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The period the element has to survive under the standard fire, as consented.
This is the period on the fire strategy, not the one on the product literature. Where an element carries different periods for different functions — loadbearing, integrity, insulation — the char check belongs to the loadbearing one.
The notional rate tabulated for this product and density in the standard you are designing to.
Rates are published per product family and characteristic density — solid softwood, glued laminated softwood, laminated veneer lumber and the hardwoods all differ, and panel products differ again depending on whether a layer can fall away. Read the value from the table rather than carrying one across from another project.
Depth of heat-damaged timber just inside the char line that the reduced cross-section method discounts.
The char layer itself has no strength, and neither does a thin band of hot timber immediately behind it. The reduced cross-section method handles that by deducting a further allowance beyond the char depth, and its magnitude is defined by the standard you are working to.
The width, depth or panel thickness being checked, before any fire loss.
Run the check one dimension at a time. A beam exposed on three faces loses two char depths across its width but only one off its depth, so the width and the depth are two separate passes with different face counts.
How many opposite faces of this particular dimension the fire can reach.
A floor panel exposed from below chars on one face only. A beam under a slab is exposed on its two sides and its soffit, so its width loses material from both sides while its depth loses it from the soffit alone. A free-standing column is exposed all round.
The smallest residual the engineer's fire check still passes at — from the design, not from this page.
This is what turns an abstract char depth into a decision about sanding. It comes from re-running the member's capacity check on the reduced section and finding where it stops satisfying the fire load combination; without it, the difference between the section you have and the section you need is unknown.
Residual section dimension
5.69 in
The headroom shown is the total that may come off this face across the life of the building, not per visit. Record what is removed at each remedial pass, because two light sandings a year apart are indistinguishable from one heavy one by the time it matters.
- Notional char depth
- 1.65 in
- Effective loss per exposed face
- 1.9 in
- Surface removable before the residual reaches the required minimum
- 0.94 in
- Proportion of the original dimension surviving
- 59.93 %
They open the calculator with your figures already in it
Mass Timber Char Depth & Residual Section Calculator: 5.69 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- EN 1995-1-2 (Eurocode 5, Part 1-2: Structural fire design) reduced cross-section method: char depth is the product's notional charring rate times the fire-resistance period, with a zero-strength layer deducted in addition. Both the rate and the layer are entered here from the standard governing your design — this page publishes neither
- The National Design Specification for Wood Construction, Chapter 16, gives the equivalent char provisions used in US practice; its charring model and its allowance differ from the Eurocode one, so take both figures from a single document rather than mixing the two
- Sanding removes material from the same surface the char depth is measured inward from, so any thickness taken off comes straight out of the residual section — the arithmetic on this page is that subtraction and nothing more
Inputs used
- Required Fire-Resistance Period (minutes)
- 60
- Notional Charring Rate (mm/min)
- 0.7
- Zero-Strength Layer Allowance
- 0.25 in
- Original Section Dimension
- 9.5 in
- Faces Exposed Across This Dimension
- Two faces — the width of a beam, or a four-sided column
- Minimum Residual Dimension Required
- 4.75 in
Intermediate steps
- Notional char depth
- 1.65 in
- Effective loss per exposed face
- 1.9 in
- Surface removable before the residual reaches the required minimum
- 0.94 in
- Proportion of the original dimension surviving
- 59.93 %
Confidence note: The headroom shown is the total that may come off this face across the life of the building, not per visit. Record what is removed at each remedial pass, because two light sandings a year apart are indistinguishable from one heavy one by the time it matters.
What this calculation does not cover
- One dimension at a time. A three-sided beam needs two runs — width at two faces, depth at one — and the residual capacity check uses both results together.
- Notional charring assumes the member keeps its shape and no layer falls away; products where a lamella can debond char faster and are covered by different provisions in the standards.
Add the equipment this sizes
This result is a specification — 5.69 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-08-30 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations3
- EN 1995-1-2 (Eurocode 5, Part 1-2: Structural fire design) reduced cross-section method: char depth is the product's notional charring rate times the fire-resistance period, with a zero-strength layer deducted in addition. Both the rate and the layer are entered here from the standard governing your design — this page publishes neither
- The National Design Specification for Wood Construction, Chapter 16, gives the equivalent char provisions used in US practice; its charring model and its allowance differ from the Eurocode one, so take both figures from a single document rather than mixing the two
- Sanding removes material from the same surface the char depth is measured inward from, so any thickness taken off comes straight out of the residual section — the arithmetic on this page is that subtraction and nothing more
Which documents these citations point at
- Eurocode 5 — Design of timber structures (BS EN 1995) (European (EN))Timber design, including the service classes and the modification factors applied for moisture and load duration.
- The Eurocodes (BS EN 1990–1999) (European (EN))The European structural design suite as a whole. A citation that names no part lands here; the numbered parts above are the documents themselves.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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