The Four D’s of Building Enclosures: Deflection, Drainage, Drying, and Durability

07.25.16   Thomas L. Bane, PE | More by this Author

The Four D’s of Building Enclosures: Deflection, Drainage, Drying, and Durability

Welcome to the second part of a four-part series on “The Four D’s of Building Enclosures: Deflection, Drainage, Drying, and Durability.” When applied correctly, the four “Ds” increase the longevity of buildings and decrease maintenance costs.

This edition of “The Four Ds of Building Enclosures” focuses on Drainage.

Water that isn’t deflected away from a building is going to wet it. Drainage helps take that water and get it off the building as soon as possible.

Good drainage is easy – provide a clear path with good slope. Unfortunately, too often drainage is insufficient or omitted altogether. Drainage is critical to performance and longevity.

  • Low-Slope Roofs Vs. Steep-Slope Roofs: Low-slope roof membranes are monolithic assemblies with sealed seams, whereas steep-sloped roofs primarily consist of loosely overlapping layers. However, because low-slope roofs drain slowly, they tend to last less than half as long and cost more than steep-sloped roofs.


Low-Slope EPDM Roofing

Steep-Slope Asphalt Shingles


(2014 RSMeans)

EPDM Membrane: $1.76/SF

R-38 Polyiso Insulation: $3.98/SF


Total: $5.74/SF

Asphalt Shingles: $1.92/SF

#15 Underlayment: $0.14/SF

R-38 Blown Insulation: $1.88/SF

Total: $3.94/SF

Average Life

(Roofing Failures by Carl Cash)

14.2 Years

35 Years

          *cost data from RSMeans Building Construction Cost Data, 2014 edition     

  • Low-Slope Roofs: Roofs with slopes less than 1/8 inch per foot pond water in the valleys and roof crickets. Ponding water leads to premature deterioration of the roofing. Ponding occurs so regularly, signs of ponding are often visible in satellite photographs. To minimize this issue, crickets should be wider to increase the slope in the valleys.

Dirt and pollution collect in the ponding areas of low-slope roofs.

  • Window Sill Pans: Sill pan are supposed to collect water and drain it out to the exterior. Unfortunately, the drainage path is often blocked by the window’s perimeter sealant joint. As a result, the pan will fill with water and leak to the interior. To avoid this issue, provide a sealant joint around the interior perimeter of the window, and provide weeps at the exterior sealant joint.
  • Sheet Metal Flashings: Sheet metal flashings often rely on sealant at the splice joints to remain watertight. Because these sealant joints are applied “blind” (covered immediately after they are applied), they often fail. If the sheet metal flashing is minimally sloped, static head pressure (Photo 2), wind-blown rain, or capillary action can cause water to penetrate the joints. Even in instances where the seals remain intact, the exposed materials may deteriorate prematurely (Photo 3). Provide a minimum slope of 1 inch per foot for flashings, and always include a back-up weather barrier behind the flashing.

A coping was insufficiently sloped and has standing water at a splice joint. This joint failed and leaked into the interior.

Lack of drainage caused the bottom of the metal panels to corrode prematurely.

  • Below-Grade Waterproofing and Planters: Plants with aggressive root systems destroy buried waterproofing systems when the roots grow into the waterproofing membrane. Roots grow toward nutrient and moisture sources. Proper drainage immediately in front of the waterproofing minimizes the amount of moisture at the level of the waterproofing, and thus repels roots.

If you’re have trouble with water in your building, or you are planning to construct a building and want to keep water out, SME can help. For more information, contact Tom Bane.

Stay tuned for our next edition of “The Four Ds of Building Enclosures,” which will focus on Drying.


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