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Terence McCann

Drainage System Question

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Bill's drawing says it all.

Kurt, thanks for getting back to the original issue. If the underground drainage can't handle the downspout discharge during heavy rain, it'll back up through the tile. Bad.

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Originally posted by homnspector

Suppose it is a blowing rain at a 45 degree angle, coming from both sides of the house??

Then you have just entered..The Twilight Zone. (cue the music)

Kurt Mitenbuler was an endlessly curious home inspector who decided to spend a weekend covering his roof with square glass tubes to collect rainwater and calculate the volume. Simple enough, and we've all thought about doing that, but on this particular weekend the rain would not only fall into square glass tubes; it would also fall into..The Twilight Zone. (action!)

Brian G.

Night of the Similtaneous But Opposing 45 Degree Winds Only a Roof Ridge Apart [:-alien]

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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?

The purpose of the sub-surface drain system is to collect and to convey water AWAY from the foundation. If you plumb the roof drainage system into the sub-surface drain system, runoff from the roof will cause the sub-surface drain system to work in reverse -- a lot of water will be applied to the soil right next to the footings. At best, this would cause elevated moisture levels in the basement or crawlspace. And at its worst, it could undermine the footings and cause structural damage to the home.

The rest of this thread had me chuckling since it reminded me of many situations I have encountered while working in an office full of engineers. At about this point in the conversation I would chime in with "let's put away the micrometers guys, I have my chainsaw and we're only cutting firewood!"

For most purposes, if you want to know the volume of water you will get running off of a roof you only need to know two things -- the "footprint" area of the roof surface and how much rain (total inches) -- and you need to make two assumptions -- the rain is falling straight down and all the rain that lands on the roof runs off. For a simple gable roof the footprint area is just the distance measured rake-to-rake multiplied by the horizontal distance measured from drip edge to drip edge (or the horizontal distance from ridge to drip edge if you want to do it for one side of the roof).

runoff, gallons = (area, square feet)x(rainfall, inches)x(1 foot/12 inches)x(7.48 gallons/cubic foot)

For Les' example, that comes out to a little more than 800 gallons per inch of rain, which is "close enough for government work".

Now, if you want to get a lot more accurate than that, you'll need to factor in things like:

-- the orientation of each roof plane with respect to the direction that the rain drops are falling (which brings in factors such as wind speed, wind direction, pitch of the roof, and orientation of the roof with respect to the wind direction)

-- the surface area and dimensions of each roof plane instead of "footprint"

-- the size of the raindrops and the intensity (rate) that the rain is falling

-- the type of roof covering material (e.g., metal sheds almost all of the water while wood shingles or shakes absorb some of it)

-- weather conditions for several days prior to the rainfall event (how dry is the roof covering when the rain began?)

-- adjust the areas within the boundaries of the roof surfaces for things that would capture rainfall instead of allowing it to run off (open plumbing vents, uncapped chimneys)

-- subtract off evaporation from the roof surface from the time the rain started until the last drop of runoff left

-- add in how much water would be released from that which is stored on the roof surface in the form of accumulated snow or ice

-- subtract off how much rain gets into the home through the leaky roof

-- How much moss is growing on the roof and how thirsty is it?

If you want to visualize how the direction the rain is falling and the orientation of the roof surface affects the runoff, you can try what I did. I took one of those 3x3 post-it notes and drew a series of parallel arrows on it that point in one direction. That's the rain falling (with intensity constant). I imagined the open top of my coffee cup is the roof surface.

When I hold the post-it over the coffee cup and point the arrows straight down, they all go into the cup.

If I rotate the post-it so the arrows move down and at an angle, and I put the lowest corner of the post-it on the rim of the cup, only some of the arrows go into the cup. The farther I move from vertical, the less arrows go into the cup. This is what happens when wind-blown rain strikes a flat roof.

Now, if I hold the post-it steady at the same angle and put it on the rim of the cup, and I start rotating the cup toward the arrows, more arrows point into the cup. And if I rotate the cup away from the arrows, less go in. In this case, the rim of the cup represents one plane on a pitched roof and it shows what happens as the pitch varies (with rainfall intensity, wind direction, and roof plane surface area held constant). In reality, as the pitch of the roof increases, the surface area of the roof plane increases along with it, so try this again but imagine that the coffee cup is increasing in diameter as you rotate it farther away from vertical.

Having said all of that, I'm sure that with not too much more effort we can get this thread to two pages. Kurt, how's your hair feel now? Oh, and I need to get some of those glass tube thingys too, and some plywood, and a garden hose ...

