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Soffits and Overhangs

Soffit treatments seem to concern a lot of the contractors I meet in the field. "How can I nail my fascia to that skinny little piece of OSB?" is a question I hear a lot. The truth is, there are several details that work, depending on the roof profile you want (Figure 4).

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Figure 4. Many soffit profiles are possible with an I-joist roof system. Shown here are dimensional lumber rafter tails (left), a level soffit (middle), and an I-joist used as backing for fascia (bottom).

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One typical detail is to sister on dimensional lumber rafter tails. You can also use plywood, an engineered rim board material, or even another I-joist for fascia backing. To frame a flat soffit, you can extend the birdsmouth cut to the end of the joist, then attach 2x4 blocking for a soffit nailer. Just about any traditional profile is possible with proper planning at the design stage. The rules for overhangs are straightforward. All the manufacturers’ design guides show many details. One point to remember is that if a birdsmouth cut has been made, the maximum allowable overhang for any of the details is 2 feet. If you want a longer overhang, use a beveled plate or sloped-seat connector.

Gable-end overhangs.

Gable-end outriggers are framed by cantilevering dimension lumber across the gable-end top plate, similar to stick-framing techniques (see ). Hips and Valleys

Hips and valleys are possible with wood I-joists, but the only practical way to frame them is to use the field-adjustable hangers mentioned above. The techniques are identical to the methods mentioned earlier, except that when the hanger is installed to the beam, the hanger must be skewed (Figure 5).

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Figure 5. Sloped-seat hangers can be skewed up to 45° for hip and valley jack rafters. Compound cuts are not necessary; only the plumb cut needs to be made.

Although hangers can get expensive, one advantage of this technique is that compound cuts are not needed on the jack rafter ends. The only cut required is the plumb cut. Header Details

Framing headered openings for skylights and dormers is also straightforward. As with dimension lumber, the size of the opening determines how many I-joists are needed to support the header. If the header hangs from a single I-joist, you nail a backer block (typically plywood) to the joist, then nail the hanger to the backer block (see ). Double I-joists require a filler between them — either plywood or dimension lumber — and a backer block for the hanger. For really large openings, it makes sense to use an LVL or Parallam beam to support the headers instead of multiple I-joists.