Concrete slab on slope

[boostaholic1]

Hey guys and girls
Before I make a huge mistake pouring this concrete slab I want to make sure it is NOT a mistake to begin with.

I am doing a monolithic pour on a 28x32 slab. We went 36" deep around the perimeter and formed it to length. When we setup the form, we have the width level from end to end (side to side). For drainage purposes, we sloped the length (back to front) at approximately 3" of drop.
I am thinking that it's going to be a pain when it comes to framing all around. Nothing is going to be square if I do this. I am by no means a builder, so I need some ideas.
Should I just readjust the forms to level before the pour? Or, am I making a MT out of a mole hill?
Any professional and productive advice is much appreciated.
Thanks

 

[Captain Spaulding]

What are you using the slab for? If a stick built building, I’d pour a footer separate from and higher than the floor to get the building materials up off the ground and reduce the chances of water infiltration. Then you can slope the floor however you like.

 

[Stuart in MN]

First thing, does it need to be sloped? The floor in my garage is flat and it's worked fine for me for many years. Second thought is water doesn't need much of a slope, if one end of the slab is 1/2 inch lower than the other, it will still run downhill. Third thought is no matter what you do with the slab, it's worth thinking about including a course of concrete blocks around the perimeter and then building the walls on top of that - it will keep the framing up away from moisture, both inside and outside, and you should be able to compensate for any slope in the slab so that the top of the blocks are level.

 

[boostaholic1]

I don't have to have a slope I don't guess. The framing should end up about 6" higher than the yard. I have lived here for 7 years and so far haven't had any standing water anywhere close to the level it will be. This would be the year that it would be tested too (record flooding around here).
Should I refrain from putting concrete anchors in when they pour then?
I thought I had everything figured out until I had this thought about the building being square.
Thanks again for advice given.

 

[ddawg16]

Unless you plan on washing cars in the garage....go flat.

You also might consider a stem wall. My garage has an 8" high stem wall that was poured at the same time as the slab. My walls sit on top of that. The really cool part, besides being way up above grade, my drywall is 8" off the floor...and it makes a great starting point for cabinets. All of my lower cabinets stop at the stem wall....NOTHING touches the ground....and the bottom plate makes a great spot for nails and screws.

Click on the Garage Build link in my signature for pics

 

[wssix99]

Please post your location in your profile so we can see where you are coming from. Often, it makes a difference for situations like this.

I have a sloped floor and don't regret it a single day. If anything, I wish I had also sloped the bays inward a bit so water would have drained out and towards the center of each door. This is also a safety feature and helps passively evacuate heavier-than-air gasses. (In some areas, it's code.)

Framing is a little bit more work, but not a big deal. Each stud is just a little bit different, which gives you a level top plate.

You might also consider a double bottom plate. Secure and properly seal the first plate to the slab and then frame the second to the studs. If you need to tweak things after you put the wall up, you an shim between the two bottom plates.

 

[boostaholic1]

Please post your location in your profile so we can see where you are coming from. Often, it makes a difference for situations like this.

I have a sloped floor and don't regret it a single day. If anything, I wish I had also sloped the bays inward a bit so water would have drained out and towards the center of each door. This is also a safety feature and helps passively evacuate heavier-than-air gasses. (In some areas, it's code.)

Framing is a little bit more work, but not a big deal. Each stud is just a little bit different, which gives you a level top plate.

You might also consider a double bottom plate. Secure and properly seal the first plate to the slab and then frame the second to the studs. If you need to tweak things after you put the wall up, you an shim between the two bottom plates.

We are from Southern IL. After much thought about the situation, I am comfortable with it being sloped. My main concern was the building looking off due to the height difference. It will be a little more work, but I have done the math already and could theoretically cut the studs ahead of time (which I won't do).
As you stated, my plan is to run a double bottom plate to help make up some difference, and maybe try to "lose" the remaining difference.
Thanks for your comment

 

[lakeroadster]

My main concern was the building looking off due to the height difference. I

A 3" slope..... if the siding overlaps the foundation, nobody will ever know.

 

[boostaholic1]

A 3" slope..... if the siding overlaps the foundation, nobody will ever know.

Yes, gonna overlap the plywood to the foundation as necessary. Then hang the siding level from there.
Thanks for your comment. Again, I am not a builder so I don't know exactly what to expect. In my mind's eye, I think I have it figured out.
Now....if it would only quit raining so we can pour the concrete!!
Trying to not get anxious.

 

[cre73]

In my opinion 3" may be a little much, but to each their own. When I built my first garage I went 1.5" and my stepfather chewed my **** when he came over to check things out with the transit before the pour. As he was the one in charge and I didn't have a clue on what I was doing. We left it that way and all worked well. This was on a 24X30.