Brandon

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The purpose of the sub-surface drain system is to collect and to convey water AWAY from the foundation. If you plumb the roof drainage system into the sub-surface drain system, runoff from the roof will cause the sub-surface drain system to work in reverse -- a lot of water will be applied to the soil right next to the footings. At best, this would cause elevated moisture levels in the basement or crawlspace. And at its worst, it could undermine the footings and cause structural damage to the home.id="blue">

Sounds a lot like my first reply to this thread[^]

Goodman may be back but He's still out!

Come on page 2

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Originally posted by Brandon Chew

If you want to visualize how the direction the rain is falling and the orientation of the roof surface affects the runoff, you can try what I did. I took one of those 3x3 post-it notes and drew a series of parallel arrows on it that point in one direction. That's the rain falling (with intensity constant). I imagined the open top of my coffee cup is the roof surface.

When I hold the post-it over the coffee cup and point the arrows straight down, they all go into the cup.

If I rotate the post-it so the arrows move down and at an angle, and I put the lowest corner of the post-it on the rim of the cup, only some of the arrows go into the cup. The farther I move from vertical, the less arrows go into the cup. This is what happens when wind-blown rain strikes a flat roof.

Now, if I hold the post-it steady at the same angle and put it on the rim of the cup, and I start rotating the cup toward the arrows, more arrows point into the cup. And if I rotate the cup away from the arrows, less go in. In this case, the rim of the cup represents one plane on a pitched roof and it shows what happens as the pitch varies (with rainfall intensity, wind direction, and roof plane surface area held constant). In reality, as the pitch of the roof increases, the surface area of the roof plane increases along with it, so try this again but imagine that the coffee cup is increasing in diameter as you rotate it farther away from vertical.

Brandon

I'm not sure I buy into that Brandon.

Let's say that we're just talking how much rain hits the earth’s surface. Let’s assume that the surface of the earth is flat as a board, same as a flat roof. In a straight down pour all the rain would hit the earth’s surface right? If the rain came in at an angle, less than 90°, all the rain would still hit the earth’s surface. Same would apply in a flat roof.

In a pitched roof, the leeward side would not pick up as much as the windward side but I can’t see how a flat roof would get less rain in windy conditions. Gravity has an affect.

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Jeez,

Nobody's found errors in my math yet? That's a surprise. It took me 4 years to make it through Algebra I so I could graduate high school, and then I only made it by 1 point. Me an' numbers don't mix.

I guess Terry has got his answer about the drains. My experience is similar to Kurt's. Most of Seattle still doesn't have storm sewers and downspout systems on most of the homes built before the 70's are inter-connected to the city's sanitary sewer system, along with the drain/waste plumbing, exterior stairwell drains and driveway drains. During rare periods of intense rain activity, the sewer system can't handle the extra volume of water and the TV news will show the occasional homeowner standing in a basement trying to stop up the geyser coming in through the toilet in a basement bathroom.

Although I've never dug one up, I'm pretty confident that these are not inter-connected to the footing drains and that the footing drains, when they are there, aren't connected to the sewers either. I think they simply extend out under the yards and allow water to disperse away from the foundation. Homes built after the 70's here are a little higher tech with dry wells to accept roof runoff. The newest have concrete wells that are built to overflow into the city sewer when/if enough rain falls to cause them to overflow.

Other parts of the city, they're in the process of converting by installing storm sewer systems separate from the sanitary sewer system and projected time for conversion for the entire city is somewhere around 2025.

The most often seen cause of water infiltration into basements and crawlspaces around here is, in my opinion, older concrete crock-style roof drain systems around foundations which have failed. These things are just 20" long pieces of 4" I.D. concrete pipe that are laid end-to-end around the foundation and then are buried. They're sealed by wrapping a piece of felt around the joint and plastering the felt with some mortar - not exactly a water-tight joint and roots from the ever-present shrubbery and settling over-dig soil causes them to separate.

About 5 years ago, during the non-compete time after I sold my inspection franchise, a customer asked me what I could do to dry up his 80-year old basement, which he said had "always" been wet, without placing unsightly elbows with long extensions and splash blocks all around his house. I took one look at the home without entering and told him, "Just bypass those concrete crock drains that are receiving that roof runoff, and get all of that water at least six feet from that foundation and it will dry out."

He seemed doubtful but hired me to do it. It was late January and had been raining for days. There was a stream of water entering his basement at the base of one wall that drained to a floor drain and then disappeared. I set about installing six bubbler pots around the home to capture the roof runoff. These are simple fiberglass devices that have a slightly rounded top with a little pop-up lid in the center and a 3 or 4 inch elbow underneath. You bury them flush with grade and connect them with a length of buried non-perforated plastic pipe to each of the downspouts.

