Pouring a Floor Lower Than the Surrounding Grade

[KDXSR5]

I got in to a discussion today with another person concerning overall building heights and floor elevations. Here is the deal:

- The code of the town that was discussed limits you to an overall building height of 18ft 6in measured from the finished grade to the peak.

- To build the hypothetical building we were discussing, one would really need 20ft 6 inches from the finished interior floor to the peak of the building.

One solution discussed was building with shorter side walls to adhere to the 18ft 6in grade to peak height, then pour the floor of the building 2ft lower than the grade surrounding the building.This would effectively create a 2ft hole inside the building. A drain system would then more than likely be built to contain any potential water that makes its way into the building.

Obviously the building design could be changed to accommodate both the code and the needs of the building owner, but that is not the point of this discussion. We wanted to see if pouring a below grade floor was a feasible solution.

So, has anyone done this? Is this a reasonable solution? Potentially a very stupid solution? Any other ideas, tips, pointers? Thank you!

 

[FullRaceMerc]

I have done work on a hillside house with height limits above grade. Part of that house is set below grade to meet the regs around the entire exterior while still keeping the interior height nice.

We remodeled another home where dropping the living room floor below grade kept a nice ceiling height under the 2nd floor above. We did additional waterproofing around it, ran raingutters everywhere, added exterior drains, & were careful with the grading around it in case the drain system ever clogged. Lots of attention to make sure water never was allowed to stand at the exterior. So far so good after several years.

 

[DougWil]

Ummmm....... basement floors are all below grade, you aren't inventing the wheel here.

 

[KDXSR5]

Ummmm....... basement floors are all below grade, you aren't inventing the wheel here.

For a garage/shop application, is there any info of value you can add? How about the slope needed to reach the lower floor? Inside the building or out?

This is slightly different from a "normal" basement garage/workshop in that there is no entrance at ground level. All entrwmces will be 2 ft below grade. All basement garages I have seen are built into hills that allow an entrance to slope away from the structure, not towards it.

I realize the basement idea is nothing new; however, I feel there are additional challenges that one might face in this situation that would not normally present themselves in a normal basement build.

 

[KDXSR5]

Ummmm....... basement floors are all below grade, you aren't inventing the wheel here.

Look at this a different way:

I have a building that is built with the concrete floor 6 inches above grade. I am now going to rip that floor out, dig down 2ft 6in, and then repour a 6 inch thick slab. What problems would you encounter once you are ready to pour the new slab? How would you overcome them?

 

[James-W]

I don't think having the garage floor 2 feet below ground level is a good idea. I suspect you would have a water issue that would be difficult to deal with. I am sure any issues could be resolved in one manner or another, but it may end up costing a whole lot more money than you would want to spend.

 

[theoldwizard1]

Look at this a different way:

I have a building that is built with the concrete floor 6 inches above grade. I am now going to rip that floor out, dig down 2ft 6in, and then repour a 6 inch thick slab. What problems would you encounter once you are ready to pour the new slab? How would you overcome them?

You need proper drainage to a sump with a pump or you are going to be hating life. Late winter/early spring will be a problem unless you have proper grading. The ground will be frozen so you external drainage system probably won't work. Then, a fast melt or a rain/sleet/ice storm could allow water into the building.

Your drainage pipe need to be set BELOW the level of the new floor. Before that, dig your trench, line with commercial landscape cloth. Add 1"-2" of gravel (for adjusting pitch) and then lay your sock covered drainage. Mack fill with gravel. The typical way to prevent the sump and pump from freezing is to place them INSIDE the building.

 

[matt_i]

It would be ideal if you had an even lower point on the property to gravity-drain all the potential water...foundation drains, and probably a slot drain in front of the garage door that would empty the waterflow from the downslope entry drive. Imo the foundation walls will have to be water-proofed just like basement walls.

 

[FullRaceMerc]

I forget that you guys get that frozen water stuff in other parts of the country. Not even something we have to think about around here. Mountain cabins, yeah, but down here, nope. No useful experience here.

 

[matt_i]

Good point, having to dig out every single snowfall, lest it fall inside if the door was opened, would get old after awhile.

 

[KDXSR5]

These are al good points guys, thank you.

The location of this build does see large snowfalls and extended periods well below freezing.

The footer depth here is a minimum of 42". So, if I am understanding correctly, the trenching and gravel would be done down at that depth and a sump installed, all inside the building perimeter walls at full footer depth.

Homes in the area discussed do not have sump pumps, even with full depth basements. The soil in the area is mostly comprised of very well draining sand. Would this have any impact on the need for this underground drain and sump system?

