I am pouring a slab for a concrete floor for an 1800 sq foot house and would like the bathroom area to be about 3/8" lower than the rest so I can tile it and wind up with the floor and seams that are level throughout once the tile is laid in the depressed area. How can I do this?
I'd pour the slab, leaving a 'hole' where the bath will be. Essentially, you will make a hole in the slab using forms.
After the slab sets, remove the forms, come back and pour your slightly-lower slab in the hole that was left.
Ideally, use some rebar in the original pour to tie in the new slab for the bath.
Alternatively, maybe consider pouring it all flat and use a transition strip between the concrete outside the bath and the slightly raised tile.
Alternative option 2: if the whole house is going to be polished concrete, why not the bath too? Maybe stain the bath concrete a different color?
I found this thread because I wanted to pour the middle of a walkway 1/2" lower than the outside boarder of the walkway. Like the OP I wanted to tile the low portion of the concrete -- in my case the low portion is the middle strip of a walkway. I came away from this thread thinking that I would have to pay for two concrete pours on two separate days but the guys I hired did it in one pour. Here is how they did it.
The forms for the outside edges of the high areas are as you would expect -- the top edge of the forms defines the top edge of the high area of concrete. But the forms for the small step into the low areas of concrete are built so that the BOTTOM EDGE of the form defines the TOP FACE of the low areas. Also, any stakes that brace the forms for this step-down are placed INSIDE the low area and remain there during the first half of the pour.
A first screed is cut with a notch on one end that matches the height of the forms minus the step depth (e.g. 3-1/2" form minus a 1/2" step = 3" notch). One end of this screed rides on the top of the high-area forms and the end with the notch rides on the top of the forms that define the step edge. This first screed is used to smooth the concrete so that the step-forms end up bracing only 1/2" of concrete on their high side.
A second screed is cut to match the height of the step forms (e.g. 3-1/2"). This screed is used to hollow out the low area so that it's finished depth flush with the bottom of the step form -- so the step-forms end up bracing no concrete on their low side.
Finally, after the screeds have been used and the concretes begins to set-up any stakes that have been bracing the step forms may be pulled so the stake-holes can be filled. At this stage, since the step forms are bracing very little concrete, they may be held in place only by cross-members screwed to the tops of the forms. Finish the concrete as usual and you're good to go.
Pour it first, after you've checked all of your height, length & width (including stud & drywall depth or minus those depending on your plans) measurements a googol times. If it isn't a full truck then have that truck empty 6' or more away, due to slump run.
You'll really want to let that setup completely before pouring anywhere near it. Cover it completely with 3/8's plywood & stake or screw any exterior wall edge.
You'll not only be able to walk on it, but the float work shouldn't be able to move anything...you can insure that with battens screwed over the seams. Still be careful not to knock or shift it while pouring around the room. But, that should work out real good with sharp right angles.
I believe your question is asking for information on how to set-up forms to pour for concrete footings and floor slab. I'll assume you have the fundamentals of working with concrete already. Here is an abridged answer for forming a room 3/8 inch lower than the rest of the slab.
It will be much simpler if the bathroom is on the periphery of the slab. Stake-out to define the bathrooms outside dimensions plus 14 inches wider. Excavate to the code described depth for footings in your area and install wood forms of 2x stock or 3/4 plywood to a level height that is 3/8 inches lower than the existing forms for the rest of the house slab. Install re-bar as per code.
Essentially you are building the forms exactly like the rest of the slab forms ,but 3/8 inch lower. If the room is located on the inside of the building you will be constructing a well or step-down 3/8 inch lower than the house slab.