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Gutter roof drains, downspouts & leaders:
These articles, sketches and photographs decribe the types of defects or leaks in the downspouts or leaders for roof drainage systems: leaks or other defects that cause basement or crawl space water entry.
Downspouts collect water from the gutters and discharge it into drains or onto the ground. Underground drains (usually made of clay tile, cast iron or plastic) become clogged or break below grade. If an underground downspout malfunctions, water problems will likely develop in that part of the basement. There are two options.
Exterior digging and repairs can be
undertaken; however, it is faster and cheaper to simply disconnect the downspout and redirect it to discharge away from the house. It’s also easier to monitor the performance, and problems are corrected easily. Downspouts should discharge above grade onto the ground at least six feet from the home. The slope of the ground in this area should be away from the house, to direct water away from the basement.
Common Downspout Defects Causing Building Water Entry or Leaks
[Click to enlarge any image]
The small diameter gutter pipes (downspouts or leaders) on the roof drainage system shown above are on flats located in Oxford in the U.K. In an area of significant rainfall we speculate that this system may not be providing adequate drainage. The roof drain pipes shown below, installed on a London building have been adequately sized and in-service since 1953.
Our list of common downspout or leader defects shown just below is followed by illustrations or photographs of many of these conditions.
Downspout empties too close to the building, concentrating water against the foundation, a very likely source of basement or crawl space water entry.
Downspouts connected to footing drains, building interior drains, floor drains, or sewer system (storm drain connections are ok). This is a common but serious mistake made by uninformed builders. Adding the load of roof runoff to building foundation drains overloads that drain system and invites basement or crawl space water entry and even flooding.
A basement flood from this mistake can happen very suddenly, often less than 10 years after new construction, when a hand full of leaves or other debris enters and clogs the footing drain system through the gutter and downspout system.
Insufficient size or number of downspouts to handle roof drainage water volume. In general, provide a downspout for every 35-40' of gutter length. More downspouts may be needed around complex roof structures.
Downspout Extension Spills Too Close to the Building
The sketch at left illustrates a major cause of wet basements and crawl spaces: the downspout ends too close to the building, backfill at the building has settled, and in addition, the original grade left from the excavation to build the foundation remains as hard-packed soil that directs spillage back towards the structure.
Our photos below illustrate downspouts emptying too close to the building.
At below left you can see erosion that is sending most of the roof runoff out of the downspout and right down the foundation wall.
At below right a downspout is terminated on the deck surface. That detail can lead to slip/fall hazards (slippery algae-covered deck boards) and basement water entry (due to in-slope grade towards the building, hidden below the deck surface.)
Downspout Empties onto Settled Backfill
Here we show the most common roof drainage system error that leads to basement or crawl space water entry: the downspout empties too close to the building foundation wall.
This mistake, combined with settled backfill that slopes back towards the building sends this concentration of water from the roof right into the building.
While there are rules of thumb for the length of downspout extensions (minimum of six feet, or more from the building), the common sense downspout rule is:
When the water leaves the end of the downspout, it should keep going away from the building, not back towards it.
Downspout Empties onto Sloped backfill over compacted in-slope virgin soils or clay
A more subtle version of the "downspout too close to the building" problem we described above is illustrated in the sketch at left.
After a building foundation and structure have been completed, backfill may have been spread around the building and may appear to slope nicely away from the structure.
But because the new backfill is less compacted than virgin soil, even though it slopes away from the building, water soaking through the backfill may encounter below-ground hard-packed original soils that slope towards the structure.
The result is a hidden in-slope grade problem sending roof spillage or downspout spillage right back into the building.
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