I started at 1:00 PM, using a trenching spade to dig narrow 4 inch wide ditches about a foot deep straight out from each downspout. Then I'd lay in a piece of non-perforated 4 inch drainpipe that connected to the downspout at one end and the bubbler pot at the other end about 8ft. from the foundation and would bury the pipe. Lastly, I broke off the lips of the crocks that projected above grade, stuffed some newspaper inside and then followed with a 5 inch thick plug of hydraulic cement.

I finished all six and had replaced all of the sod by 4:30PM and it was already dark. The rain hadn't let up all afternoon and I was soaked to the bone. I went inside and down the stairs to the basement, where I found nothing left of the stream of water except a rapidly drying damp spot on the concrete. I left and drove home. That evening the homeowner called after he got home and was ecstatic. "Mike, this basement has had water draining through it for the 12 years that I've lived here. It's hard to believe that the solution was such an easy one. Thanks a heap!"

I'd purchased six bubblers (sometimes called "pop-ups") for less than $60 at the time, along with about 60ft. of non-perforated corrugated drainpipe for about $20 and a couple of containers of hydraulic cement for less than $20. He's a doctor, so he obviously didn't want to stand out there in the rain doing this grunt work, but if he had it would have cost him about $100 plus his own time to fix, instead of the $400 I charged him.

Bottom line, roofs produce a lot of water and downspouts need to discharge either into their own separate tight-lined system, that is drained apart from footing drains and the sewer, or onto grade at least six feet from a foundation wall, or you're liable to end up with infiltration issues.

ONE TEAM - ONE FIGHT!!!

Mike

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Dang, we are going to have to use another house, the 1300 sq foot ranch was sold after they put in a B-Dry system!

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This is not exactly like the one's I used. They were designed to be used with flexible black non-perforated 4" ABS pipe, but it is essentially the same thing. The photo shows the pop-up lid extended, but it actually sits flush with the surface of the disk.

OT - OF!!!

M.

Download Attachment: icon_photo.gif pop-up.jpg

3.65 KB

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The flat roof house in my example did NOT have eaves, rakes, soffits, etc. When the pitch went up, the eaves, rakes, soffits, etc, went out. That increased the square footage (or footprint of the roof. That increased square footage catches more rain. That's all my picture showed.!!! When it was altered, those were conveniently left off.

[:-headach[:-headach[:-headach

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Erby wrote: "When the pitch went up, the eaves, rakes, soffits, etc, went out. That increased the square footage (or footprint of the roof. That increased square footage catches more rain. That's all my picture showed.!!! When it was altered, those were conveniently left off."

That was intentional. I wanted to illustrate that increasing pitch alone does not increase collection area. I like to compare apples to apples, so I changed a kumquat into an apple.

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"Simple enough, and we've all thought about doing that"

Brian, you may be alone there.

Homes in the Bahamas collect rain water into a cisern for drinking water. Do you see steep pitched roofs there? No. For collecting rain water, the best system is a low pitched roof.

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Bill,

How dare you mess with my kumquat and change it into an apple.

Next time, leave my kumquat alone and pluck your own apples.

Download Attachment: icon_photo.gif Kumquats in nature

6.98 KB

I put a lot of work into my kumquat bush so that it produces good kumquats. Then you come along and callously change one of my kumquats into a common apple.

Oh the shame of it!!![:-slaphap[:-slaphap[:-slaphap

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Hi,

They are flush to the surface and don't interfere with lawn mowers at all. In fact, the first time I saw these 10 years ago I thought they were pop-up sprinkler heads and couldn't figure out why there were so few around the home.

OT - OF!!!

M.

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B-Dry, EverDry, and fifty thousand other basement water "collection" systems use them often in Mich. Here in our local area, the pine needles screw them up, but usually do a pretty good job, if the volume is good.

Seems like I read about them in the late 90's in Fine Home Building or JLC.

Mike is right, you may have seen them and thought they are a zone valve or sprinkler head.

Just for the heck of it - try Genova PVC products.

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I know I know, a ancient thread. Not sure if erby is still around. I can't help myself. I thought that one inch of rainfall was calculated by one inch of rainfall over one acre of land, equals one inch of rain.  That is the calculation when weather men report how many inches  of rain different areas were getting, got, or might be getting. Please come back erby, and correct me if I am wrong.

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Oh yeah, I'm still around.  Not nearly so naive as I was then, but still around.

Thanks for resurfacing this thread.

I got one hell of a chuckle reading it through again.  I was so naive.

And it turned into such a humorous thread!



Yes, you're wrong.  Google "How to measure rainfall".  No one measures it over an acre.

 

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