The APA trademark stamped on a structural panel carries the information a builder needs to install it correctly: panel grade, span rating, exposure classification, thickness, mill number, and the standard it was manufactured to. The mark turns a stack of identical-looking sheets into a set of specifications, and reading it takes about thirty seconds once you know the layout. APA, the Engineered Wood Association, publishes guidance on interpreting the trademark to help builders and specifiers select the right panel for the job. Panels carrying the trademark are suitable for a variety of end uses, including subflooring, single-layer flooring, wall and roof sheathing, ceiling and deck sheathing, and siding.
Misreading the stamp is the panel equivalent of pouring concrete without checking the mix. Crews deal with concrete slump variations at the site by testing before placement, and the same logic applies to panels: verify the stamp before the sheet goes into the wall, not after. The performance data on the label applies only when the panel is installed the way the trademark says. The stamp also lists the mill number, which lets a builder trace a complaint back to the plant and the production shift, useful when a bundle arrives damaged or delaminated.
Panel Grade: What the Name and Letters Tell You
Structural panel grades are identified two ways. Plywood grades use the veneer grades on the face and back of the panel, such as A-B or B-C, while performance-rated panels use a name for the intended end use, such as APA Rated Sheathing or APA Rated Sturd-I-Floor. The grade letter describes the veneer in terms of natural growth characteristics and the number and size of repairs allowed during manufacture, the same way the properties of fine-grained concrete explain how that material behaves in a pour.
Veneer Grade Letters
The highest veneer grade commonly available is A. Grade B allows more natural characteristics, grade C permits open defects up to defined sizes, and grade D is the lowest structural grade. The minimum grade permitted in Exterior plywood is C, and D-grade veneer appears only in panels meant for applications protected from long-term weather exposure. In practice, most structural sheathing uses C-grade faces, while A and B faces appear where the wood is visible.
| Grade | Face characteristics | Repair allowance | Typical use |
|---|---|---|---|
| A | Smooth, paintable, minimal defects | Few repairs, tight limits | Architectural panels, cabinet faces |
| B | Solid surface with natural characteristics | Moderate repairs | Utility plywood, concrete forms |
| C | Open defects up to defined sizes | Larger repairs allowed | Sheathing, subflooring |
| D | Knots and voids permitted | Widest repair allowance | Protected interior applications only |
End-Use Names
APA Rated Sheathing, APA Rated Sturd-I-Floor, and APA Rated Siding are end-use names. The name tells you the panel was tested for that job, so you do not have to translate veneer letters into performance. The same panel construction can carry different names for different markets. The back of the panel tells a different story than the face. Builders who specify plywood for exposed ceilings or accent walls order the grade on both faces, because a B-C panel looks different from an A-B panel even when the structure is identical. The trademark makes that distinction explicit, which is exactly why it exists.
Span Rating: Reading the Two Numbers
The span rating on APA Rated Sheathing is two numbers separated by a slash, such as 32/16. The left number is the maximum center-to-center spacing of supports in inches when the panel is used for roof sheathing with the long dimension across supports. The right number is the maximum spacing for subflooring, also with the long dimension across supports. Choosing the right rating keeps floors stiff and roofs stable, and it matters in every framing layout, including tricky roof geometry like the shed dormers explained in fine homebuilding project guides.
Common Sheathing Ratings
| Span rating | Roof spacing (in.) | Floor spacing (in.) | Typical use |
|---|---|---|---|
| 24/16 | 24 | 16 | Light roof and floor sheathing |
| 32/16 | 32 | 16 | Standard wall and roof sheathing |
| 40/20 | 40 | 20 | Heavy roof and floor sheathing |
| 48/24 | 48 | 24 | Commercial and high-load applications |
The numbers assume the long dimension, or strength axis, runs across three or more supports. Install the panel the other way and the rating no longer applies, which is why framing crews mark the strength axis before cutting. Span ratings also set the fastening schedule indirectly. A panel rated for 24-inch spacing still needs the right nail pattern, and the APA publication includes fastener recommendations for each rating. The stamp tells you the spacing; the fastening table tells you the nails.
