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
Finally, metal straps should be nailed across the tops of the
butting joists for all roof slopes (Figure 2).
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.
A third option at the low end is a sloped-seat connector
attached at the bearing point (Figure 3).
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.
the method you use at the low end, blocking or cross-bracing is
required to prevent joist rollover.