How long must a driveway to a basement garage be?
May 10, 2010 10:01 PM   Subscribe

How long does a driveway leading down to a basement garage need to be? You can assume that the path of the driveway starts at the edge of a level sidewalk/pavement, and that the vehicles entering the garage will be up to two metres high and four metres long. I'd appreciate any numbers you can provide that will help me calculate variations on your result.
posted by Joe in Australia to Home & Garden (7 answers total) 1 user marked this as a favorite
A lot of monster homes in Toronto have these. The driveways don't seem any longer than a normal one (about 1.5 Honda Civics long). That being said, the "ground floor" of these houses are usually 5 steps above ground level.
posted by any portmanteau in a storm at 10:21 PM on May 10, 2010

I'm not sure of what the requirements will be in Australia, but there generally are regulations on how steep your driveway can be and how much clearance you need for cars. If you were doing this in the US, your driveway length would essentially have a predetermined minimum length, just due to these regulations.

So, in my area, minimum clearance is required to be 7', which is a bit over 2 meters; maximum slope is 20% (or 1 unit of rise for every 5 of run), but if you go that steep you have to have 8' long "blends" where the drive transitions from level to sloped to prevent your vehicle from cruncing on the pavement. The steepest you can get without bends is 14%. Then you'll have to assume a certain floor thickness for the ground floor of your house above (note that having the ground floor level of your house a bit higher than the curb will reduce the length of your driveway) - this will be somewhere between a foot and 18 inches (third to half a meter), making the total vertical distance required something over 8', assuming the ground level is even with the curb (it most likely isn't). Then, you probably can't start sloping much down into your basement until you get on your own property. In my area, this point is usually quite a few feet back from the curb line.

So, to go 8' down with a 20% driveway in my area, you'd need to go about ten feet back from the curb before even starting to slope down, then 8' of 10% slope at the top and bottom for the blends totalling 16' in length (which gets me down 1.6'), which would then leave me 32' of 20% driveway. The total for this example would be 58' from the curb to the end of the slope, but a 20% driveway into a cave is not too much fun to drive down.

Any slope less than 20% will increase the length of your driveway, but having a house raised up above the curb level will shorten it quite a bit - stairs tend to have much shorter runs than driveway ramps.
posted by LionIndex at 10:30 PM on May 10, 2010

Plus, if you live in an area where snow or ice are likely, remember to make it as long (i.e., less steep) as regulations allow - something the architect forgot in my last home, causing me to lose a few days of work!
posted by aqsakal at 1:08 AM on May 11, 2010

I'll just do a quick calculation without looking at LionIndex's figures and see what I get...

There arew a few useful figures in this PDF about car park construction. They give a minimum headroom of 2.1m and a maximum slope of 1:6. I would guess that the slope transitions could be 3m each at a mean slope of 1:12 each. Allowing a bit of extra height (0.5m) for good measure, these figures give you a total length of (3 + 3 + (2.1 x 6)) = 18.6m = 61 feet.

...and on preview that's not far from what LionIndex calculated. I'm not so sure about having to go back from the curb - where I live (in the UK). people's driveways tend to start sloping as soon as they're on the property (i.e. past the pavement/sidewalk). You might want to check to see what regulations exist.
posted by le morte de bea arthur at 1:22 AM on May 11, 2010

Just to put the numbers in context, the US ADA requirement for an accessible pedestrian ramp is 1:12 (or at least it was last I checked). So one inch of vertical change per foot of horizontal, or twelve meters horizontal for each meter of vertical. This comes out to an 8.3% grade. (I think in some European countries the max might be slightly steeper, more like 10%; ramps there just feel steeper to me.)

A 20% grade is steep. You might be able to drive up it just fine, as long as it's not icy, but you might want to think about doing whatever you can to lessen the incline if you have any accessibility goals, or even if you think you might be rolling some heavy furniture up or down the driveway.

My parents had a house with a 20+% grade driveway, and they had serious problems with erosion and the pavement slowly sliding down the hill over time. Unfortunately it's not something you can easily fix once the house has been sited on the lot. They also went through plow guys at the rate of about one a year; every spring the last winter's guy would say "never again!" and they'd have to find some poor naive soul to do it for the next season.

I would shoot for 10-15% max if you have room for it on the lot, plus the transitional sections on either end so you don't bottom out your vehicle at the top or bottom of the hill, or pin it on the top.
posted by Kadin2048 at 6:45 AM on May 11, 2010

I'm not so sure about having to go back from the curb - where I live (in the UK). people's driveways tend to start sloping as soon as they're on the property (i.e. past the pavement/sidewalk).

Well, that's what I meant - property lines in my town could be anywhere from 5 to 20 feet back from the curb; anything in front of that is the City's property, and they won't allow major construction like a subterranean driveway there. Even if the property line is really close to the street, there still needs to be a flat portion to function as a sidewalk, which pushes the start of the slope back a bit.
posted by LionIndex at 7:08 PM on May 11, 2010

Thanks for all this help - it's not the answer I hoped for, but it's good to have it confirmed. I'm either going to have to scratch the underground garage or try to make the driveway go down the edge of the property boundary.
posted by Joe in Australia at 8:27 PM on May 11, 2010

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