Expert: Roger Zona - 7/20/2008

Question
QUESTION: Hello,

How is a garage floor slab incorporated into a monolithic house slab? In particular, how is the formwork done? Is the house slab poured first, or is the entire thing formed and poured at the same time? The long narrow concrete perimeter edges of the garage slab (the ones that have to be flat,level and higher than the sloped garage floor, to support the garage walls) - is the formwork for them built into ( and at the same time as?) the formwork for the house slab?

The house will be built in a warm climate, where the frost line is the finished grade.

Thank you - Larry

ANSWER: Larry,

Yes, the entire monolithic slab (on a typical house) is formed and poured all at once.  Usually the garage slab steps down 8" (one block)and the slab is pitched to the overhead door(s).  Also, the form should include the recess (3/4" to 1 1/2" for the overhead door(s).
The slope is not to great - about 1/8" to the foot - or about 2".  This can be accomplished by forming the step-down from the house slab to the garage as about 6" which then slopes to 8" which will accommodate a concrete block.

If the garage slab is poured separately the footing reinforcing should be "cranked" or bent to meet the change in elevation.

Hope this was helpful.

Roger

---------- FOLLOW-UP ----------

QUESTION: Thank you very much. I am still confused about something. How is the formwork installed so that it makes the transition at the two places (left and right side edges of the sloping garage floor) where the slope of the garage floor meets the long, narrow and level portion of the house slab that supports the left and right side exterior garage walls? In case the way I asked that question was confusing, I'll ask it again in greater detail:

I'm going to try to draw a picture of what I am talking about, with words. The garage will have 4 walls. The front wall separates the house from the garage, the back wall is the one you drive through to put the car in the garage. Take those two walls out of the picture and the two side walls remain. Those side walls have to be supported by a part of the house slab that is long, narrow and level, to allow the side walls that will sit on it to be long, narrow and level. I'm going to call the long narrow part of the concrete foundation that runs along both sides of the garage, the curb, because I don't know what the correct term for it is. That concrete curb butts up ( I understand the whole thing is poured at the same time, but in order to draw the picture I am trying to draw, I'll use the word "butt") to the edge of the sloping garage floor, on both sides of the garage, along a line that reaches between the front and rear walls on both sides of the garage. The formwork has to allow the curb to be long, narrow and level at the top, and also allow the garage floor slope to run alongside it and continue to slope. I am assuming the curb is formed so the top of it is at the same elevation as the finished concrete floor of the house slab. It's all poured at the same time.

Trying to picture how this is done is a mind twister. I hope I explained my question well. Can you help me understand how it's done?

Thank you for taking the time to read through this long question - Larry

ANSWER: Larry,

I'm sorry my answer was so short and basic.  Yes, the forming of the garage area takes a longer time to create the slope in the floor yet maintain the level block coursing.  If you prefer, the garage floor slab could be poured after the walls are built.  I've done it both ways, depending on the size of the garage.  In the past 10 years, the three car garage is becoming more common.

Are you asking about this forming because you are doing the work yourself? - or are you just concerned about how it will be done?
Your concrete subcontractor, assuming he has experience, will know what to do.  I have seen many houses where the garage floor is level (without a slope) which is not a major factor in Florida except for rainwater draining off your car.  In that case I would advise a small french drain or floor drain in the center of the slab with a line to an outside drywell.  This is done by the plumber in advance of pouring the monolithic footing and slab and the concrete finisher then tapers the slab from the outside edge to the drain approx. 1/2" to 1".

Hope this was helpful.

Roger



---------- FOLLOW-UP ----------

QUESTION: Roger,

No apology necessary. Your answer helped a lot. Yes, I'm planning to do the slab prep work myself. I'll hire people to pour the concrete, because I don't think one person can do that. I know it's not easy to explain formwork with words. Pictures are much better. What I'm trying to do is learn the formal terms for the type/shape of the curb (what I am calling the curb) formwork, so I can use those terms in a good search engine to find pictures. I can't look for construction sites that are in the formwork stage, for examples, because I live about 1000 miles away from where I'm going to build. Construction practices are different here. I'll be moving soon.

I understand how the formwork for the main house is done. I'm planning to do a monolithic slab. The only area I don't understand is how to form that transition line between the sloped garage and the curb that supports the side walls. I want that curb to be poured concrete, and placed at the same time as the house slab. This way it is flush with the house slab and just as strong. With the top of the curb flush with the house slab, I'll start the garage floor slope 6 inches down from there, and have the slope continue toward the front wall, stopping there 8 inches from the top of the curb. Once the slab and curb is cured properly, I'll start the exterior walls (concrete block).

If I can't find the resources to teach me how to form that curb in one pour, I'll do two. I would like the garage floor to be sloped.

Thank you very much for your wisdom, time and patience. Have a great day - Larry

Answer
Larry,

If you would trust me with your email address, I will send you an Excel sketch showing the footing detail.  I can't add any attachments to this message.

I suggest you check with the building dept. in your new location.  They may have some different specs.  Also, are you using an Architect?  He (or she) would know the local codes and requirements.
Here in Florida we have a standard State Building Code but every city makes amendments which complicates my life.

Roger