrete slab exactly the way the engineered drawings say to.  Plan on about a 6 inch concrete floor if it's a garage and no less than a 4 inch for residential applications. (Basement floors)

Every metal building package comes with it's own set of foundation plans which the building inspector will want to see. I know for sure that Future Steel buildings require an 18" thickened edge concrete slab because we have erected their buildings before. Our Archidrome building foundation plans require a 12" thickened edge slab...so depending on the size of the building the extra concrete will cost you a lot more.

The pad is ready and the sides are sloped for the thickened edge. Basically we are going to build a big sand box. I use 2 X 6 to form with because it can be used for many other purposes later and it is a strong form. I have 12" forms already made up that I use over and over again. 

Buy straight lumber for forming! Take the lumber to the site and pile it up neatly so that the sun doesn't warp it, until you are ready to use it. You may need some 12 footers and some 14 or 16 footers...it's cheaper to buy the right lengths so you don't have to cut any more then you have to. Although you need short pieces for bracing, some stakes and pieces to strap across two 2 X 6.

Now remember...this is a very important job...so everything we do must be proper. Let's begin by laying out the desired lengths. You can stagger the joints or you can start with two 2 X 6 the same length. Lets say you start with two 16 footers...cut short pieces long enough to span across both 2 X 6. 

Keep in mind that wet concrete pushes hard on the form....especially when there's plastic involved...concrete slides down the slope pushing hard against the form. Not only that...people will walk on the form...guaranteed and the person raking concrete will be pushing concrete toward the form. Honestly...don't under estimate the force of concrete....build the form strong.

We're going to strap the 2 X 6 every 4 feet all the way around the form. The corners are the weakest...so start strapping in the corners and work your way toward the middle. Have you checked the length and the width again, making sure you're measuring on the inside of the form? 

Quite often people will measure from the outside of the form and the concrete slab ends up 3 inches to0 short. I use 3" Robertson head screws, simply because I don't want you to bang three inch spikes into the form...lol Screws are so much easier to use and the form comes apart so much easier. 

I used to use Dewalt cordless hammer drills but the battery life is too short and they take several hours to fully recharge. I switched to Ridgid with lithium batteries, recharge time is about 18 minutes. When you start tightening several thousand bolts in your new steel building you'll be glad you own a Ridgid impact driver.

Use 6 screws per strap...space them out evenly. Screws are very strong! Make sure everything is screwed tight! When the forms are built it's time to stand them up. First I want you to cut 4 pieces of rebar two feet long...then I want you to drive a rebar in each corner of the pad exactly the size of the concrete slab. Use two long tapes...measuring corner to corner until the steel stakes are perfectly square. Now bring your forms to the rebar stakes...build one corner and situate it so that the rebar is tight in the corner. 

I always start with two stakes! I say to my customers, where do you want the front of the garage to start. Mark a spot! Now what angle do you want the garage to sit on the lot. You start with two stakes and then go from there.

If you just went ahead and built the form out of place, it's a pretty heavy thing to be lifting and dragging into place. Build the other corners and connect the sides together. You will screw a 2 X 6 evenly about 3 feet long at every joint using 6 screws per scab. Leave the rebar in the corners until the bracing is done...then you can bend them inward later and they will become part of the thickened edge concrete slab.

The corners are in place, now site down all four sides and line the form up so it's close. Measure corner to corner again...use the 3-4-5 method, make sure the form is square. Cut two 2 X 4 two feet long for each corner and sharpen them.

Without moving the form...drive a 2 X 4 at the end of each outside corner. Drive the stakes in straight and don't hit the form. Drive a board nail in the very outside corner of all four corners. Now you can run a string line from nail to nail...lining up the sides perfectly. Leave the string there until the bracing is complete and everything is tight.

With the form square and the sides lined up straight, begin driving stakes between each scab or every four feet. Keep the edge of the form on the line and drive every stake all the way around the form. Set back two feet from the form and drive another stake directly out from the stake you drove next to the form.

Drive those stakes in hard and you can tip them inward a little bit. Now cut 2 X 4 long enough to go from the bottom of the outer stake to the top of the stake next to the form. Measure across from one side of the form to the other to make sure you're the slab is the right width and length. Check and recheck the plan and your work!

Set the brace in place and put one screw in the outside stake just to hold it in place. Now the fine tuning! Set the laser level up! Go around the form and find the average height of the form. If you did a good job on the pad the form should be fairly level. It's easier to pull the form up then it is to shovel some out to get it down.

On the other hand...the form is best sitting on the ground. If there are only a few places that need to be dug out, then do it. Go to each stake and sit the laser stick on top of the form...either pick the form up a bit or make it go down. When the laser flat lines...screw the stake to the form. Keep in mind that if the form has to come up at the next stake it might pull the one behind you up some more too. 

Put one screw in each stake until you are completely around the form then add 1 more per stake. I use an estwing hammer with straight claws to pull fill out from under the form, works wonderful.

The form is level, it's square and it's lined up. Now it's time to screw the brace to the top of the form. Don't be afraid to push the form in beyond the string line some, as the concrete will push it back out. Just make sure the bracing is tight and not going to move. If you're in doubt, add more bracing!

The more time you spend on forming and making the form strong the better your thickened edge concrete slab will turn out. Place gravel up against the form and pack it in with your foot, this will help hold the form in place too. If the concrete slab is out of line after the forms come off, don't panic. All you have to do is square it...chalk line it and then cut the excess off with a concrete saw. It would have to be bad to do that, depends on what type of building you're erecting. 

If you're in doubt just shoot me an email and let me know your concern.

Well, lets talk about the vapor barrier and the rebar on another page...this page is getting too long. Click the following text to learn about rebar installation for pouring a thickened edge concrete slab.


Click this text to read about Steel Building sizes

Click this text to learn about Steel Building purchasing tips

Click this text to use a concrete calculator

Click this text to read about pouring concrete



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By Tony Huggard Copyright©2012---Sussex...New Brunswick!

This is the corner of a thickened edge slab form...put three screws in each 2 X 6 and drive two stakes at the corner and two screws in each 2 X 6


Below is an 18" wide thickened edge concrete slab form I used on a Future Steel building. I just wanted to show you how thick the concrete would be. Our Archidrome buildings are designed for a 12" thickened edge concrete slab. Take notice of the strapping across the three 2 X 6.


Where you might have used three 4 inch spikes..now you can use two 3 inch deck screws.


Below is the best cordless Impact driver on the market. I use Ridgid for tightening the bolts on our Steel Buildings, several thousand bolts later they are still as good as new.


Below is my Ridgid hammer drill...I use this for drilling and sinking tapcon screws into the concrete.


This is my Stihl concrete saw...it looks new because I clean and wash it after every use. I've used them all and Stihl has the most power to cut 4 inches of concrete slab without bogging down.


I talked about cutting rebar! The rebar bender below is also a rebar cutter and will cut up to 15mm in size but be ready for a work out. The Stihl concrete saw will also cut rebar and you have to wear ear and eye protection.


Steel Building Sale!



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