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Drainage System Question


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The following is an excerpt:

Drainage system: Where porous soils are present and drainage problems are not anticipated, no sub drainage system is necessary. Where conditions warrant and the crawl space floor is below that of the exterior grade, a gravel drainage system should be installed. An optional 4-inch-diameter perforated drainpipe may be installed in the gravel. Perforated drainpipes should be placed with holes facing downward alongside the footing on either the outside or inside. Outside placement is preferred for drainage but inside placement is less susceptible to failure. Drainpipes should slope 1 inch in 20 feet and lead to an outfall or sump. A vertical clean-out pipe with an above-grade capped end is recommended to flush out the system. The pipe should be surrounded by at least 6 inches of gravel on the sides and 4 inches of gravel above and below the pipe. Surface or roof drainage systems should never be connected to the subsurface drainage system. (Optional)

What is the reason for not tying the roof drain system to the sub-surface drain system?

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Sub-surface collects water from soil that is saturated and gives up the water to the perf pipe. A roof system collects water and directs it, in toto and volume, to a particular point; where it saturates the soil. Most jurisdictions around here tie the roof discharge into storms or swales and then to storms. Sub-surface also discharges into storms or surface of ground away from foundations/footings.

Have you ever calculated the amount of water a 4/12 roof on a 1300sqft ranch would collect?

PS: I do not know the answer

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810.3896104 gallons on a 1,300 square foot flat roof per inch of rain.

144 cubic inches of water per square foot for each inch of rain.

231 cubic inches of water per gallon.

Start raising the pitch and the roof square footage goes up as does the amount of the water.

But it gets beyond my capacity (and interest) to calculate the total square foot of roof on a 1,300 square foot ranch with a 4/12 pitch.

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Well now, I still think it does. Remember we're talking a simple two slope roof, front and back.

Rain is measured by the cubic inch. If you have a square inch tube and it fills up with two inches of rain, the weather guy says we have had two inches of rain.

Let's take one slope and lay it down flat. Cover it with square inch glass tubes hinged at the bottom to pivot so they are always pointing straight up. Now raise one end. The tubes pivot slightly to stay vertical but I don't think any of those square inches disappear??? Do they???

Hell, I don't know. It makes my head hurt just to think about such anal retentive stuff.

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Hi,

There's a mathematical formula used to calculate roof runoff for cisterns. It's based on a simple formula and some givens. The givens being 1 inch of water on a 1,000 square foot roof will produce 623 gallons of water. Calculate the yield of your roof by multiplying the square footage of your roof by 623 and divide by 1000.

For Les's 1300 square foot home, you need to know the width and length of the home, in addition to the rise and run, and then the amount of rainfall during the period you want to measure (something you can get from the local weather guy.).

Use the length and width of the house and the rise to calculate the total square footage of the cover. Let's say the house is 50ft. long by 26ft. wide to get the 1300 square feet. To keep it simple, we'll assume there are no overhanging eaves. To get the total area of the roof you either get up there and measure it or you calculate it using the rise (52 inches) and run (13ft) for each half. Convert them both to inches and you get a 52 inch rise by a 156 inch run.

Take the square of the rise (24336), add it to the square of the run (2704) and you get 27,040. Take the square root of that and you get 164.43 inches or 13.7ft or 13ft. 8.4 inches. Round that up to 13'9" or 13.75ft. and you've got the chord of one half of the roof from the eave to the ridge. Multiply the length (50) by the width (13.75) and then double it to get your total roof area (1375 ft.).

Now take the area of the cover (1375) and multiply it by the amount of rainfall. Lets say for our example that it rained .4 inches over the past hour and we want to know how much water that produced. 1375 x .4 = 550. Now multiply that number by 623 to get 342,650 and then divide it by 1,000 and you should have the amount of water that drains off the roof during the past hour, which is 342.65 gallons.

Another formula that I have assumes that 12 inches of rain produces 5 gallons of water per square foot and you have to measure the roof across the flat - not the pitched surface. So, assuming 1300 sf at 5 gallons per square foot, you've got 6500 gallons for a foot of rainfall or 541.66 gallons per inch of rainfall. Since only .4 inches fell in the past hour, you're looking at 541.66 x .4 = 216.66 gallons.

So, which one would you guys go with?

Les, what's the length and width of that 1300 square foot rambler and how far does the roof overhang extend beyond the exterior walls?

