Every year, wildfire season is getting worse. In response, building codes are being updated, especially in areas where residences are located close to forests or grasslands. These areas are called Wildland Urban Interface (WUI) zones.
For those in the pressure-treated wood business, WUI has become very important. It affects how products must be treated, tested, documented, and presented to building officials. It also creates real opportunities for companies that take the time to understand the rules and design products that fit them. Trans Western Chemicals is working with customers on both sides of that equation, helping them develop new systems and also helping them navigate the code language itself.
What WUI is and how Chapter 7A works
California is a useful example because it has one of the most developed WUI frameworks. California Building Code Chapter 7A covers materials and construction methods for buildings in designated fire hazard severity zones and local WUI areas. The chapter is based on the International WUI Code, so the ideas show up in other states as well.
The process is familiar. Proposed changes go through public hearings and comment periods. A code committee reviews, then votes to approve or reject the changes. Once adopted, Chapter 7A becomes part of the main building code.
Enforcement is also familiar. The local building official:
- Reviews plans for projects inside mapped WUI areas
- Inspects the site and materials during construction
- Flags violations and can require corrections before final approval
If a project is in a mapped WUI zone and needs a permit, Chapter 7A is likely to be in play.
Can we still use exterior fire-retardant treated wood?
Wood is still allowed in WUI construction. The code does not ban it. Instead, it sets a clear bar for fire performance and durability.
For a material to qualify as a fire-retardant treated wood product, commonly known as exterior FRTW it must initially pass the identical fundamental tests required for interior fire-retardant treated wood pursuant to ICC-ES AC66. These evaluations assess flame spread, smoke production, structural integrity etc.
Exterior FRTW then has to go further. Two extra requirements matter most:
- Accelerated weathering using ASTM D2898
- A 30-minute tunnel test using ASTM E84 extended or ASTM E2768 after that weathering
ASTM D2898 is designed to approximate about ten years of harsh outdoor exposure. The protocol uses repeated wetting and drying cycles and is written to simulate about 80 inches of rain each year, or 800 inches total. Only after that conditioning does the material go back into the tunnel test, where it must still reach a Class A rating over a full 30 minutes.
If a system cannot pass the weathering and the extended burn together, it does not qualify as an exterior FRTW product for WUI work.
Simple coatings – are they compliant?
Chapter 7A also clarifies a misconception. Numerous individuals believe they can simply apply a fire-retardant paint or coating to wood and consider it compliant. However, the code explicitly denies this notion.
Section 703A.5.3 specifies that paints, stains or other surface treatments cannot replace fire-retardant treated wood for these uses. Simply put a superficial coating is insufficient. Fire resistance must be integrated into the wood to withstand exposure, to the elements.
This does not mean coatings play no role. It means they cannot be the entire fire protection strategy when you are trying to meet WUI requirements for exterior FRTW.
Durability tests that support WUI
Recent acceptance criteria add more stress tests because the goal is not only to slow fire when the product is new. The treatment has to keep working years later. Two documents matter here: AC479 and AC516.
AC479 includes:
- Freeze-thaw cycling conducted according to ASTM D7032, involves immersing samples in water at temperature for 24 hours followed by freezing for 24 hours. This sequence is done thrice.
- UV and water spray using ASTM D2898, but limited to six eight-hour cycles.
- Elevated temperature conditioning using ASTM D5516 and D5664, followed by extended E84 or E2768 burning.
- A stability test where a 4 by 4 foot panel sits at 200°F for at least 30 minutes.
AC516 focuses on durability for certain systems and also uses ASTM D2898 UV and water spray for six eight-hour cycles.
All of these steps aim at one simple question. Does the fire-retardant treatment stay in place and keep working after realistic exposure to water, sun, and heat, or does it fade away when the product is installed outdoors for years.
Closed vessel treatment and “other means”
When you read deeper into the WUI details, one requirement stands out. To follow the primary exterior FRTW path, the fire-retardant treatment must be applied in a closed vessel at a pressure of 50 psi or higher. That language describes classic pressure impregnation. It is the current standard, and it is proven.
