It depends on who you ask this question. There is no building code citation that forbids a gutter from draining onto a lower roof. So it’s a matter of what the home inspector or contractor considers acceptable. Generally, a contractor that drains a gutter onto a lower roof will say that it is an “industry standard,” whereas a contractor that rants against it will say that it's “not best building practice.” And both have good arguments to backup their opinion.
Opponents say that the concentrated water flow wears away the granules faster in the lower roof where it drains across it, which is true. But it is also true that a woven (closed) valley takes a lot of water and loses granules faster than the rest of a roof, and closed valleys are not frowned on. Although the granule loss is noticeable after about 10 years, we have never seen a roof that needed to be replaced early because the granule loss was so significantly worse than the rest of the roof.
The one thing that pretty much everybody agrees on is that at least there should be a downspout to deliver the water to the lower roof and point it in the right direction. A hole at the bottom of the gutter or an open gutter end will just splash water around in a heavy rain. It is also not a good idea to drain down a lower roof too close to adjacent wall above the roof.
Then there is the aesthetic consideration. Some homeowners feel that a gutter running across a roof, especially at the front elevation, messes with the curb appeal of the house. The photo at the top of the page is one example of a upper gutter draining into a lower one neatly, coming down tucked around the side wall, and not calling attention to itself. So it can be done unobtrusively in some situations.
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To learn more about roofs and attics, see these other blog posts:
• Why is my roof sheathing sagging between the trusses?
• Why is granule loss a problem for an asphalt shingle roof?
• What are the mistakes to avoid when doing attic improvements?
• What causes roof shingles to curl up at corners?
• What causes shingles to buckle along a line on the roof?
• What causes leaks at a fake roof dormer?
• What causes a sagging roof ridge line?
• What causes bubble-like blisters in a built-up and gravel roof?
• Why does it cost so much more to replace a steep roof than a low slope roof?
• What is "ponding" on a flat roof?
• Is an attic required to have a light by the building code?
• How can I inspect my roof for hurricane damage?
• Why is premature curl of roof shingles a problem?
• How can I tell if a roof has more than one layer of shingles?
• What are the common problems with attic insulation?
• What is the life expectancy of an asbestos cement shingle roof?
• What's the average lifespan of a roof?
• Why is it a mistake to replace a roof and not replace its flashings?
• Why is there no attic access hatch in the house?
• What is the building code requirement for an attic access hatch, scuttle, or door?
• Does a roof with multiple layers of shingles last longer?
• What can I do to prevent roof leaks?
• Are roof trusses better than roof rafters (stick framing)?
• Why is a popped nail in a shingle roof a problem? How do I fix it?
• What are the most common problems with wood roof trusses?
• What causes a lump or dip in the roof?
• If my roof is not leaking, why does it need to be replaced?
• How can I be sure my roofing contractor got a permit?
• How many layers of roofing are allowed on a home?
• What are the dark lines running parallel to shingles on my roof?
• Can metal roofing be used on a low slope/pitch roof?
• How can I make my roof last longer?
• What are the warning signs of a dangerous attic pull-down ladder?
• How can I find out the age of a roof?
• Should I buy a house that needs a new roof?
• Should I buy a house with an old roof?
• What are those metal boxes on the roof?
• What does "lack of tab adhesion" in an asphalt shingle roof mean?
• Why do roof edges start leaking?
• Why do my dormer windows leak?
• Do home inspectors go on the roof? Do they get in the attic?
• Should I put gutters on the house?
• How much of a roof truss can I cut out to make a storage platform in the attic?
• What's the difference between an "architectural" and a regular shingle roof?
• What does a home inspector look for when examining a roof?
• Do stains on the ceiling mean the roof is leaking?
• How can I tell if the house needs a new roof?
• Why does my homeowner's insurance want a roof inspection?
• What are the hazards to avoid when going into an attic?
Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.