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Unbraced Knee Walls

A lot 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 rafters. Unfortunately, I often see short kneewalls built on top of the exterior wall to provide the extra insulation space (Figure 5).



Figure 5.

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 out. 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 two walls. 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 sideways. 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.

Rim-Joist Header

It 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 it. 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.