Is a ridge board/beam required for a roof framed with rafters?
Saturday, July 21, 2018
Yes, either a ridge board or a ridge beam is necessary, and required by the building code, where roof rafters meet at the the center of their span. The difference between them is that a ridge beam is a structural member that bears half of the live and dead loads of the rafters on either side of it, but a ridge board is not structural.
The ridge board is installed to provide a bearing point where the rafters meet, and it also helps with alignment along the ridge. The ridge board is usually just a nominal one inch or two inches wide (1x or 2x lumber) and must be tall enough so that the ends of the joists make full contact with it along their face. This means that the ridge board for 2x6 rafters, for example, must be more than the nominal 6-inch height of the rafters, because the diagonal cut at the ridge end of the rafter makes the vertical dimension of the face a larger dimension. A ridge board was not always required and the photo above, of the attic in a 1920s era house, shows a roof framed without one.
A ridge beam carries the loads of the rafters connected to it and must be both strong enough to carry the weight and well supported at the bearing points at each end, to transfer the loads down to the ground. While the joists commonly attach to the side of a ridge beam, they can also sit on top of it, as shown in the diagram below, which creates an interesting look when the roof structure is exposed as part of the design.
Because a ridge beam often carries a significant area of roof load, they can exhibit structural distress due to a defect in wood, impact on the roof, or use of an undersize beam. The ridge beam shown below, at the living room of a 1950s modern-style ranch house in Gainesville, has cracked at mid-span and is slowly heading towards failure after the crack opened further after a recent repair.
One of the advantages of using a ridge beam instead of a ridge rafter is that the ridge beam carries the loads that would otherwise get transferred downward and then laterally to the end bearing walls, and exerts outward pressure at the top of the wall which, if not contained, will simultaneously cause the walls to splay and the ridge to drop lower.
When a ridge board is used, there are several choices for containing the lateral pressure:
- Ceiling rafters - Although their primary purpose is to provide a structure for supporting and attaching the ceiling, ceiling joists prevent the walls from the spreading apart by being connected to the rafters at each end of the span and creating a triangle. The ceiling joists are in “tension,” meaning that there are forces pulling on them from each end created by the roof loads.
- Collar ties - These create the same necessary structural triangle, but do it a little higher up on the rafter. They do not have to be installed at each rafter pair, and add visual interest when the roof structure is open to the room below. They need to be installed in the bottom 1/3 of the vertical rise from plate to bottom of ridge board in order to be create a large enough triangle to be effective. Ties in the bottom 1/3 of the rise are differentiated by called them “rafter ties.” To learn more, see our blog post What is a collar tie?
- Tension rods - Steel rods between the ends of the joists or top of the wall at regular intervals, when exposed, also provide an elegant solution. Steel is extremely strong in tension, so the rods can be a small diameter.
The photo below shows a home where the structural triangles were disrupted by retrofitting a small shed dormer into the attic. As you can the see, the ridge sagged at the center as a result and, when viewed from the side, the walls splay outward below the area.
Manufactured roof trusses do not need a ridge rafter or ridge beam. They have triangulation built into them, so any additional structural members to resist the lateral loads are not necessary.
Also, see our blog post What causes a sagging roof ridge line?
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