![]() Though rafter ties are commonly and historically installed on 48-inch centers, the 2012 IRC requires a rafter tie for every pair of rafters. A rafter tie connected to the bottom ends of the rafters creates the third side (the base) of the triangle, and can also function as a ceiling joist. The roof peak is the top of the triangle, and the rafters make up the two sides that slope down to the bottom points at the tops of the walls. Unlike a ridge beam, which is structural, a ridge board can be a simple 1-by (3/4-inch thick) that's at least as wide as the cut end of the rafter. The ridge board merely holds the top point of the triangle together and can be omitted altogether if the tops of opposing rafters are connected with a gusset plate. This conventional framing system can be used for roofs with a 3:12 slope or greater and works like a triangle, requiring all three sides and points to be provided, as shown in the illustration. But when using this nailing schedule, keep in mind that uplift must still be addressed using either collar ties or ridge straps (more on that later).Ī ridge board is used in a completely different method of construction. Table R602.3(1) requires that either four 16d toenails or three 16d end nails be used for the connection. However, the IRC does allow a prescriptive wood-to-wood connection of the rafters to the ridge for slopes of 3:12 and greater. In most cases that connection is achieved with manufactured load-rated hardware. ![]() In ridge-beam roof construction, the connection of the rafters to the ridge must handle the entire gravity load on the rafters. Using an engineered beam (such as an LVL) for the ridge is helpful in this regard because large spans can be achieved and span tables are usually available from the manufacturer. The ridge-beam approach is simple except for the sizing of the beam, for which the IRC does not provide guidance. In an actual ridge-beam roof-rather than in our imaginary flat surface-the center beam can be raised to any height (see Structural Ridge Beam illustration), creating a vaulted ceiling below without an attic space (although the code does allow for building an attic by adding properly sized ceiling joists). The other ends of the rafters bear on the exterior walls. ![]() Again, picture deck joists connected to a beam with hangers. The rafters are then connected to the beam and transmit their full vertical load downward. The ends of the beam must be supported as concentrated loads, generally by posts inside walls or in the middle of a room. The ridge is constructed as a beam that runs from end to end. The rest of the roof framing then follows suit.Ī roof with a ridge beam can be more easily understood if it's imagined to be a flat surface-like a deck in plan view. They're most easily identified by the type of ridge used: either a ridge beam, which is a structural or load-bearing member, or a ridge board, which is a non-structural framing member. But when it's boiled down, there are essentially two standard methods of roof construction, each having some flexibility. Understanding this history provides perspective on why the treatment of roof construction in the IRC and the associated Wood Frame Construction Manual(WFCM) allows for such broad interpretation. ![]() Combining these disparate styles and standards into a single code-the International Residential Code (IRC)-was no easy feat, and seemingly fundamental aspects of roof construction are still up for debate. Between the Eastern and Western codes, there were even different names for identical parts. In the past, methods described by the three main building codes (BOCA, UBC, and SBC), though similar, differed based on climatic factors-such as expected wind and snow loads-local habits, and available materials. Long before any building codes were published, conventional roof framing methods developed through practice, resulting in approaches to roof framing that are as varied as the framers themselves.
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