In the course of investigating hundreds if not thousands of forensic projects per year, it’s hard to find a more commonly misunderstood or incorrectly constructed item than residential roof framing. Errors and deficiencies in the construction and design of residential roof framing can lead to costly insurance claims or litigation and can also make the residence more susceptible to property damage from storm events such as snow and wind loading.
For areas throughout the United States that are not located in high wind or seismic zones, construction projects for one- and two-family dwellings do not typically require design from a licensed engineer. Instead, the International Residential Code (“IRC”) allows for the general contractor or the architect to follow the prescriptive requirements of the IRC to ensure adequate structural framing. However, in our experience, the prescriptive requirements of the IRC can often be misinterpreted or misunderstood by contractors, architects, and even building officials leading to deficient roof framing. There is often confusion with the requirements even among licensed structural engineers who aren’t familiar with residential construction. As the desire for more complex roof shapes and open ceiling configurations increases in newer construction, the risk of claims from roof framing issues will likely also increase.
VERTEX has a unique perspective in dealing with the construction and design-related issues because of our diversity in clients, the scope of services, and the diversity of experts. Whether consulting for a litigated matter, reviewing details for loss control, conducting cost estimates, or performing structural repair design, we have seen issues related to residential roof framing from a wide range of perspectives. In our experience, the most common residential roof framing issues center around a misunderstanding of the following 3 topics and components:
- Ridge Board vs. Ridge Beam
- Rafter Tie vs. Collar Tie
- Hips and Valleys
This three-part blog aims to provide an overview of these topics with the hope of providing a concise overview of residential roof framing basics as well as common deficiencies along the way. Part 1 will focus on the difference between non-structural ridge boards and structural ridge beams. Code references throughout our discussion will come from the 2015 edition of the IRC which is the most common edition currently adopted across the United States. Similar language and principles exist across other IRC editions and industry standards as well.
RIDGE BOARD:
The most common type of residential roof is a simple gable or hip configuration which generally consist of a peak (ridge) and sloped sides which resembles a triangular shape when viewed from the side. In its most common form, the roof is constructed with rafters along each slope with ceiling joists along the bottom which encloses an attic space (referred to as “conventional system” throughout this discussion). The ridge board is located along the peak of the roof in a conventional system. Its technical function can be best summarized by the following definition adopted from Wood Frame Construction Manual (“WFCM”) produced by the American Wood Council (“AWC”):
[2018 WFCM] RIDGE BOARD
A NON-STRUCTURAL member used at the ridge of a roof to provide a common nailing surface and point of bearing for opposing roof rafters.
Simply put, a ridge board is a non-structural nailing board located at the peak between the tops of the rafters. Figure 1 provides an illustration of a conventional system with ridge board.
Figure 1: Diagram of conventional system with ridge board.
Now that we have established the basic function of a ridge board, let’s move on to the specific requirements of the building code. The IRC requirements for a ridge board are relatively brief and listed as follows:
[2015 IRC R802.3] Rafters shall be framed not more than 1½-inches offset from each other to a RIDGE BOARD or directly opposite from each other with a gusset plate as a tie. Ridge boards shall be not less than 1-inch nominal THICKNESS and not less in DEPTH than the cut end of the rafter…
The IRC requirements above are focused on minimum geometry and bearing dimensions for the ridge board to function as a nailing board between the opposing rafters. To further illustrate that the ridge board is not functioning as a structural beam, it is interesting to point out that IRC allows the ridge board to be eliminated with use of gusset plates between the rafters. Even though a ridge board is not considered a structural member, the geometry and dimensional requirements listed in the IRC for ridge boards are no less important. For example, when a ridge board does not have sufficient depth to provide full bearing, the rafters can be overstressed in compression and cause crushing of the wood. In combination with other deficiencies, the rafters are also more prone to separate from ridge which can cause cracks and displacement of the rafters during an extreme snow event or overtime during cyclical, seasonal loading.
Figure 2: Example of crushing at top of rafter along ridge caused by inadequate bearing.
