What size header for a window or door opening? (IRC table + worked examples)

Header size depends on two things: how wide the opening is, and how much load is carried above it. A header is a short beam — span it too far with too small a member and it deflects, cracks the drywall, and eventually fails. The table below gives the conservative starting-point sizes for standard single-story residential construction per IRC Table R602.7.

Opening width    Header (load-bearing exterior, single story)
──────────────────────────────────────────────────────────
Under 4 ft       Doubled 2×6
4 to 6 ft        Doubled 2×8
6 to 8 ft        Doubled 2×10
8 to 12 ft       Doubled 2×12
Over 12 ft       LVL engineered lumber — requires span calc

These are starting-point maximums. They assume:
 · Single-story building, or top floor with only attic above
 · Approx. 28-ft building width, ≤30 psf ground snow load
 · Standard residential roof load (40 psf live + 10 psf dead)
 · Number 2 Southern Yellow Pine or Douglas Fir-Larch
 · 2×4 wall framing

Other conditions (point loads above, multi-story, high snow/wind/
seismic zones, non-standard species/grade) require the full IRC
Table R602.7 or a structural engineer.

Reduce one size for interior non-load-bearing partition walls.
Bump up one size (or go LVL) if there is a point load above —
a flush beam landing here, a post, or a bearing wall ending
over this opening.

What “doubled” means

A standard residential header is two pieces of the same-dimension lumber nailed together face to face. Two 2×10s nailed together make a header that is 3″ wide (1½″ + 1½″) and 9¼″ deep.

In a 2×4 wall, the doubled header is exactly 3″ wide — flush with the face of the studs. It fits without a spacer.

In a 2×6 wall, the wall is 5½″ deep but a doubled header is only 3″ wide (two 1½″ plies). That leaves 2½″ unfilled — more than a single ½″ spacer covers. The cleanest solution is a three-ply header (three 2× members nailed together: 3 × 1½″ = 4½″), which is close enough to shim flush and also carries more load. If you prefer two plies, use one ½″ spacer on each face (total 4″) and shim the remaining ½″ flush with the wall surface. A single ½″ spacer between the two plies gives only 3½″ — still 2″ short for a 2×6 wall.

Why the header gets bigger as the span grows

The header is a beam. The load from the roof and any floors above travels down through the wall framing and arrives at the double top plate. At an opening, there are no studs to carry that load straight down — so the header has to span the gap and redirect the load to the king studs and jack studs on each side.

Load path at a window or door opening:

Roof load (live + dead)
  → down the rafters
  → into the double top plate
  → through cripple studs above the header
  → into the header   ← THIS IS THE BEAM SPANNING THE OPENING
  → down the jack studs (trimmers)
  → through the sill plate
  → into the foundation

The header is in bending. The wider the opening, the longer
the span, the more the header tries to sag at midspan.
Doubling up the lumber — and choosing a deeper member —
resists that bending.

King studs and jack studs

The header doesn’t float. It has to be supported at each end by vertical members that carry its load down to the foundation.

• King stud — a full-height stud that runs from the bottom plate to the top plate on each side of the opening. The king stud is nailed to the header end and to the jack stud beside it. It transfers lateral loads and ties the assembly together, but it does not directly bear the header’s vertical load.
• Jack stud (trimmer) — a shorter stud that sits directly under the header end. The jack stud is the actual bearing point for the header. “You will always have at least one jack stud for any opening.” For openings wider than roughly 5 ft, use double jack studs to spread the bearing load over a larger area of the sill plate.

LVL headers for large openings

Once you get past roughly 12 ft, a doubled 2×12 is at the edge of its reliable range for most loads. The go-to solution is an LVL — laminated veneer lumber.

LVL is a manufactured structural member: thin wood veneers glued together with their grain aligned, then pressed into a billet. The result is stronger and stiffer than dimensional lumber of the same depth, with no knots or grain deviation to create weak spots.

LVL header sizing for wall thickness:

2×4 wall (3½" deep):
  Use two 1¾" LVL plies side by side = 3½" wide. Flush.

2×6 wall (5½" deep):
  Use three 1¾" LVL plies side by side = 5¼" wide.
  Shim the remaining ¼" or use a triple-ply with OSB spacer.

For garage door openings (14 ft, 16 ft, 18 ft):
  "For 16 ft or larger — you're going to want to go with an LVL."
  LVL must be sized from the manufacturer's span tables.
  Weyerhaeuser, LP, and iLevel all publish free online span
  calculators — input the span and the load above, read out
  the required depth and ply count.

LVL is available from most lumberyards but usually requires a few days’ lead time. Have your span and load numbers ready when you order. The yard will pull from the manufacturer’s table and confirm the size; for anything unusual, the manufacturer’s engineer will stamp a letter.

Cripple studs above the header

The space between the top of the header and the double top plate is filled with cripple studs — short studs spaced at the same on-center interval as the rest of the wall. Cripple studs are not structural in the sense that they don’t directly carry the heavy floor or roof load above the opening (the header is doing that) — but they do transfer load from the top plate into the header, provide nailing for sheathing and drywall, and maintain the wall’s lateral stiffness. They are required; don’t skip them to save a few sticks.

Non-load-bearing walls

A partition wall that carries only its own weight — no floor or roof load above it — can use a much smaller header. The IRC allows a flat 2×4 cripple header (two 2×4s laid flat, one above the other) for openings up to 8 ft in non-load-bearing walls. Many contractors use a doubled 2×4 on edge as a minimum even in non-load-bearing situations, for consistency and to give the rough opening a solid nailing surface.

Replacing a header in an existing wall

When you’re widening an existing opening — removing a window to put in a door, or removing a wall section — you must support the load above before cutting anything.

• Build a temporary shoring wall parallel to the wall you're opening, close enough to transfer the load. Use doubled 2×4 or 2×6 plates top and bottom, with studs every 12" to 16".
• Shore both sides of the wall if the ceiling joists bear on it from both directions.
• Keep shoring in place until the new header is installed, the jack studs are in, and all nailing is complete.
• Never cut an existing header and hope the framing above holds itself. It won't — it will move, and once it moves it's hard to get back.

Common mistakes

• Undersizing the header. This is the most common error. A doubled 2×6 over a 6 ft opening is undersized for an exterior load-bearing wall — use a doubled 2×10.
• Forgetting the jack studs. The header must bear on jack studs, not king studs. No jack studs means the header is effectively bearing on end grain, which crushes over time.
• Using the wrong species or grade. A Number 3 or stud-grade piece is not a structural header, regardless of size. Use Number 2 or better from a species with the appropriate Fb (bending stress) value.
• Skipping the plywood spacer in 2×6 walls. A header that doesn't fill the wall depth rattles, allows air infiltration at the edges, and doesn't transfer sheathing loads correctly.
• Not shoring the load before cutting. Cutting an existing header without shoring the floor or roof above is how ceilings crack and floors drop.
• Assuming every wall is non-load-bearing. If you're not sure, treat it as load-bearing and size the header accordingly.

Related

• Wall framing stud layout — count, spacing, and plates
• How to estimate framing lumber
• How to calculate rafter length
• BuilderCalc — construction calculators for iOS and Android

Disclaimer: This article is for general educational reference only. All structural decisions must be verified against your local adopted building code and reviewed by the building official or a licensed structural engineer before construction. Local codes, soil, snow, wind, and seismic conditions vary. Always obtain required permits.