Lowering garage floor?

[kylet]

Been tossing some ideas around how to make my garage more adaptable. We are going to have to do some significant concrete work to replace a broken sewer pipe in the future so the idea to sink the garage floor to make the ceilings taller came up. Our garage is up on a hill with an incline up to it. Slab with brick and wood panel exterior walls. Late 60’s construction.

has anyone had first hand experience doing this? What are the big pitfalls?

 

[Shiftless]

I don’t have any experience with that, but a few points come to mind.

You have a garage at the top of a hill with a sewer pipe running underneath it???

The walls no doubt have footings underneath them. If you drop the floor any appreciable amount you might risk compromising the strength of those footings.

 

[cvairwerks]

If it's an attached garage, it will be a massive project if you want to drop it more than a couple of inches. You would need to build new supporting structure for slab section that the walls sit on. I'd expect that you would need to trench inside the garage wall areas at least 2 times the depth you want to drop the floor for new supporting structure.

 

[larry4406]

Underpinning the footings will likely be involved which is a real PIA.

 

[firebirdparts]

It seems like a lot of assumptions have been made here already. I don't live in Texas, but you need to at least open your eyes and look at the building with a view to whether or not there's any evidence at all that the perimeter of the building goes below the floor. If it does, then there is some potential to remove the floor and dig down some distance. There might even be a void under the floor already. If not, it'll be rubble basically and then the original grade level. Footer depth will determine a practical distance if you disregard issues with how deep the footers "should" be.

on the other hand, if the building is on a slab, then the building is actually sitting on top of the floor. in that case, you'd have to remove part of the floor, and you'd have to leave part of the floor to include the part that the building is sitting on. I don't think this is hard to do, but it is going to leave a sort of a step on the inside.

 

[FredWanaker]

hire an engineer to give you a verbal overview. If that is too expensive, stop now while you are ahead. If the verbal overview is affordable then have (insert pronoun) convert it to drawings.

 

[kylet]

It’s detached.
No the sewer is under the driveway requiring it to be taken 90% up. There is a stem wall about 6” tall around the perimeter poured with the floor. I assume footers too based on erosion and one side exterior dirt level being below the floor grade. How far I don’t know so I’d have to investigate.

from the replies it sounds extremely cost prohibitive though.

 

[FMB4]

If you want taller then build upwards. Otherwise, as above, you're looking a ton of work just to gain several inches or so.

 

[Stuart in MN]

If the building is on footers and the floor is a separate slab it may be possible to remove the floor slab, dig down a little, and put in a new floor, but it all depends on how it was constructed.

 

[cvairwerks]

Here in the DFW area, footers are probably no deeper than 18" below grade. Best suggestion would be an evaluation to either raise the entire structure and insert a stem wall to gain the height, or pull the roof off and raise it with an extension at the top. It's going to take some engineering no matter what, and an evaluation of what will be allowed by code, if you are in a location with code.

 

[CraigStu]

"...There is a stem wall about 6” tall around the perimeter poured with the floor.." To me this is a deal killer. My garage is footers, a block stem wall w/ the stick built walls on top of the stem wall. Slab was poured last. So for mine I could just demolish the slab, dig out some of the gravel and dirt and pour a new floor lower. But for you the stem wall is part of the floor. So maybe you could saw cut the floor near the wall to remove it. But now you have a stem wall that is not properly supported but is holding up your garage. So I would want to support the garage while the stem wall is removed along w/ the floor. At that point, I say jack the garage up off the stem wall and add to the stem wall. But, not being an engineer, I don't know if that would be a good idea or not.

 

[Forgottonia]

I lowered my basement floor a few years ago. The house was built in 1915 and the basement floor was crowned at the middle--up about 6" higher than the basement floor around the sides. I only had an inch or so of head room in the middle of the floor. (I'm 6'3" tall.) I hired a guy to help and we broke up the old floor with sledgehammers and carried the rubble out in 5 gallon buckets. (I know, I know -- I sometimes do things the hard way. lol) That demo project was a hellish good workout!

Then I dug down several inches and contoured the floor for a gentle slope downward towards the middle. I put in a couple of floor drains and plumbed a bathroom while we had the dirt exposed. Finally, I tamped it all down and put a layer of sand down for the concrete to sit on. I broke out one of the basement block glass windows for the pour and hired a couple guys from a concrete crew to help finish the surface. It turned out pretty decent, if I do say so myself. I have about 10" of head room now.

 

[pbon]

I agree with being concerned about the foundation that essentially you would be doing away with. I have a barn I lowered the floor in but it had a 5’ ceiling basement so the foundation actually went way down below ground. I removed the wood floor, filled in the basement with dirt and poured a slab on top of it that was about 18” lower than the original wood floor.

You could dig up the perimeter and pour new footers, but I don’t know the details on that.

 

[dcg9381]

We are going to have to do some significant concrete work to replace a broken sewer pipe in the future so the idea to sink the garage floor to make the ceilings taller came up. Our garage is up on a hill with an incline up to it.

has anyone had first hand experience doing this? What are the big pitfalls?

Slab construction in Texas is typical concrete and steel - probably basic rebar if you have 1960s construction.
Slabs are not that thick. They usually have a "perimeter" beam of concrete around them that is very different from the slab depth everywhere else.

To get to your plumbing, they typically cut away and chip out a section of concrete that follows the drain path. This can compromise the connectivity of the steel, but doesn't always cause a problem... They'll cut that concrete out, remove the pipe, put a new one in, and then patch the concrete.

It's very unlikely that you have enough depth to your slab to be able to reduce the slab height. In the few cases where we've had to remove 1-1.5" of concrete (IE mis-poured) landing, it's very labor intensive... And then you have to re-pour a skim coat on the whole thing. It's likely that this cannot be done (at all)...