Joist

What is a Joist?

A joist is a horizontal framing member that supports surfaces you walk on or look up at. Think of joists as the ribs of a floor, ceiling, or deck: they run in parallel, carry weight, and transfer that load to beams, ledgers, or bearing walls. Subflooring, floor finishes, drywall, and decking all fasten to joists. Their size, material, spacing, and span are chosen to handle expected loads and meet building code requirements for strength and stiffness.

Where You’ll See Joists at Home

• Floor joists: Hidden under your subfloor, these carry furniture, people, and partition loads. They run between beams or walls and are capped at the ends by a rim joist (also called a band joist).
• Ceiling joists: These support drywall or plaster ceilings and can help tie exterior walls together. In some houses, they also serve as the framing for an attic floor.
• Deck joists: Exterior-grade joists support decking boards. One end often hangs from a ledger attached to the house; the other end bears on a beam or posts.

Types of Joists

• Dimensional lumber: Common sizes include 2x8, 2x10, and 2x12, typically made from SPF (spruce-pine-fir), southern yellow pine, or Douglas fir. Widely available and familiar to DIYers, but spans are limited by species, grade, and size.
• Engineered I-joists (TJIs): Look like an “I,” with OSB or plywood webs and LVL or solid wood flanges. They span longer distances, stay straighter, and are lighter than comparable lumber. Follow manufacturer rules for drilling and notches.
• LVL or LSL joists/headers: Laminated and strand products are very strong and stable. They’re more often used as beams, but sometimes as joists in heavy-duty applications.
• Open-web floor trusses: Wood members connected with metal plates create a deep, strong truss that allows easy routing of ducts and plumbing. Requires precise engineering and ordering to fit the plan.
• Steel (light-gauge C-joists): Used in commercial or specialized residential work. Straight, dimensionally stable, and fire-resistant, but require specific fasteners and corrosion protection.

Sizing, Span, and Spacing Basics

• Spacing: Typical spacing is 16 inches on center (16" O.C.). Heavier finishes (like stone tile) or long spans may use 12" O.C. Lighter loads or some ceiling applications may allow 24" O.C. Always verify with code and span tables.
• Span: The maximum clear distance between supports depends on material, size, species, grade, spacing, and load (live load like people/furniture, dead load like materials). Use manufacturer span tables for I-joists and code-approved tables for lumber.
• Deflection: Floors shouldn’t feel bouncy. A common stiffness target is L/360 for typical living areas and L/480 for tile to reduce cracking. Stiffer floors may need deeper joists, closer spacing, or engineered options.
• Bearing: Joists must have adequate bearing at supports (often 1.5 inches minimum for wood on wood, more for masonry). Where joists meet a beam or ledger, use the right joist hangers and fasteners.

Tips for Selection, Installation, and Care

• Choose the right material:
  • Interior floors: dimensional lumber or engineered I-joists. Use span tables to size correctly.
  • Exterior decks: pressure-treated lumber rated for ground contact or exterior use; use corrosion-resistant fasteners and hangers.
  • Long spans or complex layouts: consider I-joists or floor trusses for stiffness and straightness.
• Plan services early: Decide where HVAC, plumbing, and electrical will run. If using I-joists or trusses, follow the manufacturer’s hole charts. For solid lumber, keep holes centered vertically and small, and avoid notches near mid-span.
• Use proper connectors: Match joist hangers to the joist size, and use the specified nails or structural screws. For treated lumber, choose hangers and fasteners rated for the treatment chemicals (e.g., G185 galvanized or stainless steel).
• Install blocking or bridging: Mid-span blocking or bridging helps distribute load and reduces twisting and squeaks. Some codes require it; many floors feel better with it.
• Glue and screw subfloors: Apply construction adhesive on joists and use screws or ring-shank nails to reduce squeaks. Stagger seams and leave expansion gaps per the sheathing manufacturer.
• Protect from moisture: Keep joists dry during construction. For decks, use flashing at the ledger, consider joist-top tape, and maintain finish on decking to limit water intrusion.
• Inspect periodically: Look for sagging, cracks, corrosion on hangers, rusted fasteners, insect damage, and water stains. Address leaks quickly to prevent rot.
• Reinforcement options: To stiffen bouncy floors, you can sister additional joists, add a mid-span beam or wall, install blocking, or upgrade subfloor thickness.

Common Mistakes to Avoid

• Over-notching or over-drilling: Cutting large holes, notching the bottom edge at mid-span, or notching too close to supports can severely weaken a joist. Follow code and manufacturer limits.
• Wrong fasteners in hangers: Drywall screws are not structural. Use manufacturer-specified nails or structural screws for hangers and connectors.
• Over-spanning: Choosing a joist size based only on what’s on hand can cause sagging. Always consult span tables for your spacing, species, grade, and loads.
• Skipping blocking/bridging: Without it, joists can roll or twist, and floors may feel springy.
• Ignoring corrosion: Using standard hardware with pressure-treated lumber can lead to premature failure. Use properly rated connectors and fasteners.
• Poor ledger attachment on decks: Ledgers must be flashed and bolted to structure, not just sheathing. Wrong spacing or missing fasteners can lead to failure.
• Covering up problems: Don’t hide cracked, rotten, or undersized joists behind drywall or decking. Fix the issue before closing up.

Related Terms

• Beam/Girder: The larger support that joists rest on or connect to.
• Rim joist (band joist): The perimeter member that caps the ends of floor joists.
• Ledger: A board anchored to a wall to support deck or floor joists.
• Joist hanger: A metal bracket that supports the end of a joist.
• Blocking/Bridging: Pieces installed between joists to stabilize and share loads.
• Subfloor: Panels fastened to floor joists that form the base for finished flooring.
• Rafter: A sloped roof framing member; ceiling joists are horizontal.

Practical Examples

• Planning a deck: You’re building a 12-foot-deep deck. Using 2x10 pressure-treated joists at 16" O.C., you hang one end on a properly flashed ledger and bear the other end on a beam. You add solid blocking over the beam, install joist hangers with rated nails, and tape joist tops before laying deck boards.
• Fixing a bouncy floor: Your living room floor feels springy. After checking span tables, you sister new 2x10s to existing joists with construction adhesive and bolts, add a mid-span row of blocking, and reattach the subfloor with screws and adhesive. The floor feels much stiffer.
• Running plumbing: You need a 2-inch drain line across a joist bay. Instead of notching the bottom of a joist, you route the pipe through pre-approved knockouts in an I-joist (per the manufacturer’s chart) or run it under the joists with a soffit to preserve strength.
• Quieting squeaks: From the basement, you add shims where a joist lost bearing on a beam, apply adhesive, and drive screws up through the subfloor into the joist. In upstairs areas, you locate joists with a stud finder and use trim-head screws through the floor to secure loose spots.

Before altering joists or adding loads, check local codes, consult span tables or manufacturer literature, and consider advice from a qualified contractor or engineer for structural changes.