concrete over cinder blocks for foundation??

Birdman79

I am building a 4'x30' pigeon coop which has to sit on a concrete slab,and has to have a 12" rat wall.My question is would it be ok to lay cinder blocks on the soil first then pour 2" to 3" of concrete on top of it or,is it just wiser to pour the concrete only?I am trying to save time mixing concrete .

Ron6519

Is there no building code guidance where you are?
A frost line?
Use of a footing?
Ron

Birdman79

Slab has to be at least 3 1/2 " above ground,it needs a 12" footing.i am in michigan so the frost line i assume is 48".I don't need to reach the frost line do I??

Ron6519

Yes, you need to reach the frostline. If you don't, the structure will be frost heaved into a pile of rubble over time.
Ron

joed

I'm not sure what you are trying to do. Are you saying put the blacks around the perimitter as a footing? I wouldn't do that. Dig the perimtter of the slab 12" deep and pour it as one monolithic piece.



Or you could put the blocks on top of the slab to keep the bottom of the wall away from the ground.

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Daniel Holzman

You say that it it necessary that you have a slab. Is this a code requirement or a design requirement? Why not build on gravel, I mean this is a bird coop, not a house. Is this so you can clean the surface by washing?

As for the design, if you have no code requirements, you can certainly relax the need to go down to frost, after all this is a bird coop, not a house, if the slab heaves so what? Are the birds going to go on strike? Block is usually much less costly than poured concrete, so if you can substitute block for concrete, go for it, if it meets your code and design requirements.

Birdman79

Okay,sorry for the confusion.My city ordinance requires a concrete slab under the pigeon coop.They won't let me raise it on cinder blocks.

So let's say the slab is 8" thick.Cinder blocks are 4" thick.I want to lay cinder blocks first then lay the concrete on top, which will save me 4" of concrete mixing.

The footing would be all concrete though.Reason i am doing this is like i said to save on mixing all that concrete.But i want to make sure that it's stout in the long run with no cracking what soever.

Bondo

Are you excavating a hole, 'n forming this floating slab,..??

If so,.. Just excavate 4" Less in the center....
A couple pieces of rebar, 'n some wire mesh is what ya need for that...

concretemasonry

An 8" thick slab is way too thick and even 4" would be fine and convenient and more than is necessary.

If you have a narrow concrete slab 30' long, it is no more than sidewalk and as such should have tooled joints to control where it cracks, since it is guaranteed to crack. How many sidewalks have you seen that were placed on cinder blocks (which are not available, but concrete block are)?

Dick

Birdman79

yes this will be a floating slab.When you said excavate 4" less in the center,can you be more specific?.Also I am going to rebar it ,so would it be necessary to put a couple of inches of gravel under the cinder blocks??

KarlJay

The 4" less in the center is just another way of saying: 'make the edges 4" deeper' ie: dig 4" deep, then go back and dig the edges 4" deeper.
I think they are saying the cinder / concrete blocks aren't needed at all.
If you want to save some money, concider that concrete blocks are $1.50 ea (8x8x16) whereas concrete is $2.89 / 80# bag? Might want to find someone on craigs list giving away concrete chunks, bust them up and toss them in as a filler. After all, you don't need great strength.