General Question

JLeslie's avatar

How do I figure out the grade of a driveway?

Asked by JLeslie (65568points) 1 month ago from iPhone

Let’s say the driveway is 30 feet long and the difference in height from the beginning of the driveway and the end just before the garage is 3 ft. What is the slope or grade? What happens if 10 more feet are added to the driveway (house is further back on the property).

I’m just trying to understand the formula so I can plug in varying numbers.

Also, if a pick-up truck with a trailer behind needs to use the driveway, how do I figure out how steep a driveway can be and it’s ok, that it won’t hit if I back in? The wheels are more than a few feet from the end of the trailer.

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32 Answers

Forever_Free's avatar

So the slope is Rise over Run
3/30 reduced to 1/10
This means that for every 10 feet of driveway, the height changes by 1 foot.
Or differently put your driveway height changes 10 feet for every 100 feet of driveway
That said, your slope is 10%
15% may be considered steep and less that 2% might have issues with poor draining and puddling may occur.
Pickup trucks may be about 20 feet and an average car is 15. Trailers can be anywhere from 8 feet to 30 feet. So the length of the trailer is key. Some motor homes are 40 feet.
All that said, you may want to prep for 50–60 feet. That means a driveway 60 feet long would have a drop of 6 feet at a 10% slope.

JLeslie's avatar

^^The truck is 19.5 feet and the trailer will be 26 feet I think, maybe 30 at most. What do you think would be the most slope without having a problem?

Tropical_Willie's avatar

Check the building code in your city. There usually are minimum and maximum slopes, 2% to 10%.

jca2's avatar

@JLeslie Even a driveway or other flat surface that appears to be level will have some grade for drainage. The standard is ¼ inch per foot.

Tropical_Willie's avatar

Also there maybe a minimum distance from the road to the building / house.

RocketGuy's avatar

@Forever_Free – 6 ft over 60 ft sounds awfully steep. Might be too steep to hitch/unhitch a trailer.

JLeslie's avatar

It’s already under construction. I think it’s too steep.

Stressing me out.

jca2's avatar

@JLeslie This is exactly the kind of conversation you should be having with the architect/engineer.

JLeslie's avatar

^^We can’t, couldn’t. It’s too much to write.

We still are planning on building an additional garage, and now I’ll need to push it back to reduce the slope and have to hope it gets approved. This entire project is a huge risk with unanswered questions.

Tropical_Willie's avatar

We had a new house built in our neighborhood of 65–75 houses. The new house “looked’ a little low in the lot while it was being built. Well it was low, while waiting CO (Certificate of Occupancy) I saw the city at the site.
Well it was too low, The contractor had to redo the driveway (already poured) and regrade the front yard. Then re-pour the concrete – - -

The old driveway was going to send rainwater into the garage !!

rebbel's avatar

Use your iPhone’s level app.

JLeslie's avatar

@rebbel I have a level app?

I tried getting the altitude standing on the street and then standing in the garage, but I don’t think it’s accurate enough.

Edit: I just found it in the measure app. I’ve used that app to measure, I never noticed the level.

zenvelo's avatar

For comparison of slope, an 8 percent grade is considered difficult for a bicycle; 10% is considered pretty steep.

In California, cars parked on a grade of 3% or more must have the wheels curbed.

JLeslie's avatar

My current driveway is -3°. Is that 3%?

RocketGuy's avatar

Tan(3°) = .052 so it’s 5.2 ft/100ft = ~5%

JLeslie's avatar

Wow, if that is 5% I think the other driveway is more than 10%. Crap. I’ll probably go down there on Sunday.

JLoon's avatar

Using your example, and the correct formula already referenced by @Forever_Free :
3 ÷ 30 = .1
× 100 = 10

Your driveway will have a 10% grade/slope.

2018 International Zoning Code in Sec. 801.4 says :
“The maximum slope of any driveway or ramp shall not exceed 20 percent.”

So – on paper the contractor is in compliance and you should be fine. An oversize trailer or other special vehicle may create special problems, but those are your problems.

Not the builder’s.

The right time to work this out would have been before they started pouring the driveway. Now it will likely be viewed as a “change order”, and the contractor will be within his rights under a typical contract if he asks you to eat the extra expense.

Good luck.

JLeslie's avatar

^^Nothing can be changed, the driveway is too short. I’m going to build an additional separate garage, and hoping they let me push it back so it will have a longer driveway and less slope.

JLeslie's avatar

@RocketGuy So, if it’s 7° is it 133% more or 11.65% grade? Or, the math doesn’t work that way?

Tropical_Willie's avatar

I’m going to build an additional separate garage, and hoping they let me push it back so it will have a longer driveway and less slope.”.

How is that going work, side by side driveways with different slopes is a little strange? There going to be a gap . . . where the dirt falls out from under the current driveway !

JLeslie's avatar

@Tropical_Willie I hope it can work. It won’t be drastically different and thinking we will connect the driveways with a circular drive that isn’t quite level. The garages will have a breezeway between them.

jca2's avatar

I just was on a rural road this afternoon and there was a road sign showing a 9% grade and that was pretty steep.

JLeslie's avatar

My husband and I went out there again and it’s difficult to figure it out. What the level on my phone says doesn’t make sense with how high he thinks the house is compared to the road, but it’s closer to how I see it. I think the level is probably the most accurate.

I tried driving up and backing down and my car doesn’t hit in any way so it’s not so steep I need to be careful; I would have hated that. I don’t want to be careful every time I pull out of my garage, I’ve had that with other houses.

@jca2 That’s interesting. Regarding safety, the warning is about picking up speed on the hill I would think, which isn’t a problem on a driveway, but still the information for how it felt matters to me.

JLeslie's avatar

I just read the grade of the skyway bridge in the Tampa Bay area is 4%. That bridge feels very steep. It looks even steeper than it feels.

RocketGuy's avatar

@JLeslie – you would use the Tan(x) function on angle to get rise/run. Tan(7°) = 0.12 so that’s 12/100 (12% grade).

JLeslie's avatar

Oh, I see now. Got it.

RocketGuy's avatar

I was on Interstate 80 to Reno not long ago. It was a 5% grade coming down the hill from Donner Pass. Seemed awfully steep.

zenvelo's avatar

^^^ @RocketGuy That IS steep, which is why they have Westbound brake check areas for trucks before they head towards Sacramento, and have two emergency runaway truck ramps as you approach Colusa and Auburn.

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