Building Knowledge

How to choose the right substrate for exterior soffits

Jan 15, 2024

Discover products provided by National Gypsum Company that are ideal for exterior soffit applications


Whether it’s a drive-through overhang, open breezeway, porch or the underside of eaves, exterior soffits and ceilings provide shade, rain protection and architectural design elements for commercial buildings, multifamily structures and homes. It’s essential to choose the right product for the soffit substrate to prevent sag caused by exposure to high temperature and humidity.

National Gypsum Company provides many products through its affiliate companies Gold Bond Building Products, LLC, PermaBASE Building Products, LLC, and ProForm Finishing Products, LLC, approved for use in exterior soffit applications.

These products are ideal for exterior soffits because they offer mold, mildew and moisture resistance and fire resistance.

These products offer a level of convenience because they are likely already being used on the construction site to build walls and ceilings.

Wait, what happened to soffit board?

Anyone designing soffits may become frustrated at not finding a product named soffit board. Years ago gypsum soffit board, which featured moisture-repellent paper and additives to resist sag, was marketed for use in soffits. Early versions of moisture-resistant gypsum board, called “green board,” had wax in the gypsum core to make it moisture resistant but it didn’t resist sag.

Today’s mold- and moisture-resistant gypsum board found in Gold Bond’s XP® and eXP® product lines has a silicone-treated core and paper that eliminates the sag problem. Today, there is no need to look for the word soffit in the product name. Instead, choose gypsum board products specified for wet or semi-wet areas like those in the list above.

Ask these questions when selecting which product to use as the substrate on exterior soffits. Dive deeper into product recommendations for exterior soffits.

What type of finish will be used on the soffit?

Soffits contribute to a building’s exterior aesthetic by visually breaking up the structure’s facade with color, texture or contrasting material. For example, juxtaposing materials such as wood cladding on soffits for a metal building can create visual interest. And finishing soffits in stucco on a stucco building can create a sleek, minimalist look.

Soffit finishes include:

  • Paint
  • Stucco
  • DEFS (Direct-Applied Exterior Finish Systems)
  • Vinyl, wood, or metal cladding

The soffit finish influences which product can be used as the soffit substrate.

  • Painted surfaces typically pair best with XP® Fire-Shield® Gypsum Board. Using eXP® Sheathing or eXP® Interior Extreme® is also an option, but both require a skim coat of joint compound before painting. Be sure to use setting compounds approved for exterior use, such as ProForm® Quick Set Lite™ Setting Compound. (Ready-mix joint compound is not approved for outside surfaces.)
  • Stucco works best with PermaBASE® Cement Board or eXP Sheathing.
  • DEFS can be applied to glass-mat gypsum panels or cement boards. Check with the DEFS manufacturer for approved substrates.
  • Vinyl, wood, or metal cladding can be used with any type of substrate.

Is the soffit part of a fire-rated floor or ceiling assembly?

Make sure the soffit material is listed in the UL design if it is part of a fire-rated roof/ceiling or floor assembly. Most horizontal fire-rated assemblies require Type C gypsum board such as XP® Fire-Shield C™ and eXP® Interior Extreme® Fire-Shield C™.

What is the framing spacing where the soffit will be attached?

As an added layer of defense against sagging, select the gypsum or cement board thickness based on the framing spacing.

  • Maximum 16" on-center framing: Use ½" gypsum board or ½" or ⅝" cement board.
  • Maximum 24" on-center framing: Use ⅝" gypsum board.

Have additional questions about soffit design and construction? Reach out to 1-800-NATIONAL® Construction Services for technical assistance.