Sloping slab

[Beercan321]

Hi guys,

I have a question about the pouring of a new slab. Tomorrow I’m having a 30x30 slab poured for my garage. Thus far the contractor’s crew has been very friendly and answered any questions I’ve had. Unfortunately any company is only as good as their worst employees and I think I might have got the contractor’s bad employees when they were prepping the base for the slab.

2 guys showed up and put down a gravel base, tamped it down, laid a vapor barrier then rolled out wire mesh. Before they left they put a chalk line around the inside of the block wall so the cement crew knows where the top of the slab is. When I looked at the chalk line it wasn’t level. I put a bubble level on the block wall and verified it was level. I then measured from the top of the block to the chalk line. From the back of the garage to where the garage doors will be the line drops 3 ½ inches. I asked one of the guys why the line slopes. He said you want the slab to slope a little so water runs out the garage door. What?! The slab is supposed to be 5” thick. I measured from the gravel base to the chalk line and it is 5” all the way around. If they poured the slab right now it would be the correct thickness but slightly sloped. Is this normal or is this guy feeding me a line of B.S.? I assume that he didn’t level out the gravel before tamping it and it’s higher in the back of the garage then the front. He then measured 5” from the gravel base all the way around the block and by doing so the chalk line is sloped.

Should I call the contactor and cancel the pour until this is straightened out or is what these guys did correct?

Thanks for any opinions/advice!

 

[ConCretin]

It's not uncommon at all for a garage slab to pitch towards the door. It might have been nice for your contractor to inquire if that is what you wanted but there's nothing wrong with it.

 

[reds]

If the garage is attached to your house, local code may require the slope.

Water is not the issue. Dripping gasoline from a vehicle is the reason for the slope.

 

[ConCretin]

Water is not the issue. Dripping gasoline from a vehicle is the reason for the slope.

Just curious where you got this from.

 

[Beercan321]

If the garage is attached to your house, local code may require the slope.

The garage is detached.

I understand the reasoning for a slope but 3 1/2 inches over 30 feet seems like at lot to me. Is that a normal slope for a 30x30 garage? I assumed the floor would be close to level.

 

[Will67]

I have a slope of 2" over a run of 24' pitched towards door.

 

[ConCretin]

The garage is detached.

I understand the reasoning for a slope but 3 1/2 inches over 30 feet seems like at lot to me. Is that a normal slope for a 30x30 garage? I assumed the floor would be close to level.

It's about 1/8" per foot, which is actually about the minimum. In fact, you are likely to have bird baths unless your finisher gives you an exceptionally flat floor.

 

[jhelrey]

So the gas drips out of the garage, down the driveway, into the street, where a cig. could light the gas, gas follows trail back to garage, under door, into garage, into tank of car, boom. Haha

 

[reds]

Just curious where you got this from.

"If the garage is attached to your house, local code may require the slope."


The BOCA Residential building code in my area requires the slope (been awhile so I can't remember the amount) This is for garages attached to a home.

Building inspectors here, always sited gasoline as the reason.

Sure, other areas may have different codes. I was just suggesting as to why the slope.

 

[csp]

It might have been nice for your contractor to inquire if that is what you wanted but there's nothing wrong with it.

This is what should have happened, especially on a detached structure.

understand the reasoning for a slope but 3 1/2 inches over 30 feet seems like at lot to me. Is that a normal slope for a 30x30 garage?

Perfectly normal. It's about a 1% grade. Mine drops 6" over 24', which I find to be excessive.

 

[slickgt1]

Nothing wrong with it. Seems like they are doing a good job. Yea they should have asked, but trust me, you want this.

 

[ConCretin]

"If the garage is attached to your house, local code may require the slope."


The BOCA Residential building code in my area requires the slope (been awhile so I can't remember the amount) This is for garages attached to a home.

Building inspectors here, always sited gasoline as the reason.

Sure, other areas may have different codes. I was just suggesting as to why the slope.

Well if a building inspector said it, it MUST be true.

Not bustin your chops - I just think that's it a bit silly. It may even be true but it's still silly.

 

[darkk]

I have a 30x30 garage and standard slope for runoff is 1/8" per foot. So your slope angle is within those specs. It is code here in my area.

 

[Kevin54]

If you want it level, you had better get it straight with the contractor before the concrete trucks pull up. 3 1/2 does seem excessive though. I have a slight slope in mine (2") but if I had it to do over again, I would just have had it level. I always squeeqee it out anyways. Also you want to make sure the corners are up a little higher than the rest of the floor so water does not run there. Depending on how the wall foundation is designed if the slab is not pinned to the blocks or if you don't have a brick ledge on the block where the floor is gettin poured the slab will drop a little. Meaning on a free floating slab with a separate stem wall. Water will them tend to pool to the corners.

 

[KELLHAMMER]

2009 International Residential Code
Section R309 Garages and Carports
R309.1 Floor Surface
"The area of the floor used for parking of automobiles or other vehicles shall be sloped to facilitate the movement of liquids to a drain or toward the main vehicle entry doorway."

This section of the code does not distingush if the garage is attached or detached. So to me it applies to all garages.

I've been told by concrete contractors a 1/4" per foot slope is much easier to achieve. An 1/8" is just too slight and could result in flat spot where water/ liquids will collect.

Also, in my area, floor drains are not allowed due the possibility of polluting (oil and gas on or in the ground or sewer system) and a potential risk of explosion if gas were to enter a drain.

4 or 5 inches over 20 feet is barely noticeable. Think about parking a car covered in snow that will melt and tend to drain to the entry, as opposed to just sitting right on the floor.



Check with your local building official to see what is required.