How to Calculate Subfloor
Subfloor is the structural sheathing that spans your floor joists. Standard 4x8 sheets of tongue-and-groove plywood or OSB cover 32 sq ft each. Joist spacing determines minimum sheet thickness, and adhesive plus screws secure the sheets to prevent squeaks.
Sheets Needed Formula
Divide total floor area by the area of one sheet and round up. Apply a small waste factor (typically 5%) for cuts:
Sheets=⌈Area×(1 + Waste)Sheet Area⌉
Screws and Adhesive Formulas
Plan for about 1 screw per sq ft (edges at 6 in, field at 12 in for 16 in OC joists):
Screws≈Area×1.2
Construction adhesive coverage: 1 gallon per ~70 sq ft, or 1 tube (28 oz) per 2-3 sheets:
Adhesive (gal)=Area70
Sheet Size Options
4 x 8
32 sq ft
Standard, easy to carry
4 x 10
40 sq ft
Fewer seams on long walls
4 x 12
48 sq ft
Fewest seams, 2 people
Thickness by Joist Spacing
| Joist Spacing | Min Subfloor Thickness |
|---|---|
| 12 in OC | 5/8 in T&G |
| 16 in OC (standard) | 3/4 in T&G |
| 19.2 in OC | 3/4 in T&G (or 7/8 in for tile) |
| 24 in OC | 7/8 - 1 in T&G |
Fastening Pattern
- Screws, not nails - 2 to 2-1/2 inch subfloor screws (not drywall screws)
- Edges - one screw every 6 in along every sheet edge
- Field - one screw every 8-12 in into each joist in the middle
- Adhesive - 1/4 in bead on top of every joist before setting each sheet
- Expansion gap - 1/8 in at sheet ends; 1/16 in at T&G edges
- Stagger seams - end joints should not align across adjacent rows
Glue + Screw = No Squeaks
Construction adhesive (PL Premium, Loctite PL, or equivalent) on every joist top bonds the sheet to the joist for the life of the home. This single step prevents almost all subfloor squeaks. One 28 oz tube covers roughly 50 linear feet of joist, or 2-3 sheets of subfloor. Apply just before each sheet is set - adhesive skins over in 10-15 minutes.OSB vs Plywood Comparison
| Property | OSB (Sturd-I-Floor) | Plywood |
|---|---|---|
| Cost (3/4 in sheet) | $30 - $45 | $45 - $70 |
| Strength | Uniform, very stiff | High, directional |
| Moisture recovery | Poor - swells at edges | Good - dries faster |
| Fastener hold | Good | Excellent |
| Typical use | New construction (cost) | Remodels, wet rooms |
Tile Substrate Special Rules
Tile requires extra-rigid subfloor because grout and tile edges crack if the floor deflects more than L/360 (or L/720 for natural stone). Best practices for tile over wood joists:
- Double-layer - 3/4 in T&G subfloor + 1/4 to 1/2 in cement backer board
- Backer board - screw every 8 inches; tape joints with fiberglass mesh
- Uncoupling membrane - Schluter Ditra or similar allows small movement without cracking tile
- Joist check - minimum 2x10 at 16 in OC for span over 12 ft
Moisture Damage
Cover subfloor immediately if rain is expected during construction. OSB swells permanently at cut edges when wet, which causes visible bumps under finish flooring. Sand any swollen edges flat before installing tile or LVP. Never install finish floor over subfloor with moisture content above 12%.Load Ratings
Subfloor must support both dead load (building weight) and live load (people, furniture). Standard residential design: 10 psf dead + 40 psf live = 50 psf total. For bathrooms with cast iron tubs or heavy safes, verify joist size with a structural engineer.Cost Estimate (US, 2026)
OSB 4x8x3/4 T&G: $30 - $45 per sheet. Plywood: $45 - $70. Subfloor screws (5 lb box, ~400 screws): $25 - $35. Construction adhesive (28 oz tube): $7 - $12. For a 240 sq ft room at 16 in OC: 8 sheets + 300 screws + 3 tubes = ~$290 in materials.