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Knee Walls

How to handle upper-story knee walls depends on whether they’re drywalled on the back — as in a knee-wall closet — or left open, in which case the area behind them is considered part of the attic.

When the design calls for finished storage space behind knee walls, we frame them with top plates. If the area behind the wall will not be finished, we usually just nail the studs to the sides of the rafters. But there’s no need to fire block the bays, because the entire space behind the wall is considered unfinished attic.

Basement Walls

If the house has a basement, we typically frame a 2x4 stud wall inside the perimeter, holding it away from the concrete foundation about an inch or so. Even when there’s batt insulation in the bays, the back of the wall would be open at the top where the plate attaches to the joists — which means that by code fire blocking is needed.

The easiest way we’ve found to fire block this spot is to butt a continuous 2x8 to the mudsill, letting it overhang the foundation wall. We nail the 2x8 to the floor joists above, then nail the 2x4 top plate of the basement wall to the 2x8.

This technique serves another purpose too. Since the mudsills are straight and square, the overhanging 2x8s are also straight and square. We quickly plumb down from the 2x8 edge with a laser to lay out the bottom plates of the 2x4 walls, which gives us a perfectly straight, square layout without any fuss.

Separating stud bays. Because of the air space behind them, our basement walls trigger a second code requirement: R602.8 (item 1.2) states that fire blocking is needed “horizontally at intervals not exceeding 10 feet” within concealed spaces in stud walls. The rule is intended to prevent all the bays in a wall from being connected when the studs are not drywalled or sheathed on both sides. (This condition also occurs in double-framed walls, or where strapping is used to flatten a wall or ceiling.) We meet the requirement by sistering 2-by material to the sides of studs and running it back to the foundation. This blocking isn’t pressure treated, so we hold it off the concrete and get the insulator to caulk the gap when he seals and insulates the wall.

Sealing Penetrations

A new provision in the 2006 IRC (R602.8, item 4) states that fire blocking is required “at openings around vents, pipes, ducts, cables, and wires at ceiling and floor level, with an approved material to resist the free passage of flame and products of combustion.” This means you have to seal around all those wires, pipes, and ducts that run through top plates and subfloors. Energy-conscious builders often do this anyway, as an air-sealing measure. But if you work under the 2006 IRC, you now have to do it with an approved fire-blocking material. A similar requirement has long existed for fire-rated assemblies in commercial projects.

Just what constitutes an “approved material” for sealing these openings is subject to debate. Few materials that work for this purpose are prescriptively approved — that is, specifically named in the code. But materials that are not prescriptively approved may be approved as “alternative materials and methods.” Under R104.11, a material can be used if the building official finds that “the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method, or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code.” In other words, it’s up to the building official.

Tested materials. One way to get an inspector’s sign-off is to submit an ES report — a document issued by the ICC Evaluation Service (a subsidiary of the International Code Council) that shows the material in question has been third-party tested for code compliance. An ES report lists the test standards the product has met, the regulations it complies with, and the conditions under which it can be used. While an ES report is often convincing, it doesn’t guarantee that the inspector will accept a particular product.

Nearly every inspector will allow you to seal penetrations with intumescent and noncombustible sealants of the type used in “approved penetration fire-stop systems” tested to ASTM E814 or UL 1479 (R317.3.1.2). This is a higher standard than must typically be met in residential construction, where fire-rated assemblies are rare.


The red caulk around these pipes (top) is approved for use in fire-rated assemblies. The orange polyurethane spray foam around the ducts (above) is not intended for use in fire-rated assemblies, and may or may not be accepted by building officials for sealing penetrations through fire blocking.

Unfortunately, because fire-rated caulks are expensive ($8 to $16 per tube), many builders prefer to use the cheaper canned spray foams now being marketed as approved fire blocking. These foams are typically dyed orange so they can be easily identified by the field inspector, and they have ES reports indicating that they work as fire blocking. The problem is that “fire block” foam is tested to a modified version of ASTM E814, not the more stringent standard used for caulks in fire-stop assemblies. The fact that the foam will ignite may pose a problem for some inspectors. — David Frane


Most carpenters are accustomed to putting sloped fire blocks in the stud bays alongside stair stringers. What not everyone realizes is that this blocking is necessary only if the stud bays are open to a concealed space below the stair. But if the area below the stair is accessible, the bottom of the stringers and the walls below the stair must be covered with 1/2-inch drywall (R311.2.2). In that case, there’s no longer a need to install the sloped blocks in the bays, though many framers do it anyway, out of habit.


Sloped blocking beside the stringers and horizontal blocking at the landing separate the concealed space below the stairs from the stud bays above.

The enclosed space between the stringers must also be separated from the joist bays at floors and landings. The subfloor takes care of the bottom, but the top will probably need to be blocked. This is another place where we often use a single strip of sheathing or lumber, as seen in the bottom photo in Figure 1.


To separate the chimney chase from the attic, the author installs a 3/4-inch OSB fire block at the level of the ceiling. A metal fire stop fills the gap that is required between flue and OSB.


The houses we build contain manufactured fireplaces, so we don’t have to deal with masonry. We follow manufacturers’ installation instructions, but always hold framing back at least 2 inches from flues, per code. When we frame chimney chases, we install a 3/4-inch plywood or OSB lid at each ceiling the flue passes through. The fireplace installer cuts an oversize hole in the lid, runs the flue through it, and then closes the 2-inch gap with a metal fire-stopping ring.

Tim Uhler is a lead framer for Pioneer Builders in Port Orchard, Wash., and a JLC contributing editor.