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View Full Version : Formula for gutter/downspout capacity

Don't Call Me Shirley
04-19-2011, 06:13 PM
Is there a formula used to calculate how many square feet of roof space can be served by a single downspout? The downspout on the front of my house has to drain water from a large vaulted roof and a large front porch, plus half of the upstairs gutters empty onto the same roof. So that one downspout has to try to drain like half the roof area.

I know the gutters are clear, I just cleaned them. Whenever we get really heavy rain that downspout is gushing like crazy, but the gutter is still overflowing and it washes all the mulch out of my landscaping.

And while we're on the subject, are any of those products that are supposed to keep stuff out of your gutters any good?

ZenBeam
04-19-2011, 07:23 PM
This page (http://www.copper.org/applications/architecture/arch_dhb/gutters_downspouts/intro.html) has numbers for square feet of roof per square inch of downspout, for a number of locations around the US. I don't know how accurate it is, but I like that they take into account the expected rainfall for different regions.

04-19-2011, 08:02 PM
This page (http://www.copper.org/applications/architecture/arch_dhb/gutters_downspouts/intro.html) has numbers for square feet of roof per square inch of downspout, for a number of locations around the US. I don't know how accurate it is, but I like that they take into account the expected rainfall for different regions.

FTR I looked in my 2009 International Building Code book and could not find any standards for gutter and downspout sizes.

t-bonham@scc.net
04-19-2011, 10:32 PM
Whenever we get really heavy rain that downspout is gushing like crazy, but the gutter is still overflowing and it washes all the mulch out of my landscaping.There are things called downspout extenders designed to deal with this. They look like a rolled up green plastic plastic thing attached to the downspout; when filled with rainwater they unroll and let the water out through holes along their length, but slowly, so as to not mess up your landscaping.

Alternatively, many people around here are using downspout diverters to route the water into rain barrels, for use later.

chela
04-20-2011, 06:27 AM
Yes extenders would solve that for you, but I too divert rainwater from the gutters to rainbarrels. I put them under all the downspouts mainly for erosion control. Where gutterwash would create a gully, a rain barrel now stands. It can still overflow but the energy of the water is greatly reduced especially in those areas where I replaced the downspout with a rain chain.

Hbns
04-20-2011, 07:18 AM
Yes extenders would solve that for you, but I too divert rainwater from the gutters to rainbarrels. I put them under all the downspouts mainly for erosion control. Where gutterwash would create a gully, a rain barrel now stands. It can still overflow but the energy of the water is greatly reduced especially in those areas where I replaced the downspout with a rain chain.

I think the OP is suggesting that in heavy rains the gutters themselves get overwhelmed and that water washes their mulch away, rather than a problem with the discharge from the downspout.

cantara
04-20-2011, 09:01 AM
We've only got 2 downspouts for our roof (1600 sqft bungalow) and the eavestrough can get overwhelmed. I think our biggest problem is the hole for the downspout. The gutter is a one-piece trough with the holes for the downspouts cut through the bottom. The hole is only 1-1/2"x1-1/2", so I've bought a couple of bigger attachments (http://www.homedepot.ca/webapp/wcs/stores/servlet/CatalogSearchResultView?D=975709&Ntt=975709&catalogId=10051&langId=-15&storeId=10051&Dx=mode+matchallpartial&Ntx=mode+matchall&N=0&Ntk=P_PartNumber) to encourage better drainage. I haven't installed them yet.

We've had problems with our gutters collecting debris. I installed the rigid foam (http://www.thegutterstuff.com/Default.aspx) covers last year. I haven't inspected them yet to see if smaller particles have collected underneath yet. So far it looks like the larger debris sits on the top until some good wind blows it off.

Winter was a treat. The snow from the roof slid down over the gutter and sat there effectively blocking the gutter. We ended up with snowdrifts hanging over the edge of the roof and icicles hanging off the edges. I may invest in a roof rake (http://www.rittenhouse.ca/asp/Product.asp?PG=381)for next winter.