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How to Size a Header (IRC Table R602.7)
The Problem
You’re framing a wall and you need a header over a window or door. How big does it need to be? A 2x6? 2x8? 2x10? LVL?
The answer depends on four things: the span of the opening, the building width, the ground snow load, and whether the header is on an exterior bearing wall or an interior one.
IRC Table R602.7(1) gives you allowable spans for headers made from dimensional lumber. Miss a variable and you might frame a header that sags under load, or overbuild one that wastes material and eats into head height.
What the Header Actually Does
A header transfers the weight from above the opening (roof load, floor load, wall above) around the opening and down through the jack studs into the foundation. It’s a beam. The wider the opening, the more load the header carries, and the beefier it needs to be.
On a single-story house with no habitable attic, the header carries roof dead load, live load (or snow load), and a small amount of wall weight. On a two-story house, a first-floor header might carry a floor, a wall, and a roof. That’s a completely different load condition.
Reading IRC Table R602.7(1)
The table is organized by:
Building width (the direction perpendicular to the ridge). Options are typically 20’, 24’, 28’, 32’, and 36’. Wider buildings put more roof load on each wall because each wall supports half the span.
Ground snow load. The table covers 30 PSF, 50 PSF, and 70 PSF. Look up your local snow load at the building department or on the ASCE 7 ground snow load maps. Dallas is 5 PSF. Denver is 30 PSF. Buffalo is 40 PSF. Parts of the Sierras and northern Rockies hit 100+ PSF (and you’d need an engineer, not a table).
Number of supported floors. Headers supporting a roof only (one story, or second floor of a two-story) have one column. Headers supporting one floor plus a roof have another. Headers supporting two floors plus a roof need the heaviest sizes.
Header material. The table assumes #2 grade SPF (spruce-pine-fir) or equivalent. Douglas fir headers can span farther at the same size.
Some Common Lookups
For a 28-foot-wide building at 30 PSF snow load, roof and ceiling only (single story or top floor):
| Header Size | Max Span |
|---|---|
| 2-2x6 | 4’-0” |
| 2-2x8 | 5’-5” |
| 2-2x10 | 6’-11” |
| 2-2x12 | 8’-1” |
For the same building supporting one floor plus roof:
| Header Size | Max Span |
|---|---|
| 2-2x6 | 3’-1” |
| 2-2x8 | 4’-1” |
| 2-2x10 | 5’-3” |
| 2-2x12 | 6’-2” |
Notice the drop. A 2-2x10 that spans 6’-11” carrying just roof suddenly only spans 5’-3” when it’s also carrying a floor. That’s the difference between a 6-foot window working and not working.
Why Your 2x6 Might Not Be Enough
A lot of guys default to a 2-2x6 header for anything under 4 feet wide. It works in many cases, but not all.
Take a 3’-6” window in a 32-foot-wide building at 50 PSF snow load, carrying one floor plus roof. The allowable span for a 2-2x6 in that condition is about 2’-6”. Your 3’-6” window blows right past it. You need a 2-2x8 minimum.
The building width matters more than people think. Going from a 24-foot building to a 36-foot building shortens your allowable header spans by 20-30% because each wall carries load from a wider tributary area.
Snow load is the other gotcha. Framing in Texas at 10 PSF is a different world than framing in Minnesota at 60 PSF. A header that’s fine in Houston might fail in Minneapolis.
Double vs Triple Headers
The table assumes doubled headers (two pieces nailed together with 1/2” plywood spacer to make a 3.5” wide assembly that fits a 2x4 wall). For 2x6 walls, you can triple the header or use a wider material.
Tripling a 2x10 instead of doubling a 2x12 sometimes makes sense. A 3-2x10 is stronger than a 2-2x12 for many span/load combinations, and 2x10s are easier to handle.
When to Use LVLs
Laminated veneer lumber (LVL) beams are stronger per inch than dimensional lumber. A 3.5” x 9.25” LVL can replace a 2-2x12 and span farther.
Use LVLs when:
- The opening exceeds what the IRC table allows for dimensional lumber
- You’ve got a garage door header (16’ span is common and way beyond any dimensional header)
- Point loads from posts or beams above concentrate load on the header
- You want to minimize header depth to maximize head height
LVL sizing requires manufacturer span tables or an engineer. The IRC dimensional lumber tables don’t cover LVLs.
A 16-foot garage door header typically needs a 5.25” x 11.875” LVL or a 3.5” x 14” LVL depending on the load. That’s not something you solve with the IRC table.
Interior Non-Bearing Headers
Here’s where you can save material. Interior non-bearing walls (walls that don’t support any roof, floor, or wall load above) don’t need structural headers at all. IRC R602.7.2 allows a flat 2x4 as the header for non-load-bearing walls, regardless of opening width.
If the wall only holds up drywall, your “header” is just a flat piece of framing lumber to support the drywall above the opening. Framers who put 2-2x10 headers in every interior partition wall are burning material for no structural reason.
The Inspection Problem
Inspectors look at headers. They’ll check the size against the plans, and the plans should match the IRC table (or an engineer’s calc). If you framed a 2-2x8 where the table calls for a 2-2x10, you’re pulling it out and reframing. That’s an expensive afternoon.
Get the header right before the wall goes up. Checking the table takes 30 seconds. Reframing takes 2 hours and a lot of swearing.
Running Header Calcs on Site
SiteCalc has a header sizing calculator that pulls from the IRC tables. Enter the opening width, building width, snow load, and number of supported floors. It gives you the minimum header size with the code reference right next to the answer. Takes about 5 seconds, which beats flipping through the IRC book while standing in a framed wall.