While many builders have incorporated I-joist floor systems
into their standard construction, roof framing with wood
I-joists remains somewhat of a mystery. Roofs tend to be more
complicated than floors, so it’s natural that builders
accustomed to stick framing might be cautious about using
wood-I’s instead of solid-sawn rafters. But for certain
kinds of roof configurations, wood I-joists may work better
than dimension lumber — long-span cathedral ceilings are
a good example. My purpose here is to clarify the differences
between solid lumber and wood I-joists, and to provide some
hints to make roof framing with wood I-joists easier.
I-Joist Roof vs. Conventional Framing
The first thing to understand is how a roof framed with wood
I-joists differs structurally from a traditional stick-framed
In a typical roof framed with dimension lumber, the rafters
rest on the exterior wall top plate at the lower end and bear
against a ridge board at the top. Continuous ceiling joists or
collar ties span from rafter to rafter. There is no need for
bearing posts under the ridge board, which is nonstructural.
The roof loads are carried to the top plates of the bearing
walls, where the floor joists, acting in tension, keep the
rafter ends from spreading out. What you have here is
essentially a truss, built on site. The strength of the roof
system depends a lot on the connections between the joists and
the rafter ends: As long as those nails are adequate for the
loads and don’t slip, the rafters are restrained from
pushing out, the ridge board is compressed in place at the top,
and the roof doesn’t sag (see Practical Engineering,
5/96, for more on this topic).
With wood I-joists, there is no practical way to make a
strong shear connection between the floor joists and the rafter
ends. Instead, a wood I-joist roof system is framed with either
a central bearing wall or a structural ridge — a beam
that carries the roof load to posts. The load from the top half
of the roof is carried by the bearing wall or structural ridge;
the bottom half is carried by the exterior bearing walls. The
loads are primarily gravity loads, which push down, not out, on
the bearing walls. So there is no need to engineer a connection
between the floor or ceiling joists — if there are any
— and the rafter ends.
In my work as a field rep for Trus Joist MacMillan, most of
the wood I-joist roofs I see use a structural ridge beam rather
than a center bearing wall. But whether you use a ridge beam or
a bearing wall, there are two ways to support the joists at the
upper end: with hangers or a beveled bearing plate (see "").
The important point to remember is that no birdsmouth cuts are
allowed at the high end of the I-joist. This would mean cutting
through the bottom flange at the bearing point, which would
damage the I-joist.
Using Hangers at the Ridge
The most common method is to use a face-mount hanger with a
sloped seat (see Figure 1 and ",") such as the Simpson LSSU
series or the USP (Kant-Sag) TMU.
1. Sloped-seat hangers can be adjusted in the
field to match the slope of the roof. These hangers
require a plywood or lumber web stiffener on both sides
of the I-joist to fill out the space between the hanger
and the web.
These hangers can be adjusted in the field to match the
I-joist slope, and can be skewed side to side up to 45° for
hip-and-valley jack rafters.
sloped-seat hanger requires a beveled web stiffener on both
sides of the I-joist to fill out the space between the hanger
and the web. You can rip stiffener material out of plywood (the
thickness depends on the I-joist size), then production-cut it
on a chop saw to the right length. For larger I-joists, use
2x4s for the stiffeners. Make sure you check the
manufacturer’s literature for the proper stiffener size
and thickness — it may vary from brand to brand.
Plywood stiffeners should be attached with three 8d nails
with points clinched. For 2x4 stiffeners on the larger
I-joists, use three 16-penny nails. It’s a good idea to
drive two nails from one side.
It’s important to install stiffeners with a gap at the
top (we recommend 1/4 inch). This prevents the top flange from
prying off the joist web under load.