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We're about to level our front yard. It will be partly sown with grass, and partly planted with trees & shrubs, though it may not be completely relevant to this question. What is a suitable slope to gently drain water away from the house? I think one of the tradespeople I spoke to suggested a 1 in 65 slope would be adequate, which by my calculations would be a drop of roughly 15mm for every horizontal metre. Is this about right, not enough, or too much?

Also, I had a go at pulling a string taught from one side of the yard to the other. Holding my spirit level against the string didn't really work very well for working out what was level. What is the best equipment for determining what's level?

I think I have a small spirit level that can attach to a string (from a picture framing kit) which might be better, though I don't know how accurate it will be (I can calibrate it against my big spirit level probably).

Background info: we have high rainfall sometimes in the winter, though the soil should have reasonably good drainage qualities (no clay). The size of the yard isn't very big. The max distance from the house to the edge of the yard is probably about 7 metres.

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The best equipment for determining level is a transit. I've done with preliminary work with string and bubble level only to find out when it was checked with the transit that the early work was very off. It's a two person job, but if you set it up properly you can make precise, accurate measurements over a pretty good distance -- 7m is trivial. –  bstpierre Mar 2 '12 at 1:42
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For simple slope comparison of this nature, I'd use a laser. You could make sufficient estimates as simply as pushing a marked stake into the outside edge of your yard, setting a laser pointer on top of the stake aimed at the house, and adjusting level with toothpicks or something as shims. That should give you enough info to work with, at least. –  Scivitri Mar 2 '12 at 17:16
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Or, use a water level. It probably has the best accuracy per buck (at a distance), if a laser level isn't at hand and isn't likely to find further use. –  Ed Staub Mar 2 '12 at 19:40
    
Thanks very much, @EdStaub. For around NZ$8 I got about 12m of hose and a bucket and made a water level. Next time, I'd use slightly larger hosing (it was about 6mm I think; very thin) as the water took a bit to long to reach its equilibrium. –  Highly Irregular May 12 '12 at 10:04

2 Answers 2

up vote 6 down vote accepted

The minimum slope that I recommend is two percent, or 1/4 inch per foot. As an example, the grade at a spot four feet away from your house should be one inch lower than the grade next to your house. This will do for most houses with soil that will absorb water readily.

For your 7 metre garden (~ 22.9 feet) this would be no more than a 6" drop.

Keep in mind that too much slope means difficulties with mowing equipment. 12.5 degree of slope or a 22.2 percent slope is as much as you want to have close to your house.

@Scivitri's comment: I agree, for the purpose of grading a laser level would do the job.

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All you need is the equation; rise/run=slope. For the bulk of my design work all I used was a 'pea-shooter'...grin. Using the height from the ground to your eye you can make very accurate readings in grade. Enough to compile a map of contours for your site. Use the main floor of your home as ZERO. Or whatever number makes sense to you. You have to go across property lines for this to be useful.

A 'pea-shooter' is basically a level with a magnified eyepiece. Find one at a surveying equipment store. Grin. Let us know if this makes any sense to you. I have a feeling you'll get this right away.

Your numbers were about 1.5% slope...which is okay. The devil is in the details; how much rain, what are your soil properties, what is happening on the other side of your property lines, what kind of plantings do you have and what kind of pervious/impervious surfaces are in play on your site and off...see if this makes sense, let me know if not. You'll be able to know where each drop of water landing on your site will go and be able to direct it where you want it to go!

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