Build Knee Walls
Creating usable space from sloped ceilings. Knee walls transform the awkward triangular areas where rafters meet the floor into clean, finished walls with hidden storage beyond.
Why Knee Walls Matter
Attic rooms have a unique challenge: sloped ceilings that eventually meet the floor at impossibly low heights. Without knee walls, you'd have unusable floor space where the ceiling is too low to stand or even sit. Knee walls solve this by creating a vertical surface at a practical height - typically 4 to 5 feet - that defines the room's usable boundary. Behind these walls lies valuable storage space in the eave triangles. Well-built knee walls also provide critical structure for attic rooms: they anchor the sloped ceiling framing, support drywall installation, provide backing for furniture placement, and create the thermal boundary between conditioned living space and unconditioned attic. The height you choose affects both the room's character and how much floor area qualifies as "living space" under building codes.
Tools & Materials You'll Need
Step-by-Step Guide
Determine Knee Wall Height
Choose the height that balances usable space with room character:
4-Foot Knee Walls:
- + More dramatic sloped ceiling
- + Larger triangular storage area
- + Room feels more spacious
- - Less wall for furniture
- - Beds/desks need careful placement
5-Foot Knee Walls:
- + More usable wall space
- + Easier furniture placement
- + More floor area at full height
- - Less dramatic ceiling
- - Smaller storage triangles
Code Note: Building codes require at least 50% of finished floor area to have 7-foot ceiling height. Calculate how knee wall height affects your compliant floor area before deciding.
Lay Out Bottom Plate
Mark and install the foundation of your knee walls:
- Measure from exterior wall to where you want knee wall face
- Mark this distance at both ends of the room
- Snap chalk line between marks - this is your bottom plate location
- Use pressure-treated lumber for bottom plate (moisture protection)
- Position plate with outside edge on chalk line
- Fasten to floor joists with 3-inch screws every 16 inches
- Mark stud locations on plate (16 inches on center)
Planning Tip: Consider electrical and HVAC needs. If outlets will be in knee walls, space allows. If ducts run along the eave, position knee wall to clear them.
Cut Angled Top Plate
The top plate must match your roof slope for a tight fit:
- Use bevel gauge or speed square to measure rafter angle
- Common roof pitches: 4/12 (18.4°), 6/12 (26.6°), 8/12 (33.7°)
- Set circular saw blade to matching angle
- Rip full length of 2x4 at this angle
- Beveled edge will sit flat against rafter undersides
- Test fit a short piece before cutting all plates
Pro Tip: Cut the top plate bevel so the wide face is toward the room - this gives you more nailing surface for drywall and studs. The narrow edge faces the eave.
Install Studs
Cut and install vertical studs between plates:
Cutting Studs:
- Measure from bottom plate top to rafter underside at each stud location
- Heights will vary if rafters aren't perfectly straight
- Cut bottom square, top angled to match roof pitch
- Angle cut allows stud to sit flat against rafter
- Cut each stud individually - don't assume uniform height
Installing Studs:
- Position stud on bottom plate mark
- Toe-nail through stud into bottom plate (2 nails each side)
- Check plumb with level on stud face
- Align top with rafter and toe-nail to top plate
- Nail or screw through top plate into rafter
Alternative Method: Build knee wall sections flat on the floor (like standard walls), then raise into place. Works best when rafter spacing matches your 16" stud spacing.
Add Blocking and Access
Complete the knee wall with blocking and storage access:
Horizontal Blocking:
- Install at mid-height for drywall support
- Add blocking at 48" for chair rail
- Block around access door openings
- Use 2x4 blocks between studs
- Face-nail or use joist hangers
Access Doors:
- Plan at least one per room
- Size 24"x24" minimum
- Frame with double studs on sides
- Add header and sill blocking
- Use insulated access panels
Storage Tip: Install plywood floor in the eave storage area before finishing walls. Add a light and outlet for convenience. Consider built-in drawers accessed from the room side for frequently used items.
Pro Tips
- ✓Run electrical wiring through knee walls before insulating - it's much easier to add outlets now than after drywall
- ✓Install blocking for future TV mounts, shelving, or grab bars at appropriate heights
- ✓Consider making knee walls slightly taller in children's rooms - kids use the floor space where adults wouldn't fit
- ✓Photo-document all framing before covering - you'll need it for hanging heavy items or future modifications
- ✓Build closets into knee wall spaces - the sloped area above is perfect for storing hanging clothes
Frequently Asked Questions
What is the best height for attic knee walls?▼
Most attic knee walls are built 4-5 feet tall. This height provides usable wall space for furniture placement while maximizing floor area. Taller knee walls (5 feet) create more usable floor space but reduce the sloped ceiling area that gives attic rooms character. Shorter walls (4 feet) maximize the room's volume but may limit furniture placement. Building codes typically require at least 50% of finished floor area to have 7-foot ceiling height, so knee wall height affects how much floor area counts as "livable."
Do knee walls need to be insulated?▼
Yes, knee walls must be insulated on the attic side (the cold side in winter). Install R-19 to R-30 batt insulation between studs with vapor barrier facing the conditioned room. The triangular space behind knee walls should also be air-sealed and may need floor insulation to prevent heat loss through that unheated space. Don't insulate the room side of knee walls - that would trap moisture.
Should I include access doors in knee walls?▼
Yes, access doors are highly recommended. The triangular space behind knee walls provides valuable storage for seasonal items, luggage, and holiday decorations. Install at least one access door per room, sized large enough to move boxes through (typically 24x24 inches minimum). Use insulated access doors to maintain energy efficiency. Also consider that you may need access to HVAC equipment, plumbing, or electrical components in the eave space.
Can knee walls be load bearing?▼
Standard knee walls in attic conversions are not designed to be load-bearing - they simply enclose the space where the sloped ceiling meets the floor. However, if your structural engineer specifies knee wall posts or if you're using knee walls to support a modified roof structure, proper sizing and connections become critical. In these cases, posts within the knee wall may need to transfer loads to floor joists below, requiring careful engineering.