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Buried Downspouts & Underground Drains for Roof Runoff
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
This article describes problems that occur in buried downspuout or roof leader drains, including broken or clogged lines that cause basement flooding. This article series discusses how to choose, install, diagnose & maintain roof gutters & downspouts, & roof drainage systems to prevent building leaks and water entry.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Buried or "Below Grade" Downspouts
Also see DOWNSPOUT LEAKS or start this topic at DOWNSPOUT / LEADER DEFECTS; also see GUTTER DEFECTS LIST for other sources of wet basements or crawl spaces caused by problems with the roof drainage system.
Photo of a Buried Drain Line that Should be Abandoned
The buried downspout drain line shown at left is one that we'd recommend abandoning - it connects to a drain line that enters the building through the foundation wall and passes below the basement floor, incorporating a basement floor drain, ultimately connecting the the sewer line.
Buried or below-grade downspout extensions in new construction are (or should be) directed either to "daylight" downhill from the structure so that water will drain away from the building by gravity, or they are connected to a storm drain.
For flat sites it may be necessary to drain into a drywell or "French drain" system or into a pumping chamber that sends drainage to a remote storm drain or drywell. See FRENCH DRAINS for details of handling roof runoff at flat or poorly-drained sites. |
When a buried downspout line has become blocked (or if you're not sure if it has become blocked) we recommend that you disconnect the downspout above grade and install an above-ground extension to direct water away from the building. This step can stop basement flooding from several hidden downspout drain problems including
- Clogged or broken buried downspout drain line
- Buried downspout line (stupidly) connected to the building footing drains
- Buried downspout line connected to drains that run through and/or under the building basement floor
- Buried downspout line connected to local septic or sewer piping
Sketch courtesy of Carson Dunlop Associates. |
How to Detect a Clogged Buried Downspout Line
Some visual clues that indicate a clogged buried downspout drain line include:
- Soil was areas visible around the top of the buried drain line
- Water flowing out over the top of the buried drain line during heavy rain
- Water stains, moisture, leaks, or actual water entry inside the building originating near the buried drain line
- Gutters overflowing from above.
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 The dramatic roof drainage leak shown at left includes several interesting observations made at this nearly-new home:
- The roof gutters were overflowing, sending water down and behind vinyl siding - you can see water pouring out of the "J" channel at the center of our picture of this Pawling New York Home.
- The buried downspout line (visible under the deck) was blocked
- The ground under the deck was lower than surrounding soils (the deck was constructed before final backfill and grading were performed), forming a "pool" of water against the building rear foundation wall.
These conditions are likely to lead to basement flooding, if not immediately, as soon as the footing drains clog or a crack opens in the poured concrete foundation wall. |
Don't Connect Downspout Drains to In-Building Drains & Floor Drain Systems
The sketch at left illustrates several reasons why it's a bad practice to connect roof drainage downspouts to drains that run through the building or under the building floors.
- Clogs in the vertical section of the downspout can lead to leaks through the building foundation wall.
- Blockages in the under-floor drain combined with high volumes of roof runoff water entering the system send the entire roof runoff into the building basement or crawl space
- Debris flowing down the downspout drain (leaves from gutters) can block floor drains or even the main building sewer line, leading to sewage backups into the structure.
- Debris flowing down a downspout drain that has been (foolishly) connected to the building foundation drain system can block that drain leading to basement flooding from either roof runoff or surface and subsurface runoff, or both.
Sketch courtesy of Carson Dunlop Associates. |
Don't Use Perforated Drain Piping for Buried Downspout Drains (near the building)
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 Some builders consider that if they use perforated 4" drain pipes to transport roof drainage away from the building, the water will "soakaway" harmlessly into the soil even if the end of the buried drain line is blocked or never taken to daylight. Both of those are mistakes.
- Perforated buried drain lines near the building allow roof spillage to leak against and into the foundation wall.
- If the downspout line end is not taken to daylight where water can continue draining away from the building (or to a storm sewer), the blocked line will eventually back up in times of heavy rainfall or wet soils, leading to building leaks and water entry.
If we see perforated drain lines close to the building (photo at left) we expect that the building will eventually have a basement water entry problem. A "fix" for this problem is to abandon the buried drain line entirely. See Buried or "Below Grade" Downspouts for more notes on abandoning buried drain lines. |
Don't Bury Aluminum Downspout Drain Lines
Buried aluminum downspouts (photo, below left) corrode through and leak fairly quickly as the aluminum metal is thin and when exposed to corrosive soils or even acidic water from leaves and vegetation (or salt from ice-melts used nearby) the material will be damaged.
The results are hidden leaks below ground that lead to basement or crawl space water entry.
A better design uses plastic for buried downspout lines; the plastic should be solid piping and should be brought above grade for the downspout connection (photo, above right).
Under-sized Buried Downspout Drain Lines
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As our photo illustrates (left) it is usually a mistake to connect a downspout line to a buried pipe that is too small - in this case obviously smaller than the downspout drain itself.
In periods of heavy rain the small-diameter plastic pipe shown here won't handle the volume of water running off of the roof. |
OK to Share Buried Downspout Lines
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 On occasion we find that a buried downspout line has been used to dispose of basement or crawl space sump pump water. That's probably fine. But don't use your buried downspout line to dispose of graywater from sinks, laundry systems, or showers. In most communities it is illegal to dispose of gray water on to the soil surface or into nearby waterways.
Our photo (left) shows a basement sump pump drain line connected to a buried downspout drain.
And be sure that the buried downspout drain line that you are using to carry off sump pump water is properly installed and that it is not connected to the building footing drains.
Watch out: We have found buildings whose sump pumps were busily pumping away trying to get rid of basement water while discharging the water to an improperly installed drain that simply sent the water right back into the footing drain and right back into the basement.
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Downspouts Used for Air Conditioning Refrigerant/Condensate Lines or Wiring
Don't mistake as part of the roof drainage system an aluminum downspout that has been used as a cosmetic cover for HVAC refrigerant lines as we illustrate in our photo at below left.
Some A/C installers use an aluminum downspout to cover the refrigerant and condensate lines that are routed from an attic-mounted air handler down to an outdoor ground-level compressor. But do not use downspout materials as electrical conduit.
In our second photo of downspout materials used to cover air conditioning refrigerant piping (above right), notice that the extension and elbow on the water-carrying downspout (the right-most downspout of this pair) has fallen off and we have a splash area on the ground at an "inside foundation corner" - conditions likely to cause basement water entry.
Frequently Asked Questions (FAQs)
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
- John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: johncranor@verizon.net
 Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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- Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF - ** Particularly useful text **
- Dampness in buildings, Diagnosis, Treatment, Instruments, T.A. Oxley & E.G. Gobert, ISBN 0-408-01463-6, Butterworths, 1983-1987 [General building science-DF]
- Certainteed Weatherboard fiber cement siding and trim products - see certainteed.com/ or see certainteed.com/resources/sidingandtrimspecsheet.pdf
- "Flashing: the plain solution to leaky walls", Thomas E. Remmele, Manager, Technical Services, Sto Corporation, Building Standards, November/December 1999 p. 21-25.
- "Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
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