Install Plywood Subfloor
Create a solid, squeak-free foundation for your finished flooring by installing tongue-and-groove plywood over attic joists.
Why a Proper Subfloor Matters
The subfloor is the foundation of your finished floor and directly impacts how your attic feels underfoot. A properly installed subfloor eliminates squeaks, provides a level surface for finish flooring, adds structural rigidity, and helps with sound transmission to rooms below. In attic conversions, subfloor installation also ensures adequate support for furniture and foot traffic that typical attic framing may not have been designed to handle.
Tools & Materials Needed
Step-by-Step Installation Guide
Check Joist Spacing and Condition
Before installing subfloor, verify your joists can support it:
- •Measure joist spacing - should be 16" on center for 3/4" plywood
- •Check for damaged, cracked, or sagging joists that need repair
- •Use a long straightedge to check if joists are level
- •Verify joists are rated for floor loads (40 psf), not just ceiling loads
- •Consult a structural engineer if joists seem undersized
Important: Many attic joists are only designed for ceiling loads. Adding a living space requires joists capable of supporting 40+ pounds per square foot.
Install Blocking Between Joists
Add blocking to support plywood edges and strengthen the floor:
- •Cut blocking from same size lumber as joists (typically 2x8 or 2x10)
- •Install blocking at 4-foot intervals where plywood seams will fall
- •Nail or screw blocking securely between joists
- •Keep blocking flush with joist tops - use a straightedge to verify
- •Add blocking around stairway openings and at knee wall transitions
Apply Construction Adhesive
Adhesive is key to preventing squeaks:
- •Use subfloor adhesive rated for your conditions (check temperature range)
- •Apply 1/4" bead along each joist where plywood will rest
- •Apply adhesive on blocking and around openings
- •Work in sections - only apply adhesive you can cover in 10-15 minutes
- •Apply to tongue-and-groove joints for even stronger bond
Lay Plywood Sheets
Install plywood with proper orientation and staggered seams:
- •Start with groove edge against wall, tongue facing out
- •Run sheets perpendicular to joists for maximum strength
- •Stagger end joints by at least 4 feet between rows
- •Leave 1/8" gap at walls for expansion
- •Use a block and hammer to tap tongue-and-groove joints tight
Pro tip: Cut plywood in a staging area below to reduce dust in the finished attic space and make handling easier.
Fasten with Screws
Secure plywood with screws for a squeak-free floor:
- •Use 2" coarse-thread deck screws (not drywall screws)
- •Space screws 6" apart along joists and sheet edges
- •Space screws 8" apart in the field (interior of sheets)
- •Keep screws 3/8" from sheet edges to prevent splitting
- •Set screw heads slightly below surface without breaking through
Pro Tips
- ✓Acclimate plywood in the attic for 48 hours before installation to match humidity levels
- ✓Snap chalk lines on plywood to mark joist locations - makes fastening faster and more accurate
- ✓Use a impact driver for faster screw installation, but switch to drill mode near edges to avoid splitting
- ✓Mark electrical boxes and other obstructions before covering with subfloor
Frequently Asked Questions
What thickness plywood should I use for attic subfloor?▼
Use 3/4-inch (23/32 actual) tongue-and-groove plywood for attic subfloors with joists spaced 16 inches on center. If joists are 24 inches on center, you may need 7/8-inch plywood or to add blocking. OSB is an acceptable alternative but plywood is preferred for its dimensional stability.
Can existing attic joists support a living space floor?▼
Attic joists designed only for ceiling loads (10-20 psf) may need reinforcement for living space loads (40+ psf). Consult a structural engineer or building inspector to verify your joists can handle the additional weight. Common solutions include sistering new joists alongside existing ones or adding support beams.
Should I use screws or nails for attic subfloor?▼
Screws are strongly recommended over nails for attic subfloors. Use 2-inch coarse-thread screws every 6 inches along joists and 8 inches in the field. Screws prevent squeaks, hold better as wood expands and contracts with temperature changes, and are easier to remove if you need access to wiring or plumbing below.
How do I handle uneven attic joists?▼
Check joists for level using a long straightedge or laser level. For minor variations (less than 1/4 inch), use leveling compound or shims. For significant unevenness, sister new lumber alongside existing joists to create a level plane. Never sand or plane joists as this weakens their structural capacity.