The code also allows “other means” of treatment. This is where some confusion, and opportunity, begins. These other means cannot be simple paints, stains, or standard coatings, because surface films are already excluded. Instead, the idea is to leave room for other types of treatments that still behave like a built-in system. These could be intumescent or non-intumescent chemistries. They might use dipping, flood coating, or other processes as long as the treatment penetrates, stays in the wood, and survives weathering.
For these alternative approaches to gain acceptance, they need to live in a controlled manufacturing environment. That means clear standard operating procedures, documented quality control, and third-party inspection. If a system can pass the weathering and burn tests described earlier, the producer can then apply for a formal evaluation report, such as:
- ICC-ES ESR
- UL evaluation
- Intertek CCRR
The evaluation body checks the data, confirms which acceptance criteria apply, and publishes a report. Once that exists, the product can be presented to building officials as an alternative equivalent to exterior FRTW. Final acceptance still rests with the local code official, so clear communication and documentation matter. TWC helps customers prepare for that step and explain their systems in language that officials can work with.
Fire marshal listings and the need for clear data
In states such as California, many inspectors and fire officials also use State Fire Marshal listing programs. CAL FIRE’s Building Materials Listing (BML) is one example. These databases show which products have some form of approval for specific uses.
The helpful part is obvious. A listed product is easier to look up. The risk is that not all listed products reach the same level of performance. Some products show only a Class B rating. Others show Class A based on a ten-minute tunnel test, not a 30-minute extended test. Some listings relate only to roofs or to non-WUI conditions.
Because of this, a siding, decking, or door product can appear acceptable at a glance while still falling short of the full combination of D2898 weathering plus extended E84 or E2768 required for exterior FRTW in WUI zones.
One practical way to avoid confusion is to prepare a clear information packet for code officials in the areas where the product will be used. That packet can include:
- A simple description of the chemistry and treatment process
- The plant quality control outline and third-party inspection details
- Copies of ICC-ES, UL, or Intertek reports
- Copies or summaries of ASTM D2898, AC479 or AC516, and E84 or E2768 test results
TWC often helps customers assemble and organize this type of documentation so they can walk into meetings with building officials prepared. The objective is to show that the treatment has passed the full set of durability and fire tests, not just a minimum burn test. The focus is on protecting buildings and lives, not only placing more volume into the market.
Market size and where demand is going
Today, the industry estimates that about 850 million board feet of fire-retardant treated wood are produced each year by pressure impregnation. Only around 18 percent of that is for exterior FRTW.
As WUI language becomes more common, that ratio will most likely change. Standard single-family homes historically used limited treated wood. Now, in mapped WUI zones, requirements can push projects toward either noncombustible materials or wood that meets exterior FRTW rules in all exposed locations. That list includes:
- Decks and balconies
- Siding and soffits
- Fascia and trim
- Fencing close to structures
- Exposed window and door components
- Other exterior elements in both new construction and repair work
Engineered wood items like LVL, glulam, CLT and OSB are complicated. Many of these materials are incompatible with water-based pressure treatment due to their adhesives and composition. They need treatment techniques that comply with WUI standards while preserving the integrity of the product.
The expected growth in exterior FRTW demand should create space for both more pressure-treating capacity and new treatment technologies geared toward engineered products and specialty applications.
How Trans Western Chemicals supports customers
Trans Western Chemicals sits at the junction of chemistry, wood products, and codes. We supply raw materials for fire retardant formulations and work with customers who treat wood, make engineered products, or plan to enter the exterior FRTW market.
Our role is twofold. We help customers design and refine chemistries that can survive durability and fire tests tied to WUI. We also help them understand the regulatory path, from AC66 and D2898 through AC479 or AC516, E84 or E2768, evaluation reports, and local approvals.
TWC helps guide customers through the WUI regulations, so they do not have to interpret the rules alone. We work with them to align product design, testing programs, and documentation with what building officials expect to see in the field.
WUI regulations raise the bar for exterior wood, but they also keep wood in conversation as a viable material in wildfire-prone regions. With the right treatment, testing, and guidance, pressure-treated and engineered wood products can meet those expectations and continue to serve as key components in modern construction.
