What you Need to Know about Internal Gutter Systems

Time: 2021-07-21

There are two main types of gutter systems: external and internal. Aesthetics aside, the major difference between the two styles involves what happens when the gutter fails. Gutter failure can occur from something as simple as a clogged downspout or as complex as extreme rainfall that exceeds what the system was designed to carry.

Regardless of the cause, when failure occurs external gutters deposit water outside the building envelope and internal gutter systems deposit water inside the building envelope.  

External gutter systems commonly involve installing a gutter at the low end (or eave) of the roof outside the building footprint.  Downspouts are then connected to the gutter and allow any water that flows into the gutter to drain outside of the building.

Conversely internal gutter systems function by funneling water to a series of troughs within the roof system. These troughs then drain water into tubes located inside of the building.

At first blush, this difference may seem minor but the implications can be potentially serious.

Pitfalls of Internal Gutters

Internal gutters are most common in flat roof applications; however, they are also occasionally designed for metal roofing systems. Their use in metal roofing systems tends to occur when either the building design isn’t conducive to external gutters or when the designer or owner wants to avoid the visual appearance of gutters.

While internal gutter systems aren’t necessarily rare, it’s routinely recommended to avoid them if at all possible. The reasons for this recommendation are numerous.

First, as mentioned earlier, internal gutters deposit water inside the building envelope. When the tube drainage system fails to function properly, water infiltration can lead to serious damage to the building infrastructure and occupants below in a variety of ways:

  • Life safety issues such as mold and employee injuries due to slips and falls on wet surfaces
  • Water damage to building contents and inventoried products
  • Irreparable damage to the entire roofing system, including but not limited to the insulation.  Once insulation becomes wet, the insulating value is greatly reduced.  Further, damage can occur to other building components that are in contact with wet insulation.

Consequently, whenever possible, it’s preferred to use designs that allow water to exit outside of the building envelope.

Secondly, internal gutters can be incredibly difficult to repair or replace because the roofing material is commonly installed on top of the gutter system. When shorter life roofing systems, such as membranes and TPO systems, utilize an internal gutter, replacement of the gutter is less critical as they tend to wear out at the same pace as the roofing system.

However, when internal gutter systems are used with long life products such as metal roofing, which can routinely last 50-60 years, internal gutter repair or replacement can become a major issue.

In fact, owners often find themselves replacing roofing material which is still functional simply to address internal gutter replacement. Consequently, internal gutter systems should be avoided if at all possible in metal roofing projects.

Tips for Internal Gutter Use

For projects where an internal gutter cannot be avoided, consider the following tips:

  • Address internal gutter repair or replacement during the initial building design.
  • Internal gutters should be made of a long life material such as stainless steel.  
  • In snow prone areas, a snow guard system should be used to allow the gutter to remain free to drain melted snow.
  • Internal gutters should be designed with a continuous support structure such as plywood.

In summary, internal gutters have been in use for centuries so it’s clear they “can” work.  But it’s also clear they represent a big risk of failure that’s better avoided whenever possible.

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