 

[bad_idea]

One more vote for a separate stem wall (I had mine built at 16" above grade) and pour the slab inside the stem wall at whatever slope you desire. I hired the foundation work done as that is a set of skills I do not have. My contractor built a cinder block wall down 12" (frost line where I am at) on a footer, then built the wall 2 rows of block above grade. Then came back later and poured the floor within the cinder block wall. The walls are built on top of the cinder block. The anchor bolts are set into the footer.

The taller stem wall serves a few purposes:

• Gets the framing up away from the dirt - prevents rot and possible flooding.
• Gives the building cheap height - block is cheaper than wood framing.
• Prevents damage to the inside wall covering - floor jacks and the like will bang into block instead of sheetrock. Can wash the floor without water damage to sheetrock.
• Makes yard work around the building easier - can string trim against the block.
• Gives a level surface to build the walls on.

Are you sure you want a slope inside the garage? What is it's primary use going to be? Parking daily drivers - then you want a slope for snow melt (you all get that there right?). Work shop - much nicer to work on a flat, level floor. Easier to setup jack stands on a flat floor too.

You want to set your anchor bolts into the wet concrete at time of pour for uplift strength. The anchor studs should have a large flat washer on the bottom of them to prevent them from being pulled out. The ultimate goal of the anchor studs is to keep the roof on the building in high winds.

 

[Gladiator]

Ddawg has the right idea. That also keeps your sill plates up above grade

 

[rieferman]

Back in my pole barn salesman days, there was about 1 sloped floor installed every 5 years or so, the rest being level.

One of those was for one of my customers - made the interior finished trickier, but he was planning on constantly detailing cars inside so it was worth the added time/expense in his case.

Later, he reported being annoyed by the sloped floor and wished he'd gone level and just used a push broom to push water out on the rare instances that he ended up actually detailing his vehicles indoors.

 

[Falcon67]

I never slope. Buy a squeegie. I like a flat floor. I about never GET one, but I prefer one.

 

[TTMotorsports]

Flat is all relative when talking concrete. Flat to you or I is totally different than flat to concrete guys. Lol

 

[ClappedOutBport]

I've worked in a shop for years built on a salvage pad that was a mistake. The concrete is rough, titled in every direction, there is a 6" step in the middle, and the upper half is about 3" over 7 feet sloped. That's a bit of an extreme case, but it means everything must be leveled. Welders, drill presses, cabinets, parts washer, even the wood stove. You can't have anything on wheels. In your case, that wouldn't be an issue, but it still makes it harder to level everything. I like the idea of a stem wall.

 

[steveo1o9]

You might want to think about what your needs are and the intended use of the garage. If you only intend on parking cars in it then the slope will be nice for rain and snow melt. If you plan on having a workshop the sloped floor may create a headache when trying to level everything. A 3" drop across that slab is not much so you might never even notice it.

 

[ford33]

I would suggest a flat floor with a level stem wall. Makes it easy to build and gets the wood off the ground or away from moisture.

OP do a search and you will find instances where the near ground level wood sill decayed over the years due to soil build up around the foundation.
Good luck.

 

[pcmeiners]

Separate stem wall, instead of sloping the slab front to back, slope towards center of garage slightly, place a drain in center, very easy to do.

 

[Chilliwack Murray]

I sloped mine toward a drain near the front and happy I did, I can wash the floor and nothing runs away under cabinets or into corners.

I only sloped it about 1" over 32' and it's not really self draining but it's enough to keep it from going the wrong way. Wash it, hose it, broom or squeegie into the drain. I don't think I'd want any more slope, it's still noticeable pushing cars around or fabricating things on the floor. I have a foundation wall around the edges that is higher than the slab as others do so it didn't impact my wall framing and it has little impact on interior walls. Take a measurement at each point you have a stud and cut so the top of the wall is level. Non issue with my amount of slope.

Regarding the floor drain, most places won't allow a drain in a shop, mainly for spill potential but also to prevent flammable vapours from collecting. If you put one in, you will need to strategically pile your building materials on inspection day, but more important, always, always take care not to allow any flammable vapour to collect in the drain. I keep a piece of heavy, flexible rubber around that I lay over the drain if there's any risk.

 

[ZRX61]

3in over 32ft isn't a slope, it's a hill. 1/2-3/4in over 32ft would be a slope.

 

[Chilliwack Murray]

I just measured mine, 3/4” over 32’ and it’s enough.

 

[roc_on_the_rocks]

I just measured mine, 3/4” over 32’ and it’s enough.

There are many ways to do it... I'm just curious, how are you guys measuring this slope? String and line level?

 

[Chilliwack Murray]

Top of the foundation to the top of the slab. The foundation is level.

 

[paredown]

Agree with the separate stem wall, and that the slope seems excessive.

However, stick framing if the slab is already poured is not really a problem. Set your plates, then one corner, then a temp corner. Tie a chalk line between them at the correct height, level. Grab a 2x, put it on the plate, touch the chalk line at the appropriate spot--that's your height for the cut. It does mean you can't prefab the walls easily and tilt them up, so framing will be slower...