Download PDF version (363.7k) Log In or Register to view the full article as a PDF document.

Beveled Plate Details

The other method used at the ridge is to nail a double-beveled plate on top of the ridge beam (or bearing wall top plate) to provide the sloped bearing surface for the joists. (A single-beveled plate can be similarly used at the low end of the rafter.) Once the plumb cuts have been made, the I-joists are installed by butting them at the ridge and nailing them to the beveled top plate with at least two 10d nails. Blocking. In order to provide lateral stability for the I-joists, you must also install blocking between them on each side of the ridge. This can be metal cross-bracing, dimensional lumber, or I-joist material (see ). Probably the simplest and sturdiest is to use I-joist blocking, installed at the angle of the roof. This provides flange-to-flange support without the need for any filler pieces, and also makes a good shear block for transferring forces from the roof diaphragm to the ridge beam. In a cathedral ceiling, where continuous roof ventilation is needed, you can use narrower-width dimension lumber, metal cross-bracing, or engineered rim joist material notched to allow airflow.

Strapping required.

Finally, metal straps should be nailed across the tops of the butting joists for all roof slopes (Figure 2).

ijoist3-2.jpg (13462 bytes)

Figure 2. When supporting I-joists on a beveled plate, install metal straps across the tops of the butting joists for all roof slopes. When using hangers, straps are necessary for slopes above 7/12.

As an alternative, some manufacturers show a plywood gusset connecting the webs of the butting joists. Both methods will work, but metal strapping is faster. Bottom Bearing Details

You can use a variety of details at the exterior wall plate, depending on the the roof profile you want.

Birdsmouth.

The most common detail is to make a birdsmouth cut at the plate (see ). But be careful: There’s definitely a right and a wrong way to do this (see ""). When laying out a birdsmouth, make sure the seat cut does not overhang the inside face of the bearing wall. The bottom flange must get full bearing on the plate. If this cut is not made properly, the joist’s strength can be significantly reduced. With a birdsmouth cut, you’ll have to use web stiffeners on both sides of each I-joist, as at the ridge.

Beveled plate.

Another option at exterior walls is to use a beveled plate instead of a birdsmouth. This can save time, because there’s less cutting to do and web stiffeners are usually not necessary (except in cases of very large roof loads). Using a beveled plate also provides more design flexibility, as the joists can cantilever up to one-third of the rafter span. The only possible drawback is the additional cost and availability of the beveled plates, although these can be ripped from dimensional stock on a band saw.

Sloped-seat connector.

A third option at the low end is a sloped-seat connector attached at the bearing point (Figure 3).

ijoist4-2.jpg (12176 bytes)

Figure 3. Sloped-seat connectors like the Simpson VPA or the USP TMP can provide bearing at the top plate if the roof loads are not too great.

These metal connectors, such as USP’s TMP or the Simpson VPA, provide a field-adjustable sloped bearing surface. Depending on the manufacturer and type of connector, the allowable slopes range from 1/12 to 12/12. Installation varies by manufacturer, so always check the instructions for the specific connector you are using. The advantages of these connectors are that no birdsmouth cut and no web stiffeners are needed. The disadvantages are that their loading capacity is somewhat limited (see manufacturer’s catalog for specifics) and that installation can be time consuming.

Blocking.

Regardless of the method you use at the low end, blocking or cross-bracing is required to prevent joist rollover.