Addressing the Roof Truss Design Note: “Provide adequate drainage to prevent water ponding.”

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All Things Wood
Issue #11234 - January 2019 | Page #46
By Frank Woeste and Scott Coffman

The purpose of this article to examine the significance and implementation of a typical note that appears on metal plate connected roof-truss drawings when a top-chord-pitch of ¼:12 or less is specified by the Construction Documents. Three questions will be addressed:

  1. What is the mechanism of “water ponding”?
  2. How does the building code (IBC) address the ponding subject whereby the project design team can establish that the installed trusses and roofing system will “provide adequate drainage to prevent water ponding”?; and
  3. To whom is the instruction “Provide adequate drainage” directed?

1. Visualizing Roof Ponding Behavior—A Safety Issue

The loading mechanism of trusses by “water ponding” creates a path for the possibility of a catastrophic structural failure. Unlike a uniform load, such as a 10 psf Dead Load (DL) that acts on the entire span and does not change based on the deflection of the span in-service, loading by water is a dynamic process whereby any additional deflection by the water produces more deflection.

To help understand and visualize the process, a Florida carport that experienced a ponding failure is depicted in Figure 1. The carport in this example is designed with a slope for drainage to a gutter on the left side. The blue arrows show the flow of rain water that took place for decades due to a sloped roof. In contrast, the red arrow depicts the flow of water toward the center section of the span which produced substantial panel deflections due to the weight of the accumulated water.

Originally sloped for drainage, incremental deflection of the span near the center slowed the rainwater flow to the left and allowed water accumulation within the span. As the water accumulated near the center of the span, the roof panels continued to deflect, creating a deeper and heavier pond of water. While the carport ponding example may not have been life-threatening, ponding can cause a catastropic failure of a roof due to an increased load from water.

Ironically, cases when only leaks are observed in low slope roofs can be fortuitous. That event can prompt an examination of the roof surface, and the evidence of water not draining (or residing) may be revealed as a precusor to structural ponding (see Figure 2).

2. Building Code Requirements that Address Ponding

Structural design to prevent water ponding is a complicated issue when a “design roof slope” is set at ¼:12 by the design team, typically the Architect-of-Record and Owner. While roof drainage and ponding are a concern for roof system design, ponding is a structural issue for the roof trusses that are engineered based on IBC load provisions and the referenced IBC standard for wood trusses, ANSI/TPI 1–2014.  

Several definitions from the IBC are relevant to the ponding issue. For discussion purposes, we assume the applicable code for a project is the 2015 IBC.

From the 2015 IBC:

TABLE 1604.3
DEFLECTION LIMITSa, b, c, h, i

CONSTRUCTION

L

S or Wt

D + Ld, g

Roof members:e

 

 

 

     Supporting plaster or stucco ceiling

l/360

l/360

l/240

     Supporting nonplaster ceiling

l/240

l/240

l/180

     Not supporting ceiling

l/180

l/180

l/120

Footnote “e” on Roof Members:

“e. The above deflections do not ensure against ponding. Roofs that do not have sufficient slope or camber to ensure adequate drainage shall be investigated for ponding. See Section 1611 for rain and ponding requirements and Section 1503.4 for roof drainage requirements.” (Underlined for emphasis.)

The use and reliance on Table 1604.3 for maximum member deflections is only one aspect of a code-conforming design. To understand the second sentence in the footnote, a definition is needed.

From IBC Chapter 2 Definitions:

“[BS] SUSCEPTIBLE BAY. A roof or portion thereof with:
1. A slope less than 1/4-inch per foot (0.0208 rad); or…”

The key words in this definition are or portion thereof directing the structural designer to consider the slope of the framing member along the entire length of the span. The sloped line in Figure 3 depicts a “design slope” of ¼-inch per foot and a 30-ft. truss span.

Once a truss is installed with sheathing and a roofing system that conforms to the truss top chord slope, a specified design slope of ¼-inch per foot on the lower part of the truss span/roof has, in reality, become less than ¼-inch per foot. Thus, without intervention by the design team to maintain a ¼-inch per foot roof slope, the roof structure has at least one SUSCEPTIBLE BAY (due to Dead Loads only).

