Understanding why doors leak

A flowchart showing decisions related to exterior door thresholds.
A flowchart showing decisions related to exterior door thresholds.

Accessible doors
Since the Americans with Disabilities Act (ADA) first passed in 1990, codes and standards have evolved to improve accessibility for the disabled. Compliance with the new 2010 ADA Standards for Accessible Design has been required since March 15, 2012.

For doors along accessible routes, the following requirements pose a challenge for waterproofing:

  • thresholds at doorways cannot exceed 19 mm (3/4 in.) in height for exterior sliding doors or 13 mm (1/2 in.) for other types of doors (e.g. the swing doors discussed in this article); and
  • landings with a slope not to exceed 0.25 units vertical in 12 units horizontal (i.e. approximately a two-percent slope).

In addition to the 2010 ADA Standards, accessibility is governed by the International Building Code (IBC), International Residential Code (IRC), the Fair Housing Act, and other national codes and standards, not to mention local amendments and ordinances. (See Building Codes Illustrated by Francis D.K. Ching and Steven R. Winkel, FAIA, PE [John Wiley & Sons, 2012].) These documents generally agree on the dimensional requirements for accessible doors, but they impose different criteria for where exceptions apply. Figure 2 maps the exceptions permitting the installation of a higher threshold or even a step, the latter of which completely solves the water penetration issue.

The American Architectural Manufacturers Association (AAMA) has played a leading role in recognizing the issue of water penetration over accessible door thresholds. The group stated:

The current issue concerning the threshold requirement is that the Department of Justice [DOJ] has issued a Fair Housing Act interpretation that requires doors to a deck or balcony from an apartment to meet the accessibility criteria for a Type B unit …This means thresholds higher than 1/2 or 3/4 inch [12 or 20 mm] above the interior floor level are not permitted, even though a step-down of up to 4 inches [102 mm] is permitted between the interior floor and the walking surface of the exterior deck or balcony … A threshold height of 3/4 inch is only sufficient to resist water infiltration in areas of low wind and exceptionally low rainfall. (For more information, read “AAMA Seeks Code Modification for ADA Threshold Height. AAMA Official Website.”)

AAMA proposed the following code change as a second exception to IBC 1008.1.7, which was approved by the International Code Council (ICC) during a code development hearing in 2012:

In Type B units, where Exception 5 to Section 1008.1.5 permits a 4 inch [102 mm] elevation change at the door, the threshold height on the exterior side of the door shall not exceed 4 3/4 inches [120 mm] in height above the exterior deck, patio or balcony for sliding doors or 4 1/2 inches [114 mm] above the exterior deck, patio or balcony for other doors. (Read “ICC Approves Revisions to IBC Egress Codes” on the Door & Window Manufacturer Magazine.)

This code change would allow another exception for the installation of a higher threshold, but only for a limited number of doors in multi-family construction. For that matter, all the exceptions apply to a small number of exterior doors, and they are often ignored. The authors have observed designers default to requiring accessible exterior doors (with their attendant waterproofing challenges) even where they may not be strictly required. This is rational given the complexity of code requirements, the risk of liability, and changing code interpretations (e.g. the Department of Justice [DOJ] requirement in the aforementioned AAMA citation). In other words, low thresholds are here to stay.

Rainwater accumulation
The authors’ research indicates there has yet to be an accurate measurement of how much water accumulates outside a door threshold during rainfall. Neither the average, typical rain nor the thousand-year flood should be the basis of design. However, as the climate warms and intense storms become more frequent, exterior doors should be better designed to resist water and require parameters that are more severe than the current leading standard methods.

Three standards specify a horizontal spray of five gallons of water per square foot per hour, which equates to a vertical rainfall rate of 203 mm (8 in.) hourly:

  • ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference;
  • ASTM E547, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Cyclic Static Air Pressure Difference; and
  • AAMA 501.1, Standard Test Method for Exterior Windows, Curtain Walls, and Doors for Water Penetration Using Dynamic Pressure.

This amount would be an unprecedented flood if the rain continued for an entire hour, surpassing the roof drainage capacity of even the most stringent codes. However, the peak rainfall intensity over shorter durations is much higher and, of course, leakage at a marginal door threshold is most likely during these peak events. Spraying the water horizontally at a test door may appear to compensate for a lower volume of water, but the testing discussed later in this article shows that is not the most severe spray angle.

The threshold was set at 13 mm (1/2 in.) above the platform to match the maximum permitted by code. The platform was hinged to adjust the slope up to five percent—more slope did not improve drainage.
The threshold was set at 13 mm (1/2 in.) above the platform to match the maximum permitted by code. The platform was hinged to adjust the slope up to five percent—more slope did not improve drainage.

Accessible doors are required to have a low-slope walking surface outside, which compounds the leakage problem by creating a slow-draining surface. Two-percent slope is so small it can easily be erased by normal construction tolerances and slab distortion from post-tensioning, resulting in flat spots—or even negative slope toward the threshold.

Further, exterior doors typically are at the bottom of tall walls that increase the collection field for water that eventually cascades over the threshold. Water flows turbulently down walls, resulting in eddies and vortices. (See Turbulent Liquid Flow Down Vertical Walls by H.H. Belkin et al [Carnegie Institute of Technology, 1959].) Wind compounds this problem, pushing water against the door, but even on still days, water can stagnate in front of a door threshold due to the fluid dynamics required to turn the direction of the water flowing down the wall onto the low-slope walking surface.

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