(Architecture) Climate Change and Building Gutters by Ben Haavik

By Nancie Ravenel posted 06-01-2019 12:45

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Historic New England, where Ben Haavik works as team leader for Property Care, and Shelburne Museum, where I work as objects conservator, have lots in common: two of them are climate and historic structures that need care. Resiliency in the face of intensifying storm patterns is something that is of mutual concern.

In 2017, Historic New England received a grant from the state of Maine to undertake a study of 21 roof drainage systems, consisting of gutters, outlets and downspouts, at 9 properties in Maine. The purpose was to determine how well these systems operate now, how well they could be predicted to perform in the future, and what modifications could be made to improve performance. After reviewing the basics of roof drainage systems, how standard calculations for gutter capacities are made, and how storm intensity is described, Ben shared the process and results of that study, which are also published in a white paper available on Historic New England's website.

Here are a few of my take-aways from Ben's findings:

The rain intensity assumptions used in industry standards to calculate gutter size date are more than 40 years old, and no longer appropriate since rainfall intensity in Maine is now 25% greater than it was then.

Overall, 48% of the gutters studied failed the 10-year storm projection (1:10 chance of a storm of that particular intensity for a 5 minute duration) and 76% failed the 100-year storm (1:100 chance of a storm of that particular intensity for a 5 minute duration) projection.
- For gutters made of wood, the results were especially grim: All but 1 wooden gutter failed the 10-year storm projection and all of them failed the 100-year storm projection.
- Wooden gutters replaced with painted copper resembling wood performed well under both assumptions.

The size of the outlet (the hole between the gutter and the downspout) matters: under 2 inches in diameter failed every time regardless of storm intensity; over 2 inches was successful nearly every time.

All downspouts passed regardless of storm intensity.

What they're doing about the problem:

doing calculations up front to determine if gutters need to be changed,

trying out using 25-year storm size assumption as a standard in calculations rather than the 10-year storm-size,

considering changing the size of the outlet first when damage is primarily at that location
- the outlet isn't typically seen and would not contribute to an aesthetic change in the structure,

adding grade drainage to get moisture away from building foundations,

experimenting with what details (proportions, materials) of a failing system can be changed without sacrificing appearance/aesthetics in the service of improving preservation on a case-by-case basis.