A slot drain at any doors would definitely be a smart idea to prevent a large ingress of water. A fairly large slot would probably be necessary to prevent ice and snow from clogging the drain inlet.

The build area in the discussion is currently petty flat with minimal elevation change. To provide drainage at a lower elevation than the lowered building floor, a reservoir of sorts would need to be dug elsewhere on site, or the building would need to be built on a small built up hill of sorts.

I appreciate the input guys, thank.you.

 

[wssix99]

So, has anyone done this? Is this a reasonable solution? Potentially a very stupid solution? Any other ideas, tips, pointers?

It's probably not going to be a cost effective or the most maintainable solution.

Your biggest problem is an explosion hazard. "Digging in" the garage creates a bath tub like structure where heavier-than-air gasses (like gasoline) can congregate in an enclosed space. Codes typically specify that garages have sloped floors directed towards the doors, above grade due to this phenomena. (You may also see people elevating sources of spark/ignition, like water heaters, etc. in garage spaces for the same reason.)

This topic comes up on this site every so often when people look to store cars in basements,(using elevators) put living space under garages, install maintenance pits, or in-floor drains. This can be done - but, like anything, it just takes a big pile of money. Some additional commercial mechanical systems would be required to detect and evacuate gasses, which you'd need to operate, maintain, etc.

Then... you would be prone to flooding, so that's another nuisance that would add more design and cost.

For all the fuss, your money is probably better spent on a good lawyer who can slam-dunk a zoning variance for you. I would expect that the cost to do that would be less than your construction and equipment cost.

 

[Showkey]

Assuming your not going to have a "step down drive way" ? Or the slope of the drive way is inside the building ?

If the inspector measures the grade at the front of the entrance to the garage .........and the entrance is level with the floor the height would be the same. As opposed to the side or back where the height might be two feet difference.

Before doing this "get around plan"...........get them to above the plan.......or your could be screwed.

I have seen this lowered floor in remodeling in a way to gain inside height for RV entrance and storage.

 

[KDXSR5]

It's probably not going to be a cost effective or the most maintainable solution.

Your biggest problem is an explosion hazard. "Digging in" the garage creates a bath tub like structure where heavier-than-air gasses (like gasoline) can congregate in an enclosed space. Codes typically specify that garages have sloped floors directed towards the doors, above grade due to this phenomena. (You may also see people elevating sources of spark/ignition, like water heaters, etc. in garage spaces for the same reason.)

This topic comes up on this site every so often when people look to store cars in basements,(using elevators) put living space under garages, install maintenance pits, or in-floor drains. This can be done - but, like anything, it just takes a big pile of money. Some additional commercial mechanical systems would be required to detect and evacuate gasses, which you'd need to operate, maintain, etc.

Then... you would be prone to flooding, so that's another nuisance that would add more design and cost.

For all the fuss, your money is probably better spent on a good lawyer who can slam-dunk a zoning variance for you. I would expect that the cost to do that would be less than your construction and equipment cost.

These are all good points. Any detection/evacuation controls/equipment I can source at or near cost and install/program myself due to my line of work. This will ease the money side a bit, but the rest would still be pretty expensive and a hassle as you have pointed out. Tha k you for the input.

 

[KDXSR5]

If the inspector measures the grade at the front of the entrance to the garage .........and the entrance is level with the floor the height would be the same. As opposed to the side or back whwere the height might be two feet difference.

Before doing this get around plan...........get them to above the plan.......or your could be screwed.

This is a great point and something that would have to be addressed before performing any work. The other option we had discussed was sloping the floor inside the garage instead of outside. This would provide the necessary grade level outside and allow the lowered floor. Thank you.

 

[DougWil]

For a garage/shop application, is there any info of value you can add? How about the slope needed to reach the lower floor? Inside the building or out?

Underground parking garages etc,,, are below grade.

You would want to have drainage below or right at the bottom of footing, waterproof the stemwalls (which are now retaining walls), sump pump, drywell, etc.. All of this is do able and has been done forever it just cost a lot over having a conventional building with the slab above grade.

And if you aren't around and the sump pump fails, you got a disaster.

 

[wssix99]

These are all good points. Any detection/evacuation controls/equipment I can source at or near cost and install/program myself due to my line of work. This will ease the money side a bit, but the rest would still be pretty expensive and a hassle as you have pointed out. Tha k you for the input.

