The entire purpose of a roof is not just to keep the inside of the house dry but also to divert water away from the area of the building or the footprint above the building towards other areas where it can be managed away from the building. Today in this coming week we’re going to look at a few examples of how the path of what are diversion, particularly here in Washington DC were buildings are built close to one another, may take the water diversion draining from a roof system, off the property.
The interior of a house, and the foundation of building a particularly susceptible to water entry. We generally, in almost all cases, think of foundation water entry and associated types of problems as being separate and different from the issues of exterior facade and roof water and associated leakage. They are generally different, but in some cases, these two different realms of water management and leakage are related. The roof water generally has to get down from the upper parts of the roof to the ground and run away without short-cycling back into the building.
In the picture below, you can see a downspout that runs from the gutter at the eave of the roof above, going down the side of the building, but the side of the building is directly on the public sidewalk. That sidewalk is on public space, not private property.
In this relatively unique case, the case style gutter drains into an outlet in the gutter to a 4-in diameter round pipe that although painted and looks similar a typical aluminum or metal downspout, is actually made of PVC. It’s somewhat disguised by the fact that it’s painted a color similar to the other accents of the building.
Gutters are typically a long, slightly sloped channel installed along the roof’s edge. As rain falls on the roof, it flows down the roof following the slope and into the gutter. Normally, a roof system, particularly low slope roof systems but also high slope roof systems, will terminate at a drip edge or similar metal termination which transitions the roof system to a strip of flashing or directly into the gutter channel. The gutter’s slight inclination, usually about 1/4 inch per 10 feet, directs the collected water towards downspouts.
Downspouts, also sometimes referred to as leaders, are vertical pipes connected to the gutters at specific points. They serve as the conduit for water to travel from the elevated gutters to ground level. At the base of the downspout, the water is often directed further away from the building’s foundation through extensions or underground drain systems. Here in Washington DC, the location of gutters is often dependent upon the location of the historic, drain lines underground. Here in Washington DC, it’s common for these historic drain lines to be made from cast iron in many cases, the historic cast iron drain lines are damaged and or defunct from years of being clogged, and or damaged by roots and debris and or all green or snaking of the drains. If relatively delicate drain lines are not cleared or all good properly, when needed, it’s easy for these drain lines to become damaged and broken underground. Over many decades, the cast iron pipe can become very fragile because it’s not completely in permeable or unaffected by oxidation and slow deterioration from exposure to water, even though it’s intended to drain water as its primary function.
The effectiveness of the gutter and downspout system depends on proper sizing, which is particularly important in areas like Washington DC, where rainfall can be significant. Calculating the appropriate gutter and downspout sizes involves considering the roof area, local rainfall intensity, and the roof’s slope.
For a low-slope roof in Washington DC, the calculation begins with determining the roof’s drainage area. This is typically the horizontal projection of the roof, not its actual surface area. For instance, a 30 by 50 foot rectangular roof would have a drainage area of 1,500 square feet. However, it’s crucial to consider how this area is divided among different gutters and downspouts. If this roof has gutters on both long sides, each gutter might be responsible for draining 750 square feet.
Next, we consider the rainfall intensity. In Washington DC, the typical design rainfall intensity for a 5-year storm event is about 5.5 inches per hour for a 5-minute duration. This is a standard used for residential and small commercial buildings, balancing between common occurrences and extreme events.
To calculate the water volume, we multiply the drainage area by the rainfall intensity. For our example of 750 square feet draining to one gutter, this would be:
750 sq ft × (5.5 in/hr ÷ 12 in/ft) = 343.75 cubic feet per hour, or about 43 gallons per minute.
This volume helps determine the required gutter size. Standard residential gutters are often 5 or 6 inches wide, with 6-inch gutters capable of handling about 360 gallons per minute. For our example, a 6-inch gutter would be sufficient.
Downspout sizing follows a similar logic but focuses on the water’s vertical flow. A typical 2×3-inch rectangular downspout can handle about 600 square feet of roof area in areas with rainfall similar to Washington DC. For our 750 square foot example, we might opt for a 3×4-inch downspout or two 2×3-inch downspouts.
The roof’s slope also plays a role, although it’s less significant for low-slope roofs. Steeper roofs tend to shed water more quickly, potentially requiring larger gutters and downspouts. For low-slope roofs, the standard calculations are usually sufficient without major adjustments.
It’s important to note that these calculations provide a baseline. Factors like the presence of valleys (which concentrate water flow), local building codes, and architectural considerations may necessitate adjustments. Additionally, in areas prone to leaf fall or other debris, oversizing gutters and downspouts by about 10-20% can help maintain efficiency even with some blockage.
Regular maintenance is crucial for any gutter system. In Washington DC, where autumn leaf fall can be significant, cleaning gutters at least twice a year is recommended to ensure they function as designed.
The next picture below shows a typical aluminum downspout which runs down the side of a building and goes through the fence. In this particular case that downspout crosses the property line. We generally recommend against crossing the property line because there can be other implications such as rights and rules against crossing the property line.
We thought this particular circumstance was a little bit interesting though because unlike typical cases where the water would just be dumped onto a walkway, here they took what was a pretty big problem, the fact that the underground rain leader was broken or clogged, and made an almost acceptable solution.
Even if this is not technically acceptable, it’s a little bit better than just dumping the water onto the sidewalk. At least here, the water flow is broken or slowed by landing into a bucket of rocks with a perforated bottom. Although it’s not the right way to do things, by the book, it might be making the best out of a bad situation.
We recommend every building owner in DC who values the longevity of their roof (and their investments) and building use a contractor who values the simple and important principles of proper roof construction like Dupont Roofing DC. Learn more about our company and the proper techniques of working with roofing on historic buildings in Washington DC here on our blog at DupontRoofingDC.com, and you can call us at (202) 840-8698 and email us at dupontroofingdc@gmail.com.
