Flame Spread vs. Burn Through vs. Fireproof

While LP® FlameBlock® Fire-Rated Sheathing offers both flame-spread and burn-through resistance, it’s important to remember these are different concepts as they relate to construction and the code. Flame spread is the propagation of flame across the surface of a material and can be minimized with “fire retardants” that delay ignition. Burn through is the penetration of flame through an assembly and is countered by the “fire resistance” of the assembly. And then there is the notion of “fireproof” materials. As a refresher, let’s ask a few experts the definition of each term.

What is the difference between fire resistance and being fireproof?

“‘Fireproof’ is an old term that has fallen out of favor in the fire standards. However, as a simple analogy think about ‘waterproof’ vs. ‘water resistance,’” explains Lance Olson, director of technical field sales for LP Building Solutions. “Waterproof means impervious to water transfer as with a membrane from one side to the another. It’s going to stop water. Having water resistance, or being water resistive, means a material is only going to resist the water for a period of time but is not going to stop the water for long.”

Therefore, fire resistance is the ability of a fire-resistive material or assembly to provide protection from fire—for a time. Given enough time, fire will ultimately burn through or penetrate an assembly. 

“I don’t like the word fireproof. It’s undefined in the code. What are we proofing? For most people it’s a proxy for ‘non-combustible,’” says Jeff Yelle, director of OSB/EWP Technology. “There are code-recognized standards for determining if a material is ‘combustible’ or not.”

What is the difference between fire resistance and fire retardant?

“A fire retardant imparts some level of protection against combustion to some materials. For instance, add product A to product B and now product B is more resistant to flame spread,” explains Yelle.

Therefore, a fire retardant is something that is applied to a building material—like a coating—to make it more resistant to flame spread during a fire event.

“Depending on the type of building, wood must be treated or coated with a fire retardant to help protect the structure from a fire event. Fire retardants can be chemicals, coatings or other treatments that help prevent the spread of fire by inhibiting ignition,” says Olson.

What is the burn-through resistance of building materials?

Burn through is the penetration of fire through a wall or floor/ceiling assembly. The resistance to burn through is determined by test and is represented by a fire-resistance rating for the assembly, expressed in minutes or hours. In the test, assemblies are exposed to standardized conditions and given a rating so that they can be compared and specified by an architect and so a building official can verify an assembly meets the required rating. The required fire-resistance rating for an assembly is specified in the code based on building type and occupancy. LP FlameBlock sheathing is approved in both one- and two-hour fire-resistance–rated wall assemblies. So what does this mean?

“For example in townhouses, a fire-resistance–rated wall assembly is intended to contain a fire from burning into adjacent units,” explains Olson. “It protects one living space from another. But remember that fire-resistance ratings are only that—a rating. They do not represent a ‘safe’ time in a real fire,” says Olson.

How does LP FlameBlock sheathing resist flame spread and burn through?

LP FlameBlock sheathing is manufactured to resist flame spread and burn through using a non-combustible fiberglass-reinforced Pyrotite® treatment that covers one or both sides of the panel, depending on the assembly.

“In heat, the coating releases water,” explains Yelle. “It sweats to stay cool, releasing water to slow down heat transfer to keep the product cooler for a period of time.” 

Related: How Fire-Rated OSB Outperforms FRT Plywood. 

Why is it so important to be clear when defining terms for fireproof, fire retardant and fire resistance in construction?

“Almost any material, when given the proper heat, will burn,” cautions Josef Chen, product manager of siding innovation for LP Building Solutions. “The problem is when a homeowner hears something about fireproof or fire-retardant building materials, they think it will prevent their house from burning down. It creates a false sense of security.”

To learn more about meeting fire code requirements on a tight budget, refer to this article.