In the threads where we have discussed this before, I haven't come across any resources for this type of thing. They are out there - for every underground parking garage, Jiffy Lube, etc. but I'm not sure if we have that knowledge out here yet.

I'd still look at the variance. Just the machine time and excavation to dig in your garage should be half the bill for that.

 

[wssix99]

I suppose a movable roof is out of the question?

 

[Chaznsc]

Good point, having to dig out every single snowfall, lest it fall inside if the door was opened, would get old after awhile.

Whats snow?

 

[wssix99]

Whats snow?

Angel dandruff.

 

[Firebrick43]

Have you looked into a variance?

 

[K13]

My neighbour has an underground garage (probably 5 feet below grade) and we get 6 months of winter a year. He has a grate that runs the width of his door at the very bottom of his driveway and a drain in the garage. In 15 years of me living here he hasn't had a single issue with flooding.

 

[bczygan]

I got in to a discussion today with another person concerning overall building heights and floor elevations. Here is the deal:

- The code of the town that was discussed limits you to an overall building height of 18ft 6in measured from the finished grade to the peak.

- To build the hypothetical building we were discussing, one would really need 20ft 6 inches from the finished interior floor to the peak of the building.

One solution discussed was building with shorter side walls to adhere to the 18ft 6in grade to peak height, then pour the floor of the building 2ft lower than the grade surrounding the building.This would effectively create a 2ft hole inside the building. A drain system would then more than likely be built to contain any potential water that makes its way into the building.

Obviously the building design could be changed to accommodate both the code and the needs of the building owner, but that is not the point of this discussion. We wanted to see if pouring a below grade floor was a feasible solution.

So, has anyone done this? Is this a reasonable solution? Potentially a very stupid solution? Any other ideas, tips, pointers? Thank you!

Yes! We call them enclosed swimming pools!

All kidding aside, sure, why not?

Better yet, put a berm around a building. How is finish grade defined?

Putting the slope inside solves the problems of code measurement at the door, as well as drainage problems. Of course it will eat space.

Bill

 

[KDXSR5]

Have you looked into a variance?

This is mainly a hypothetical between several of us that live in said town right now, so no. To obtain a variance, we would have to prove a hardship. This would probably be difficult.

My neighbour has an underground garage (probably 5 feet below grade) and we get 6 months of winter a year. He has a grate that runs the width of his door at the very bottom of his driveway and a drain in the garage. In 15 years of me living here he hasn't had a single issue with flooding.

This sounds promising!

Yes! We call them enclosed swimming pools!

All kidding aside, sure, why not?

Better yet, put a berm around a building. How is finish grade defined?

Putting the slope inside solves the problems of code measurement at the door, as well as drainage problems. Of course it will eat space.

Bill

The berm idea may just be the best solution if they will allow the doorway to be at a lower elevation than the berm. Otherwise there would basically be a large tabletop speed bump to access the building.

 

[Stuart in MN]

My neighbour has an underground garage (probably 5 feet below grade) and we get 6 months of winter a year. He has a grate that runs the width of his door at the very bottom of his driveway and a drain in the garage. In 15 years of me living here he hasn't had a single issue with flooding.

My neighbor has a tuck under garage in their house here in Minnesota that was built in 1909. It's set up the same way - the driveway slopes down to the garage door, there's a grate at the bottom, and then there's a drain in the garage floor. As far as I know they haven't had any problems in 107 years.

 

[Jlbc212]

Drains in a garage can be a problem if any gasoline, oil or other substance considered hazardous to the enviroment spills onto the floor. Some codes require engineered drainage systems to keep these substances from contaminating the ground water. Have you considered altering the shape, size of the building to lower the height of the peak?

 

[bczygan]

Flat roof.

 

[Hilltopmasonry]

I would do a flat roof instead of all that....whole lot cheaper


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[Red05GT]

What about shortening the sidewall height by 2' and then have scissors trusses designed to gain some of the lost height back. I didn't see what roof pitch is currently planned or
required. Lowering pitch by 1 might do it if you're talking 48' wide building. Can you get the floor drain to daylight if you drop the floor 2 feet?

 

[shedfullatools]

This guy did it

, skip to about 2:50 and the build starts. He did his doors above the floor and lowers tools and equipment in but similar concept to what you seem to be think about. Lots of other videos on his channel about the building too Edit: that building isnt used for automotive garage purposes either so explosion and environmental risks wouldn't have posed an issue for him I suppose...