Warning, I'm a math idiot and calculations of any sort just make my head explode. Don't be surprised if someone whose better at math than I am finds these calcs rife with errors.

ONE TEAM - ONE FIGHT!!!

Mike

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Rake is 13" and overhang is 24". It has cedar shingles, cut valleys, ice and water shield and a 24"x36" brick chimney in the center, two 4" plumbing vents and five standard alum roof pot vents. Wind speed is 0mph and barometric pressure is 29.5.

This is better than poetry!!

Kurt - how is your hair?

Erby, you are on the right track, continue inserting different facts and criteria.

Chad - convert the rainfall to snow and a 60 degree day.

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Picky, picky,

Add 2ft. to the chord you end up with 180" or 15'

Add 13" to either end of that 50' structure and you end up with 52'2". Round it up to 52.25' or 627 inches long.

180" x 627" = 112860" on one side. Multiply it by the two planes and you've got 225,720 inches or 1567.5 sq. ft. (raw).

Toss the vents - negligible.

Figure the area taken up by the chimney at the ridge by calculating the chord: run = 12 inches rise = 4 you get 12.649 inches. Round it up to 13" times the length, which is 36", and you get 468 sq. in. or 3.25 square feet. Double that and you end up with 7.5 sq. ft. Deduct the 7.5 sq.ft. from the raw roof area of 1567.5 and you get exactly 1560 square feet of roof area.

Multiply the area of the roof by the amount of rainfall during the time that you want to calculate, then multiply that by 623 and divide by 1000.

For the example above I used .4 inch over the past hour.

1560 x .4 = 624

624 x 623 = 388752

Divided by 1000 and ignoring evaporation, you get 388 gals and 3 quarts of water, give or take a few ounces.

I ain't even gonna bother trying to re-figure the other one, 'cuz you gotta figure worst case.

You're evil Les. I can see that now.

ONE TEAM - ONE FIGHT!!!

Mike

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On a serious note - This is exactly the sort of thinking and kibitzing that sets the environment for education. I think the water volume is answered.

Now let's move on to why you wouldn't want to dump 388 gallons of water into a foundation drainage system. It is just too much water, it would fill several hundred feet of 4" pipe even if you accounted for a pipe sock, sand and double perfs.

Where was Goodman, when we needed him?

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Originally posted by Terence McCann

....

What is the reason for not tying the roof drain system to the sub-surface drain system?

Check out Chapter 11 (Storm Drainage) of the International Plumbing Code and you will see many of the issues that relate to your question.

It is very complicated to figure out the affects of how the sub-surface drainage conditions, roof area, pipe sizes, rainfall maps, grade, and soil materials all combine together. The less variables, the better chance that the system(s) will be designed correctly and will work properly. Additionally, if there are drainage problems, keeping them separate makes it easier to figure out what is wrong.

It is better to keep them independent.

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It's as simple as a power outage; if the power goes out in a storm, the tile backs up into the sump, ergo, the bsmt. get's filled w/water.

I'm very familiar w/that arrangement here in Chicago; it's how every old house is built. All the roof drains tie to the sanitary; combined sewers.

If it was good enough for the foolish ancients, it's good enough for Chicago.....

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RE Erby's diagram, there's more square footage of roof, but the volume of rainfall per square foot is going to be diminished. The amount of rain falling on a 12" x 12" patch of earth is finite during however long the measuring period is. If it's a gallon of water per square foot, the slope of a roof is moot. Sure, the pitch creates more roof area that becomes wet, and it also accelerates the velocity of the rain's descent onto the ground, but the volume of water will not change. If I drop a gallon of water on the 12" x 12" square, regardless of the angles it encounters on its way down, the amount of water still equals one gallon.

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Originally posted by Bain

RE Erby's diagram, there's more square footage of roof, but the volume of rainfall per square foot is going to be diminished. The amount of rain falling on a 12" x 12" patch of earth is finite during however long the measuring period is. If it's a gallon of water per square foot, the slope of a roof is moot. Sure, the pitch creates more roof area that becomes wet, and it also accelerates the velocity of the rain's descent onto the ground, but the volume of water will not change. If I drop a gallon of water on the 12" x 12" square, regardless of the angles it encounters on its way down, the amount of water still equals one gallon.

Erby, John's explained it very well above. Surely you don't believe that more rain falls on a roof as its slope increases?

- Jim Katen, Oregon

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