Roof Gutters, Supports, And Seams – Part I

Learn How Roof Gutters, Supports, and Seams Work

Roof gutters simply collect rainwater from rooftops and channel it away from the building.  It’s a simple concept, yet methods of installation, interconnectivity to the roof and ground or underground and downspout systems, leaf protection and or maintenance are all important considerations.  

The picture below shows the rear of a historic rowhome in the city.  The rear termination of the modified bitumen roof system is essentially missing.   Instead of installing a proper termination with a metal angle and drip edge, the roofing contractor or roofing installer took a major shortcut and just ran the modified bitumen roll roofing a bit extra long.   It flaps over the edge of the roof deck onto the edge of the gutter.  This type of configuration essentially works to divert water but it’s not a stable type of installation because the membrane is loose and detached at the end.  The membrane could flap around in the wind and be easily lifted up by rodents.  As well, in circumstances of ice damming, water will easily wick upwards behind the edge of the roof membrane. 

rear of a historic rowhome

Gutters are not used or necessarily required at all roof installations, some sheds and even actual buildings are built without gutters and limit the splashback effect of rainwater at the facade by having extended roof eaves.  This type of gutterless roof systems can be very problematic though, in the DC climate zone, so it’s almost always better to have a properly built, adequately sized and well maintained gutter system at your rooftop.   

As a side note, it’s important to realize that buildings with extended eaves often have a higher first cost of construction because the amount of additional roof area outweighs the savings in limiting or omitting gutters.  As well, the life cycle cost is also driven much higher by not having gutters to direct the water collected at the rooftop away from the building foundation through a guttering and downspout system.   Also, the context is a bit different in the urban areas of Washington DC because we simply live in a concrete jungle where there aren’t always good options for rain water absorption in the landscape or hardscape.  The fact that most DC buildings are built with basements also changes the context because undiverted rainwater becomes a problem for underground foindations and associated waterproofing systems when that water is not proactively managed.  

In the next picture below, you can see that the water is completely pooled up and filling the gutter.  It cannot flow down the drain because the drain is clogged with seeds pods from the nearby Princess tree, also known as a Paulownia tree.  The Paulownia tree is very common, in Capitol Hill and Washington DC.  Its widely considered a nuisance tree and am invasive species from China.  The species is very fast growing and often found growing in places where it’s undesirable, such as in interstitial hard to reach places between buildings where the hardy and fast growing trunk can cause structural damage to buildings by pushing structural elements out of place.

blocked roof gutters

How Rooftop Gutters Are Installed

Gutters are installed along the edges of the roof and are designed with a slight slope to allow for passive drainage. Like a flat roof, a minimum slope is required for passive drainage at all points.   Gutters can be also installed close to horizontal or flat, but should have about three times as much slope as the roof itself, at minimum because a gutter is a channeling which can become a concentrated bottle neck point of water flow.

In the picture above you may notice a ridge running perpendicular to the gutter, underneath of the roof membrane at every 24-inches.   Ridges that occur at approximately 24 inches beneath a modified bitumen membrane like this, or any type of single-ply roll roofing membrane are a sign that the previous roofing contractor neglected to install the roof properly by preparing the substrate completely and did not remove the substrate standing seam metal roof, nor did they infill the space between the seams with 2″ polyisocyanurate or similiar material boards before overlaying the old roof.  They simply took a cheap shortcut which will lead to problems later.

how rooftop gutters are installed

When rain falls on a rooftop, it flows down the surface of the rooftop membrane, and passes over a flat roof termination which both flashes the spanning eave or fascia area of the roof and then directs the water into the gutter.   We often see circumstances of improper construction where the flashing or drip edge at the rear termination of the roof does not extend all the way into the top of the edge of the gutter.  This span and coverage is important because otherwise the wood substrate of the fascia board is exposed to water flow or water at a area of high flow.  During heavy winds, coincident with rainfall that area of facet will be unnecessarily hydrated and wet with rainwater.  Excessive moisture exposure against wood trims such as fascia boards or even rake boards will invariably lead to accelerated deterioration and rot.

The Paulownia tree seeds and leaves are clogging up the gutter outlet, as shown in the next picture below.

tree seeds and leaves blocking roof gutter

Ideal Size of A Rooftop Gutter

The gutters act as a collection channel, preventing the water from simply dripping off the roof and causing potential damage to the building’s foundation or landscaping. The gutters are positioned just below the roof’s edge to catch the water effectively.   It’s important that a calculation is taken into account to recognize the potential for maximum waterflow at heavy rains, when picking the size of the gutter.  Gutters can be purchased in relatively small sizes of only 4-inches wide or much larger app to about 6-inches for residential installation and even significantly larger than that for commercial installation.  In the next picture below, we show an example of two roofs, side-by-side.  The roof to the left is much larger and even though the overall width or of the building or run of the gutter is the same between these two different buildings, the building to the left requires a wider gutter to accommodate a heavier waterflow during the heaviest downpours.

ideal size of gutters

In an upcoming article we will talk about the deleterious effects of blocked gutters and associated hanger failure and the nonlinearity of deterioration.

Maintaining Roof Gutters and Flat Roof Systems

The upkeep, maintenance, and general care of flat and mansard roof systems should be driven by an understanding and passion for historical methodologies, waterproofing principles, engineering and building science. Washington DC, a city built with both vintage charm and contemporary modernities, residential and commercial buildings of substantial value. 

The roofs of these buildings are their defensive shield from the harsh elements of nature. To our clients, as well as all readers of this article and our blog, we emphasize the importance of quality construction and active building maintenance.  Our website includes informative resources you can use to understand and learn about best practices on preservation of your building.  If you are in need of further guidance on the roof and its associated systems for your Washington DC property, we are here to assist, where we can. Simply complete the webform below and drop us a line and we’ll respond if we can help.

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