 

[KDXSR5]

What about shortening the sidewall height by 2' and then have scissors trusses designed to gain some of the lost height back. I didn't see what roof pitch is currently planned or
required. Lowering pitch by 1 might do it if you're talking 48' wide building. Can you get the floor drain to daylight if you drop the floor 2 feet?

This is a good idea. The hypothetical design we were discussing includes a gambrel roof on 10 ft sidewalls. Unfortunately, changing the slopes and/or ratios of gambrel roofs can make them look very odd and disproportionate in a hurry.

 

[KDXSR5]

This guy did it

, skip to about 2:50 and the build starts. He did his doors above the floor and lowers tools and equipment in but similar concept to what you seem to be think about. Lots of other videos on his channel about the building too Edit: that building isnt used for automotive garage purposes either so explosion and environmental risks wouldn't have posed an issue for him I suppose...

I have seen this guy's build before. It is very impressive, and a very nice little workshop. I will review a few of his videos again. Thank you for the link!

 

[KDXSR5]

I would do a flat roof instead of all that....whole lot cheaper


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A building with a normal A frame 4/12 or greater roof could easily be built to do what we had discussed and meet the town's requirements. One of us was looking at plans for a gambrel truss building that begins to look funky if you mess with proportions, hence the idea of dropping the floor to keep the gambrel trusses the shape desired and get a first floor with adequate head room all while meeting the town's height limit.

 

[tthornto]

Could you *excuse me, Could this "hypothetical" person who may or may not be you, build the building at the current grade with a foundation that extends 2-3 feet out of the ground, and then regrade or bring in fill to bring the finish grade up a few feet?

Is there any slope to the property? If so could it be built into the slope to lower the height?

have you considered other roof designs to get the interior height you want; Flat, low slope, gambrel?

You may be able to find a loophole by finding out exactly what the definitions of "finish grade" and "building height" are according to the city, county, homeowners association, or whatever governing body imposes that limitation.

 

[ItsNemo]

I don't think I've seen it asked but my question would be why you need 20 feet instead of just 18 feet? What finished interior height do you actually require? Can you just do different trusses to give you extra height? Realistically the most anyone could ever use in a standard residential garage is 14 feet, maybe 16 feet.

 

[lakeroadster]

What about shortening the sidewall height by 2' and then have scissors trusses designed to gain some of the lost height back. I didn't see what roof pitch is currently planned or
required. Lowering pitch by 1 might do it if you're talking 48' wide building. Can you get the floor drain to daylight if you drop the floor 2 feet?

This is a good idea. The hypothetical design we were discussing includes a gambrel roof on 10 ft sidewalls. Unfortunately, changing the slopes and/or ratios of gambrel roofs can make them look very odd and disproportionate in a hurry.

Why a gambrel roof?

As for other alternatives, have you considered a Ridge Beam, or the use trusses spaced, say 12 feet with purlins such as those on pole barns and steel frame buildings?

 

[KDXSR5]

Why a gambrel roof?

As for other alternatives, have you considered a Ridge Beam, or the use trusses spaced, say 12 feet with purlins such as those on pole barns and steel frame buildings?

Why not a gambrel roof? During the original discussion, several different roof types were discussed. Like I said in the first post, changing the building design would definitely be the easiest, but if it is changed to something other than what a person really wants, why bother at all? Mainly I am more curious about the potential issues with a below grade floor than anything. This way I have a few talking points when I meet up with the other guys again this weekend. Maybe, since all of us live in the same town, we should just start a petition to get the height restriction changed, haha.

 

[theoldwizard1]

This guy did it

, skip to about 2:50 and the build starts.

Very interesting. Ir appears that the grade to the right and back falls off so he does not have much issue with drainage.

 

[Aberdale]

I have an existing barn on my property built in the 1930s/1940s. Architecturally it's pretty nice, but the ceiling height on the bottom floor was 9 1/2 feet above grade. I needed at least 10 feet ceilings to get my tractors inside, so I excavated down 2 feet and poured a 6" concrete floor with no drainage about 20 years ago. It has worked out well for me.

In my case, the foundation walls were poured, and I left the foundation at each door entrance raised about 6 inches to help keep water out during a hard rain. I built small ramps inside and out to drive over them. In addition, I excavated around the barn to create a swale that would direct water away from the barn on all sides, and added a french drain at the perimeter of the swale to keep water from pooling.

 

[wssix99]

This is mainly a hypothetical between several of us that live in said town right now, so no. To obtain a variance, we would have to prove a hardship. This would probably be difficult.

This is not difficult if you pay an experienced attorney. (If you are paying an attorney, that should be hardship enough!) They know all the methods to exhibit this.