On this page
Home / Guides / How it works / Char rate
Char rate
Wood chars from the outside in. AWC’s 2024 Fire Design Specification uses a nominal char rate of 1.5 inches per hour on an ASTM E119 furnace clock. The leftover core is what still carries load. That is why a thick member can outlast a 2x. It is not Oregon R327’s 4-inch heavy timber definition, and it is not a wildfire ember test.
You are holding a 2x4 next to a 6x6 post. The 2x is already thinner than one hour of that nominal char. The 6x still has a core. Neither stick is an E119 assembly until someone runs the calculation for the actual faces, load, and protection.
No g3-passed exhibit matches this object. MECH-003 is Type X chemistry. Do not reuse it as a timber hour. The scene is the two members.
Why it matters
People hear “heavy timber survives fire” and write 4x tails onto the eave list, or they treat a char-rate hour as a WUI covering. R327.2 is a size. FDS is a furnace calculation. Type X is water in gypsum. Those three objects do not substitute. Using the wrong one is how a porch post gets sold as a 1-hour wall.
How the documents split
| Object | What it measures | What it is not |
|---|---|---|
| FDS nominal char rate 1.5 in/hr | Linear char on an E119/UL 263 exposure, for remaining-section design | A wildfire clock |
| Effective char depth | 1-hour example in FDS Table 3.3.1.2. achar 1.5 in. aeff 1.8 in. | Remaining lumber on a 2x4 |
| R327.2 heavy timber | Smallest minimum nominal dimension 4 inches | An E119 rating |
| Ignition-resistant | 30-minute E84/UL 723 | Char rate |
| Type X | Gypsum behind covering | Charred wood |
This page does not reprint AWC’s full tables.
The paths
1. Know the exposure. FDS Chapter 3 is the ASTM E119 or UL 263 time-temperature curve. Embers in the five feet are home ignitability, not that curve.
2. Char is a shrinking section. At the standard 1.5 in/hr nominal rate, one hour of furnace exposure is 1.5 inches of char and 1.8 inches of effective char (the extra accounts for heated wood behind the char). Design uses what is left. A 1.5-inch-thick 2x has no leftover core on a one-hour, one-face version of that clock.
3. Heavy timber on an Oregon WUI wall is still R327.3.3 item 3. Four-inch smallest nominal. Planks close together. Log is item 4. See heavy timber and log walls. Do not file FDS Table 3.3.1.2 as the wall covering.
4. Protection is a different calculation. FDS also assigns time to Type X membranes and insulation. That is protected-member design. It is not a spray coating. Coatings are not an R327.3.3 path. See the coatings finding.
5. After the fire. FDS says these procedures are not intended to evaluate wood members for continued use after a fire. Charred posts get an engineer, not this page.
What people mix up
- Treating 1.5 in/hr as a wildfire survival time.
- Treating R327’s 4-inch definition as a 1-hour rating.
- Treating a 4x eave tail as FDS heavy timber.
- Treating char rate as ignition-resistant E84.
- Pasting FDS tables onto a Bend permit as if they were R327.
How to check
- Open FDS 2024. Find 3.2.1.1. 1.5 in/hr. Find Table 3.3.1.2.
- Open R327. Find R327.2 heavy timber. Compare the objects.
- Measure the smallest nominal dimension on the drawing. If it is 2x, it is not R327 heavy timber.
- If someone quotes TR-10, open TR-10. This session’s TR-10 landing page did not open. FDS is what opened.
- Ask whether the official wants a WUI covering path or an E119 hour. They are not the same stamp.
Limits
This page does not run an NDS fire calculation. It does not rate a CLT wall. AWC’s TR-10 PDF did not open here. FireApproved is not AWC and not the AHJ. Confirm the document that binds the job.
Related records
Questions
Does 1.5 inches per hour mean a 2x4 lasts one hour in a wildfire?
No. AWC FDS 2024’s 1.5 in/hr is a nominal char rate for structural fire-resistance calculations under ASTM E119 or UL 263 furnace exposure. A wildfire is embers, flame, and radiant heat at the exterior. A 1.5-inch-thick 2x member can be consumed on that E119 clock. It is not a WUI survival time.
Is R327 heavy timber the same as a 1-hour rating?
No. R327.2 defines heavy timber as sawn or glued-laminated wood with a smallest minimum nominal dimension of 4 inches. That is a WUI wall, floor, and underfloor-exception path. It is not an E119 hour. See heavy timber and log walls.
Why does remaining wood still work after it chars?
FDS computes remaining section properties after an effective char depth. For a 1-hour rating at the standard 1.5 in/hr nominal rate, char depth is 1.5 inches and effective char depth is 1.8 inches. The leftover core is what the calculation uses. A thin 2x has little or no leftover core on that clock.
Can I use this instead of Type X or E2707?
No. Char-rate design is structural fire resistance of exposed wood members. R327.3.3 still wants noncombustible, ignition-resistant, heavy timber, log, Type X behind covering, a 1-hour wall, or ASTM E2707. Pick the path the code named.