Understanding the header depth estimate
The suggested depth is a rough planning figure that scales with opening width and load condition — it is not a code-compliant header size on its own.
| Load condition | Load factor | Typical use |
|---|---|---|
| Non-bearing wall | 0.5× | Partition wall carrying only its own weight — smallest header needed |
| Single-storey bearing wall | 1.0× | Wall carrying one floor or roof level above the opening |
| Two-storey bearing wall | 1.5× | Wall carrying two levels of structure above the opening — largest header needed |
- This is a rule-of-thumb estimate for early planning, not a code-compliant header size. Actual required header depth depends on the specific roof/floor loads, header span, lumber species and grade, and must be confirmed against the local building code's prescriptive header table or an engineered calculation.
- Very wide openings, openings carrying concentrated point loads (such as a girder truss or a post from above), or openings in high snow-load or seismic regions typically require an engineered header (LVL, glulam or steel) rather than a solid-sawn lumber header sized by rule of thumb.
What is a door or window header and how is it sized?
A header is the horizontal structural member placed above a door or window opening to carry the loads that would otherwise pass through the wall studs removed to create the opening — it transfers roof, floor and wall loads above the opening out to the jack studs (trimmers) on either side. Header depth requirements grow with the width of the opening and with how much load the wall above is actually carrying.
This calculator applies a widely used carpentry rule of thumb: header depth increases by roughly 25 mm (1 inch) for every 300 mm (1 foot) of opening width on a single-storey bearing wall, with a lighter estimate for non-bearing walls and a heavier one for walls supporting a second storey. This rule gives a reasonable starting point for planning, but it is not a substitute for an actual span table or engineering calculation, which accounts for the specific loads, header material and lumber grade involved.
How to use this door header calculator
- Enter the rough opening width — the width of the door or window opening the header must span.
- Select the wall's load condition: non-bearing (carries only its own weight), single-storey bearing (supports one floor or roof level above), or two-storey bearing (supports two levels above).
- Read the suggested header depth as a rule-of-thumb starting point, the total header length including bearing at each end, and the number of king studs recommended per side for wider openings.
- Verify the suggested depth against your local building code's prescriptive header span table, or have the header sized by a licensed engineer, especially for openings that carry significant roof, floor or point loads.
The rule of thumb behind header depth estimates
The rule of thumb used here scales header depth with opening width: depth (mm) ≈ (opening width ÷ 0.3 m) × 25 mm × load factor, rounded up to the nearest 25 mm and never returned below a 100 mm practical minimum. The load factor is 0.5 for non-bearing walls, 1.0 for single-storey bearing walls, and 1.5 for two-storey bearing walls, reflecting the increasing load the header must carry as more structure bears on it. Header length adds 90 mm (roughly 45 mm of bearing at each end) to the opening width, and openings wider than 1.8 m get two king studs per side instead of one.
Worked example (calculator defaults): a 0.9 m wide opening in a single-storey bearing wall. Depth ≈ (0.9 ÷ 0.3) × 25 mm × 1.0 = 75 mm, rounded up and floored at the 100 mm minimum, giving a suggested depth of 100 mm. Header length = 0.9 + 0.09 = 0.99 m. Since the opening is under 1.8 m, one king stud per side is suggested.
Common mistakes
- Treating this rule-of-thumb depth as a final, code-compliant header size rather than a starting estimate to verify against a proper span table or engineer.
- Selecting the wrong load condition — for example, treating a wall that carries a second-storey load as single-storey, which understates the required header depth.
- Forgetting that wide openings need additional king studs and jack studs (trimmers) to transfer the header's reaction load down to the foundation, not just a bigger header.
- Ignoring concentrated point loads from above (such as a girder truss bearing directly over the opening), which this simple width-based rule of thumb does not capture.
Perguntas frequentes
How thick should a header be for a 3-foot door opening?
Using the common rule-of-thumb of about 25 mm (1 inch) of header depth per 300 mm (1 foot) of opening on a single-storey bearing wall, a 0.9 m (about 3 ft) opening suggests a starting depth around 75–100 mm, but this must be verified against a local building code header span table for the actual species, grade and load involved.
What is a king stud vs. a jack stud (trimmer)?
A king stud is a full-height stud running from the sole plate to the top plate on either side of an opening. A jack stud (trimmer) is a shorter stud nailed to the king stud that actually supports the header's ends, transferring the header's load down to the floor. Wider openings commonly get two king studs per side for added support.
Why does header depth depend on whether a wall is bearing or non-bearing?
A non-bearing wall carries only its own weight, so the header over an opening in it needs to resist very little load. A bearing wall carries roof and/or floor loads from above, so its header must be sized to safely transfer that additional load to the jack studs — the more storeys of structure bearing on the wall, the larger the header typically needs to be.
Is a rule-of-thumb header size safe to build with?
A rule-of-thumb estimate like this one is useful for early planning and rough material lists, but it is not a substitute for the local building code's prescriptive header span table or a licensed engineer's calculation, both of which account for actual loads, lumber grade and header material in a way a simple width-based rule cannot.
Referências
- International Code Council (ICC) — International Residential Code (IRC), prescriptive header span tables for wood-framed openings.
- American Wood Council (AWC) — Wood Frame Construction Manual, header sizing and jack/king stud framing conventions.
- American Wood Council (AWC) — span tables for engineered wood headers (LVL, glulam) by manufacturer.