Single-Number Ratings
Sturd-I-Floor underlayment and APA Rated Siding panels carry a single number, such as 24 oc, because those panels are designed for one support condition. Sturd-I-Floor ratings run from 16 oc to 48 oc, and the rating is based on the panel spanning three or more supports with the strength axis across them.
Bond Classification: Exterior vs. Exposure 1
APA trademarked panels come in two moisture-resistant bond classifications: Exterior and Exposure 1. The classification reflects the severity and duration of weather and moisture exposure the panel is designed to withstand, along with the wood materials and adhesives used in manufacture. Getting this right is a structural decision, not a cosmetic one, in the same way an outrigger structural system defines how a high-rise building transfers loads.
Exterior Panels
Exterior panels have bonds that withstand repeated wetting and redrying and long-term exposure to weather or conditions of similar severity. Use them wherever the panel will face the elements for years: exterior siding, decks, and other permanently exposed locations.
Exposure 1 Panels
Exposure 1 panels resist the effects of moisture on structural performance during construction and similar conditions. They are made with the same exterior adhesives used in Exterior panels, but other compositional factors can affect bond performance, so only Exterior panels should be used for long-term weather exposure. Exposure 1 works in protected locations such as the underside of panels at roof overhangs, where the appearance characteristics of the grade should also be considered. Moisture questions come up at every inspection. A common field error is using Exposure 1 panels where the drawing calls for Exterior, because the two look identical on the face. The bond classification appears in the stamp, and inspectors look for it in the same place they look for the span rating.
Matching the Class to the Location
Choose the bond class the way you choose a fastener: match it to the application. Just as hollow wall anchors are selected for the right drywall application, bond class is selected for the exposure. Wall and roof sheathing in most climates uses Exposure 1, while decking, siding, and any panel left exposed to weather calls for Exterior.
Thickness, Finishes, and Panel Construction
Performance Rated Panels come in a range of thicknesses, commonly 15/32, 19/32, and 23/32 inch, plus thicker engineered options for specialty floors. The trademark lists the nominal thickness so the installer can verify it against the drawing without a tape measure. Performance Rated Panels are covered by PRP-108, the performance standard for wood-based structural panels, and the stamp references the applicable product standard: PS 1 for plywood and PS 2 for other wood structural panels such as OSB. When the drawing names a standard, the stamp must match it.
Finishes and Edges
Panels can be sanded, textured, or left unsanded, and edges can be square or tongue-and-groove. Sturd-I-Floor panels with tongue-and-groove edges are the standard choice for single-layer flooring, where the floor system carries both structure and finish without a separate underlayment layer.
Siding and Appearance Grades
APA Rated Siding panels carry appearance requirements along with structural ratings, because the panel is the finished surface. Texture, groove spacing, and grade all affect the installed look, so the stamp matters for aesthetics as much as for structure.
Putting It Together at the Jobsite
Panel selection interacts with the rest of the wall assembly. A panel that performs on paper still depends on the framing, fastening, and adjacent layers, the way an exterior foam system such as the matrix wall system changes how the sheathing sheds moisture. Read the whole assembly before you change one component. Subflooring and sheathing are usually specified by the structural engineer, but the builder picks the day-to-day details: which bundle goes to which wall, how sheets are stacked, how scrap is cut. Those details decide whether the rating on the stamp is real in the finished building.
End uses covered by the trademark:
- Subflooring and single-layer flooring
- Wall and roof sheathing
- Ceiling and deck sheathing
- Siding and exterior finishes
Checklist before installation:
- Confirm the span rating matches the framing spacing on the plan.
- Confirm the bond class matches the exposure at that location.
- Verify the thickness against the specification.
- Install with the strength axis across three or more supports.
- Check the stamp on every sheet; grade can change between bundles.
Verification at Delivery
Spot-check panels at delivery, not after they are in the wall. A wrong bundle delivered in the morning costs one hour to catch and a full day to fix. Compare the stamp on a few sheets against the submittal, and photograph the bundle labels for the record.
Finally, make sure the project is permitted before the panels go up. Residential building permits are required for most structural work, and inspectors will check that the installed panel matches the stamp. The trademark is the fastest way to prove compliance on the spot, so keep the submittal sheet in the trailer until the final inspection.