Inspiration12 min

UL Fire Ratings for Wall Assemblies Explained

Whether it’s a townhome, a “tall and skinny, an apartment complex or a zero-lot-line single-family application, finding the proper code-compliant fire-resistant-rated wall assemblies and UL fire-rated wall systems is often a challenge. Navigating UL wall assemblies can add another layer of complication. Ensuring the proper UL evaluated assembly to use grows more complicated depending on the project's size and scale.

In today’s building and code environment, it’s key for architects, engineers and specifiers to maximize space while maintaining the cost-effectiveness of the building's construction and design. There's a lot to know about UL fire-resistant assemblies and OSB assemblies, so let’s get started.

LP® FlameBlock® Fire-Rated Sheathing, UL Evaluated Fire-Resistant Assemblies

With these design challenges in mind, LP Building Solutions created LP® FlameBlock® Fire-Rated Sheathing for low- and mid-rise wood structures and light gauge steel framing. LP FlameBlock sheathing allows fire-resistance rated walls to be assembled faster than common alternatives, potentially reducing labor costs while providing exceptional strength, durability and consistency. Additionally, Exposure 1 classification signifies that LP FlameBlock sheathing is intended to resist the effects of moisture on structural performance during active construction.

UL fire-resistance rated wall assemblies with LP FlameBlock sheathing provide several benefits to architects, engineers, builders, developers, contractors and even homeowners. LP FlameBlock sheathing wall assemblies provide design flexibility by combining structural capacity and fire resistance in a single panel. It may eliminate a layer of gypsum to simplify and expedite the construction process. The cost savings may allow designers and builders to redirect the funds into design elements their clients are seeking, increasing the value of the property.


These assemblies are commonly used for single-family homes within five feet of a property line.  

UL Design No. V340 (1-hour from either side)

UL Design No. U348 with stucco (both 1-hour from either side)  

UL Design No. U348 with brick (both 1-hour from either side)


  • No exterior gypsum is required 
  • Consistent wall thickness with unrated walls 


These assemblies are typically used in dividing walls in townhouses.  

UL Design No. V340 (Two 1-hour walls, back-to-back)


  • No code restrictions for mechanical, electrical and plumbing.

UL Design No. U350A and U350B (Common 2-hour wall)


  • Alternative to a shaft wall system
  • Uses conventional framing practices
  • Sound Transmission Class (STC) rating of 61


These assemblies are commonly used for Type III and Type V exterior walls. 

UL Design No. V340 (1-hour from either side)


UL Design No. V340 (2-hour interior, 1-hour exterior)

When using UL Design No. V340 assemblies for Type III exterior walls, 2-sided LP FlameBlock is required. 


  • No exterior gypsum is required for exterior 1-hour resistance

UL Design No. V337 with gypsum and V337 with stucco (2-hour from either side)


  • Potentially reduces the amount of exterior gypsum required.
  • Base layer of interior gypsum may be replaced with 2-sided LP FlameBlock for double-sided shear walls.  

UL Design No. U349 (2-hour interior)  

UL Design No. U349 (2-hour interior, 1-hour exterior) 

UL Design No. U349 (2-hour from either side)  


  • LP FlameBlock sheathing does not require any strength reductions.
  • Some water resistive coatings and roof underlayments may be used in conjunction with LP® FlameBlock® Fire-Rated Sheathing

Code compliance does not have to limit design flexibility, budget or schedule. LP FlameBlock Fire-Rated Sheathing assemblies offer code-compliant solutions for a variety of building needs, including plenty of assemblies by UL for fire-resistance rated wall systems. Additional assembly options, including steel studs, are available at

Do you have a project in the works? Check out best practices for choosing fire-rated OSB.

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