Rafter

A rafter is a sloped structural beam that runs from the roof’s ridge (peak) down to the wall plate, supporting the roof deck and transferring loads from shingles, snow, and wind to the walls; rafters are installed in pairs or sets to shape the roof and create its slope (pitch).

Rafter

What is a rafter?

A rafter is a sloped beam that forms the skeleton of many roofs. It typically runs from the ridge (the top of the roof) down to the wall plate (the top of a wall), where it bears on the structure below. Rafters support sheathing or decking, and ultimately carry the weight of roofing materials, wind, and snow down to the walls and foundation. In traditional “stick-framed” roofs, rafters are cut and installed on site; they may also be factory-made from engineered lumber.

Where you’ll see rafters in DIY projects

  • Building a shed, tiny house, or porch roof
  • Converting an attic where existing rafters and ties define usable space
  • Repairing a roof after a leak or replacing a damaged member
  • Adding skylights or roof vents (requires careful reinforcement of rafters around openings)
  • Extending eaves or adding a small awning

If you’re framing a small structure, you’ll likely layout, cut, and install rafters yourself. For larger homes or heavy snow/wind areas, a permit and engineering may be required—always check local codes and load requirements.

Types of rafters

Rafters appear in several roles depending on roof shape:
- Common rafter: The standard rafter in a gable roof that runs from wall plate to ridge.
- Jack rafter: Shorter rafters that meet a hip or valley rafter instead of the ridge.
- Hip jack: Runs from wall plate to a hip rafter.
- Valley jack: Runs from a valley rafter up to the ridge.
- Cripple jack: Connects between a hip and a valley rafter.
- Hip rafter: Diagonal rafter forming the outside corner where two roof planes meet.
- Valley rafter: Diagonal rafter forming the inside corner where two roof planes meet.
- Lookout rafter (or lookout): Short framing used to support roof overhangs at gable ends.

Related members often paired with rafters:
- Ridge board/beam: The top member where rafter tops meet. A ridge board is non-structural alignment stock; a ridge beam is structural and supports loads.
- Rafter tie: A horizontal member near the bottom of opposing rafters that resists the outward push on walls.
- Collar tie: A horizontal member higher up that helps keep rafters from separating under uplift.
- Purlin: A horizontal support under rafters between wall and ridge to reduce span.

Materials and sizes

  • Lumber species: Common choices include SPF (spruce-pine-fir), Douglas fir–larch, southern yellow pine. Choose graded structural lumber (e.g., No. 2 or better).
  • Sizes: 2×4, 2×6, 2×8, 2×10, 2×12, selected based on span, spacing, pitch, and loads. Heavier loads or longer spans need deeper rafters.
  • Engineered wood: LVL or LSL rafters can carry higher loads and stay straighter.
  • Steel: Occasionally used in special designs or retrofits.

Spacing is typically 16 inches or 24 inches on center (distance from the center of one rafter to the next). Closer spacing increases support but uses more material.

How rafters are laid out and cut

Rafter cuts are described by two basic angles:
- Plumb cut: The vertical cut at the top (and sometimes bottom) of the rafter where it meets the ridge or wall.
- Seat cut and heel/birdsmouth: A notch near the bottom that allows the rafter to sit flat on the wall plate, with a vertical “heel” and horizontal “seat.”

Rafters are sized using the roof pitch, expressed as rise over run (e.g., 4:12 means 4 inches of rise for every 12 inches of horizontal run). You can:
- Use a framing square with pitch marks
- Use a construction calculator or app
- Refer to span tables approved by your building code

Fastening typically includes nails or structural screws at the ridge and wall plate, plus hurricane ties or metal connectors where required.

Tips for selection, installation, and care

  • Match span to size: Use code span tables for your area, considering snow/wind loads.
  • Pick straight stock: Sight down the edge and orient the natural “crown” (slight curve) upward. Keep crowns all the same direction.
  • Mind the birdsmouth: Do not cut deeper than allowed (often no more than one-third the depth of the rafter). Over-notching weakens the member.
  • Use proper connectors: Install required metal hangers, ridge straps, and hurricane ties. Follow manufacturer nail/screw schedules.
  • Ventilation and moisture: Provide soffit and ridge venting as designed. Moisture trapped around rafters leads to rot and mold.
  • Protect exposed rafters: If rafters are exposed (porch or pergola), seal or paint and inspect yearly. Consider treated lumber where they may get wet.
  • Permits and inspections: Roof framing often requires permits. Inspections protect you and help resale.

Common mistakes to avoid

  • Guessing sizes: Undersized rafters can sag or fail. Always consult span tables or an engineer.
  • Mixing up pitch and angle: Pitch is rise over run, not degrees. Use the right reference for layout.
  • Over-deep birdsmouths: Removing too much wood at the wall seat compromises strength.
  • Skipping rafter ties: Without ties, walls can spread under load. Install as designed.
  • Irregular spacing: Inconsistent on-center spacing complicates sheathing and weakens the roof.
  • Poor ridge detail: A ridge board is not a beam. If the design calls for a ridge beam, it must be sized and supported like a beam.
  • Ignoring connectors and uplift: High winds can lift roofs. Use required straps and nails.

Related terms and how they differ

  • Rafter vs. truss: A rafter is an individual sloped member cut on site. A truss is a pre-engineered triangular assembly that replaces rafters and ceiling joists; trusses are not designed to be cut in the field.
  • Rafter vs. joist: Joists are horizontal members supporting floors or ceilings. Rafter ties may act like ceiling joists at the exterior walls.
  • Ridge board vs. ridge beam: A ridge board aligns rafters but does not carry vertical load. A ridge beam carries load and must be sized and supported.

Practical examples

1) Framing a small gable roof for a 10-foot-wide shed, 4:12 pitch:
- The run is half the span: 5 feet.
- Rise at 4:12 is 4 inches per foot of run. Over 5 feet, rise = 20 inches (1 ft 8 in).
- Rafter length (excluding overhang) is the slope distance: roughly √(5² + 1.67²) ≈ 5.27 ft (about 5 ft 3 in). A calculator or framing app can do this instantly and give plumb-cut angles.
- Layout the plumb cut at the top, mark the seat and heel for the birdsmouth to fit the wall plate, and add an overhang as desired.
- Install hurricane ties at the plate, and nail opposing rafters together over a ridge board (or to a ridge beam if designed that way). Add rafter ties near the bottom where required.

2) Replacing a damaged rafter from a leak:
- Shore the roof with temporary supports under the sheathing on each side of the damaged area.
- Remove the bad section and sister a new, full-length rafter alongside the old one, extending well past the damaged area (often 4 feet or to the next bearing point) with structural screws or nails in a staggered pattern.
- Restore connectors, replace any compromised sheathing, and fix the leak source (flashing, shingles, or underlayment).

Quick checklist for DIYers

  • Determine pitch, span, and loads; verify with code tables
  • Choose straight, properly graded lumber
  • Keep spacing consistent (16 or 24 inches on center)
  • Cut accurate plumb and seat cuts; avoid over-notching
  • Install ties, straps, and ridge details as designed
  • Provide ventilation and moisture control
  • Inspect fasteners and exposed members annually

Understanding rafters helps you plan safe, long-lasting roofs for sheds, porches, and more. With good layout, correct sizing, and proper connectors, your framing will support the roof covering and protect the structure beneath for years.

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