Unbraced Knee Walls
of builders want to provide extra ceiling insulation over the
eaves walls. When using trusses, this can usually be
accomplished by raising the heel. When using conventional
rafters, the best way to add room for insulation is to carry
the ceiling joists to the outside of the wall, add a rim joist,
then fasten a 2x6 on the flat to provide nailing for the
Unfortunately, I often see short kneewalls built on top of
the exterior wall to provide the extra insulation space (Figure
This short kneewall
(top) is designed to add extra insulation space over the
exterior wall, but it is framed only half-right. The 2x4
diagonals nailed between the top of the wall and the ceiling
joists brace against outward thrust, but the wall needs to be
sheathed to prevent racking. In a similar situation (bottom), a
kneewall has been added to accommodate two different ceiling
heights. The joists themselves provide resistance to any hinge
action, because they are tied to an existing wall. But the wall
needs drywall on both sides or plywood sheathing on the attic
side, 4 feet in from each end, to prevent racking.
The problem here is that the kneewall creates a hinge point
that could kick outward when the roof is loaded. The kneewall
can also rack laterally.
The framing in the photo, above left, is done half-right.
Every other ceiling joist has a 2x4 diagonal fastened to the
top of the kneewall. With three or four common nails in each
end, this diagonal bracing will keep the kneewall from kicking
The kneewall can still rack from side to side, however.
Luckily, the simple solution is to sheathe it with plywood,
just like the full-height wall it sits on. Better yet, run the
sheathing so it overlaps the joint between the plates of the
The other kneewall in Figure 5 also needs sheathing to keep
it from racking. In this case, outward thrust is not a problem,
because the ceiling joists it supports are tied back into a
wall. But it still needs lateral support so it won't fall over
The wall will be finished on one side with drywall, but that
won't help. Drywall on both sides may be enough to provide the
lateral bracing the wall needs. But with an open attic space
behind it, the easiest solution is to use plywood to sheathe at
least 4 feet in at both ends.
sometimes happens that there isn't enough room over a window or
door for a proper header. That's the case in the photo of the
top of a patio door opening (Figure 6).
igure 6. When there is not enough
room for a conventional header, the rim joist can be doubled to
take the load. Use joist hangers on 2x4 walls, where the
doubled rim joist doesn't leave enough bearing surface. Also,
be sure that the inner rim joist is long enough to bear on the
doubled king studs; otherwise, the plate may crush.
Fortunately, this builder didn't rely on the flat 2-bys to
carry the load - a situation I see all too often. Instead, he
scabbed an LVL onto the rim joist and fastened his I-joists to
There are several important points to remember when using a
rim joist as a header. First, depending on whether the wall is
framed with 2x4s or 2x6s, you may have to use joist hangers
because there may not be enough plate left for the joists to
sit on. Secondly, the scabbed rim joist has to be long enough
to bear on the king studs at both ends of the opening. Most
openings will have double king studs to add stiffness; if not,
you should double them anyway to increase the bearing area for
the header. The header over the first-floor opening in this
photo, for example, must also carry the load of a second story.
Such a large compression load concentrated on the plate above a
single king stud could cause the plate to crush.
David Utterback, formerly District Manager for Western Wood
Products Association, is field representative for the
California Redwood Association. Based in Overkland Park,
Kansas, he inspects building sites all over the country.