Below is a Great article on Knee Wall Fires by Howard Rinewalt.
The presence of knee walls holds the dangers of hidden fire and rapid fire progression. Identifying knee walls early in the fire will allow for actions to decrease the chance of hidden fire finding you before you can find it . Let’s discuss knee wall construction.
Typically found in the half story of a home. The knee wall is in place to basically take a room from looking like a triangle to a box, much more livable. Also found in A frame houses. They will be upstairs on the story and a half and down stairs on A frame homes. A lot are used for storage so the presence of heavy contents and hidden fire creates a large risk for rapid fire progression. We should do some things to improve interior conditions if we feel fire is present in the knee walls.
Identifying the presents of knee walls is fairly easy. Your first clue will be dormers. We are looking for real dormers not faux. If dormers are present we should report to the gable end and check for windows. Our confirmation of a half story will be a steep pitch roof with real dormers and gable windows. We have knee walls without a doubt.
Any working fire in a half story that has high heat and zero visibility requires aggressive roof top operations coordinated with aggressive engine operations. This is not the place to drag ass. We want a crew inside opening ceiling on division one and a crew opening walls on division two. We have a good opportunity of slowing the fire from the underside and fully extinguishing it from the same level of the knee wall, this is only completed by opening the voids.
Ventilation in a half story should be placed directly over the knee wall. This action can be achieved by placing your top cut even with the area that the top of the dormer attaches to the roof. This is on dormers that are towards the eave. Dormers that come directly off of the ridge, use the same concept. But, make your top cut even with the bottom of the dormer. This will allow heat in the knee wall void to exit the structure, this will make things safe for the crew opening the voids. After this is complete make a second vent hole by cutting normally or busting the dormer windows. MAKE SURE THIS IS COORDINATED !!!! Your knee wall heat hole should be priority!
Do not take any half story for granted as a bread and butter fire. You can literally not find fire in a story and a half and unknowingly have it burning out of control right next to you in a knee wall. Open the voids, vent low so the heat is released from the voids and be an aggressive firefighter. This is not the time to drag your feet or have no plan. Knee walls are dangerous! But, so is being a firefighter. Do your job, know your job and know how the building will kill you. Be safe