Figure 3: Example of rafter exceeding depth of ridge. (Note separation as evidenced by withdrawn nails)
So, if the ridge board is not a structural beam, what is holding up the peak of the roof in a conventional system? Due to the self-support provided by the triangular shape (“truss action”), the tops of the rafters are held up by the equal and opposite force of the two rafters pushing against each other. At the same time, the bottoms of the rafters are attached to ceiling joists which resist the outward thrust at the end of the rafters. Part 2 of our series on Residential Roof Framing Basics will provide more insight on the behavior and connection requirements for resisting outward thrust at the bottom of the rafters. For now, it’s useful to mention that in the context of ridge boards, the truss-action that holds up the ridge does not perform well at lower roof slopes because the thrust force at the rafter-to-ceiling connection increases and often fails, leading to subsequent displacement at the rafters and a deflection at the ridge. As such, Section R802.3 specifies the following:
[2015 IRC R802.3] Where the roof pitch is less than THREE UNITS IN 12 units horizontal (25-percent slope), structural members that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams.
Therefore, regardless of whether there are ceiling joists or similar horizontal ties, the IRC states the ridge should always be designed and installed as a structural beam (not just a nailing surface) when the roof pitch is less than 3:12. This requirement repeatedly matches our experience of lower sloped roof framing behavior in the field. For example, a common roof deflection issue observed during snow loading events occurs at low-sloped roofs, even when ceiling joists are installed. The ridge tends to deflect downward, and the rafters push outward as illustrated in Figure 4.
Figure 4: Example of sag at ridge that occurred during snow loading.
(Note roof had low-slope (less than 3:12) with conventional system)
RIDGE BEAM (or STRUCTURAL RIDGE):
What happens in an open (“cathedral”) ceiling system when we remove the ceiling joists or similar horizontal ties? In that situation, the ridge is functioning as much more than a nailing surface and must be designed as a structural member (“ridge beam” or “structural ridge”). Again, we look to the WFCM for a useful definition of a ridge beam and associated open ceilings as follows:
[2018 WFCM] RIDGE BEAM
A STRUCTURAL member used at the ridge of a roof to support the ends of roof rafters and transfer roof loads to supports.
[2018 WFCM] OPEN CEILINGS
Where ceiling joists and roof ties are omitted and rafters are used to create an open (cathedral) ceiling, rafter ends shall be supported on bearing walls, headers, or ridge beams.
As indicated in the above definitions, a ridge beam with closely spaced rafters is akin to a floor girder along the centerline of a house that takes loading from joists and transfers the load to supports at its ends. Figure 5 provides an illustration of a ridge beam in an open ceiling system.
Figure 5: Diagram of open ceiling system with Ridge Beam
As can be seen in Figure 5, there is no longer a complete triangle, and the tops of the rafters must be supported by a structural beam. Regarding building code requirements, the IRC provides the following brief requirements regarding ridge beams.
[2015 IRC R802.3.1] Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder DESIGNED in accordance with engineering practice.
Thus, in open ceiling systems, the ridge beam must be properly sized as a structural beam by an engineer. In addition, the connection between the rafters and ridge also must be designed by an engineer because the prescriptive connections in the IRC are only applicable for a ridge board. As is the case with most of residential construction, a licensed engineer is not always involved, and the contractor often provides framing sizing in accordance with the prescriptive requirements (usually provided in tables) throughout the IRC. In the case of ridge beams, there is no specific tables that provide the required sizing and connections for specific spans and loading like there is for floor girders. Without this direct guidance, the contractor may rely on lumber suppliers or just their own experience to size the ridge beam, supports, and connections. Alternatively, an architect may provide sizing and detailing for the ridge beam. It is common that the architect, lumber supplier, or contractor will provide inadequate sizing or detailing either due to a lack of understanding about ridge beam behavior and/or the code specified loading and deflection requirements. In addition, the actual site conditions may differ from that assumed by the architect or lumber supplier. This often results in either undersized ridge beams (see Figure 6) or convoluted and overcomplicated framing layouts that do not serve their intended purpose (see Figure 7).
Figure 6: Excessive deflection of ridge at open cathedral ceiling caused by undersized ridge beam.
Figure 7: Example of overly complicated roof system indicating lack of understanding of residential framing. (Note large (7×20) LVL ridge beam and support posts installed in a conventional system with ceiling joists).
SUMMARY:
Some of the most common residential roof framing deficiencies seen at VERTEX stem from a general misunderstanding above the function of ridge boards and ridge beams as well as their basic code requirements. A ridge board is non-structural member located at the peak of a conventional closed ceiling system that serves as a nailing surface between the tops of opposing roof rafters. The ridge board is dependent on the truss-action of the closed conventional roof system. A ridge beam on the other hand is a structural member at the peak of an open (cathedral) ceiling system that must be designed by an engineer to support the rafters along its length and transfer the roof loads to supports.
In Part 2 of this series, we will discuss the difference between rafter ties and collar ties as we continue our overview of residential roof framing basics.
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