Referring to the 2015 IBC, Section 1608 Snow Loads:

“1608.3 Ponding instability. Susceptible bays of roofs shall be evaluated for ponding instability in accordance with Section 7.11 of ASCE 7.” (Underlined for emphasis.)

The emphasis of the IBC language is structural (ponding instability) and points to the need for additional structural work when “susceptible bays” exist on the completed roof construction.

Referring to the 2015 IBC, Section 1611 Rain Loads:

“1611.2 Ponding instability. Susceptible bays of roofs shall be evaluated for ponding instability in accordance with Section 8.4 of ASCE 7.” (Underlined for emphasis.)

Again, the emphasis of the IBC language is structural (ponding instability) and points to the need for additional structural work by the design team.

To summarize—based on the IBC code provisions cited above, the typical truss design note that appears on a truss design drawing with a top-chord design slope of ¼:12 or less is clearly a structural issue, requiring additional consideration and design work by the design team.

3. To whom is the instruction “Provide adequate drainage” directed?

The intended recipient of the note to “provide adequate drainage” is not clear. In addition, the note in total does not give guidance as to what actions or professional work is required to prevent water ponding.  Knowing the structural complexity to evaluate a roof having “susceptible bay(s)” for ponding instability, uncertainty of the actual dead loads (uniform and concentrated) on the completed roof construction, and final roof-surface profile, we believe clarity in stating the truss design assumptions is critical information for the design team members in their review and approval of the proposed truss design drawings.  

Other typical design notes, such as wind load design data, special live loads, concentrated loads, snow loading, IBC referenced load standard and edition used, and so on, are a statement of “design assumptions” that can be reviewed and approved by the structural engineer and/or architect and general contractor and returned to the Component Manufacturer (CM). Even if the truss designs are not reviewed and approved by the structural engineer and/or architect and returned to the CM, the stated design assumptions stand alone as the basis of the proposed truss design.

Suggestions and Guidance

We offer the following ideas for educating and communicating the substance and importance of this low-slope roof ponding issue.

  • Consider revising the “water ponding” truss note to be a truss design assumption consistent with other notes that spell out the parameters of the design. For example, this type of language could be used:

“This design does not include loads that could occur from the ponding of roof water in susceptible bays (2015 IBC, 1608.3 and 1611.2).”

This could be a truss design assumption made consistent with the content of most truss design notes.

  • In most truss cases investigated by these authors, documented evidence that a design professional (other than the truss design engineer) reviewed the project truss design drawings was not available. At least for the case of ¼:12 roof trusses, CMs should consider steps to:
    • educate their customers and related project design-team members about the ponding instability issue,
    • require review and approval of the truss design drawings by a design professional, and
    • document the reviewed and approved truss design drawings.

We believe these steps could benefit all parties involved, including the project owner.

  • Hold in-house discussions on the ponding note with respect to its meaning and purpose, and encourage design technicians to share available resources on roof design to prevent the likelihood of a ponding instability. Indeed, the use and specification of a top chord “design slope” greater than ¼:12 is one example of a possible remedy that can be proposed.  
  • A readily available source of design education on low-slope roofs to prevent ponding is a two-part article by the co-author Scott Coffman. The articles are published in Structure Magazine (2017):

Please consider these suggestions, as well as other ideas you may have in practice, to mitigate the unintended effects of ponding. Comments are welcome and can be shared with the Editor or authors.

About the Authors

Frank Woeste, P.E., is Professor Emeritus, Virginia Tech and a wood construction consultant. Frank, along with his colleagues, have developed and presented continuing education programs for more than 30 years. For example, visit http://www.cpe.vt.edu/sdwnds/index.html for our next course.

Scott Coffman, P.E., SECB is a forensic engineer with Construction Science and Engineering, Inc. Scott has over 35 years in structural wood design experience that includes engineered wood building components.

You're reading an article from the January 2019 issue.

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