Concrete Forms

[markrahn]

Anyone ever built your own concrete forms for a slab with foundation. Will be a monolithic pour.

The land has a slope to it with a 23" grade corner to corner diagonally.

My main question is. Does the top of the forms have to be level with the 4" slab that will be poured? Or can the forms be higher? and if they are higher how does the finishing crew ensure that the slab is level and true when they screed and finish?

thanks for your knowledge
Mark

On edit, added example picture.

 

[beppington]

In '92 me & my dad built the forms the monolithic slab for the 1,600 SF house I still live in. We used 2x12's & secured them to 2x4 stakes about every 4 or 5 feet.

You do need the top of the forms to be at the exact height you want the finished slab surface to be at - The top of the form is exactly what the concrete finishers will use to determine the height of your slab's surface. They use screed boards, either a long straight 2x4, or a straight aluminim piece, to drag along the top of your form boards. They continue dragging the screed board until the concrete has worked its way down to being the same height as your forms.

Put another way, if you make your form boards higher than you want your slab, your slab will be higher than you want it

Should look like this, with the top of the gravel in the pic exactly 4 inches lower than the tops of your forms, so that the concrete all across the slab is 4 inches thick everywhere except for where its thicker in the footings:

 

[Duffster]

My main question is. Does the top of the forms have to be level with the 4" slab that will be poured? Or can the forms be higher? and if they are higher how does the finishing crew ensure that the slab is level and true when they screed and finish?

No they don't need to be level.

They will simply mark the froms in the corners for level using a transit and then snap lines from mark to mark.

 

[rockinmywaypa]

Don't listen to that last bit of advise. You don't save five minutes to spend an hour fixing. As soon as the concrete hits the line it's covered and gone. It's not hard to level your forms, starting with a transit and 2by4 stakes drive in the stakes along a string line until they are all at the right depth and level with finished slab, then using a sledge as a shock absorber nail on your boards flush with the tops of the stakes and back fill. job done and you can race against time when it counts and the concrete needs to be poured in a hurry and trowelled before it dries.

 

[Duffster]

Don't listen to that last bit of advise. You don't save five minutes to spend an hour fixing. As soon as the concrete hits the line it's covered and gone. It's not hard to level your forms, starting with a transit and 2by4 stakes drive in the stakes along a string line until they are all at the right depth and level with finished slab, then using a sledge as a shock absorber nail on your boards flush with the tops of the stakes and back fill. job done and you can race against time when it counts and the concrete needs to be poured in a hurry and trowelled before it dries.

Yes because the advice from someone that thinks concrete "dries" is much more reliable.

You have obviously never poured a basement floor.

If the finishers can't pour off a chalk line then you best gets some new finishers.

 

[beppington]

If the finishers can't pour off a chalk line then you best gets some new finishers.

Why make it harder than it has to be. Any ol' shmoe can make a slab level if the forms are installed level & set at the right height. Using a transit & following a chalk line takes a higher level of skill altogether, & more time. Just makes no sense to me when it's so easy to have the forms sitting there at the right height to start with.

 

[markrahn]

Thanks yall, I am going to start building the forms this weekend. I don't have a builders transit but I do have a PLS180 laser level and detector.
I am planning on using it the same way as a transit, only I can do it myself and check grades, etc. Is OSB board OK to build the forms with, by building a 2X4 frame (with bracing every 3 or so feet)

Thanks yall.
Mark

 

[markrahn]

Oh, yeah.
another question.

My ground is sandy loam on top of red clay. you start running into red clay about a foot down.
How deep should my footings go down into the ground. In other words, how much digging do I have to do?

thanks
Mark

 

[wvpolekat]

Ask the person(s) finishing your concrete what they want. They are the ones you want happy.

 

[beppington]

Is OSB board OK to build the forms with, by building a 2X4 frame (with bracing every 3 or so feet)

You can make forms out of just about anything as long as you support it well enough with stakes like 2x4's pounded in the ground (like in the pic I posted). And form material doesn't really need to be water proof, just water resistant long enough for the concrete to set up a little bit.

When I formed my pole barn for pouring, I used every different kind of material I had laying around, from 2x12's, to stacked 2x6's, plywood, particle board, etc. Looking at it now it looks exactly like anybody else's slab done with say all brand new 2x12's.

My ground is sandy loam on top of red clay. you start running into red clay about a foot down.
How deep should my footings go down into the ground. In other words, how much digging do I have to do?

I've heard horror stories about building floating around on & sinking into clay, & things to "try" to mitigate the problems, etc, but I don't have any actual answers myself.