DP rating (Design Pressure)
A DP rating (design pressure) is the wind and structural load a window or door is rated to withstand, expressed in pounds per square foot (psf) under the North American Fenestration Standard. A performance grade (PG) is the DP plus every other NAFS requirement passed at that pressure, so PG is the number to specify. Higher means a stronger assembly, for the size and configuration that was tested.
Design pressure (DP) is the positive and negative wind or structural pressure, in psf (or Pa), that a window assembly is rated to withstand without failing, set by laboratory testing under NAFS. You see it on a spec or schedule as a line like "DP 50" or "CW-PG50", and a code reviewer compares it with the design wind pressure of the site. The number belongs to a tested assembly at a tested size, not to a brand. NAFS is the North American Fenestration Standard, AAMA/WDMA/CSA 101/I.S.2/A440, in its 2008, 2011, 2017 and 2022 editions (FGIA performance class overview, accessed 2026-11-02).
What is the difference between DP and PG?
DP is the structural pressure a product is rated for; a performance grade (PG) means the product was tested to that design pressure and also met all the other requirements of the standard, so DP alone is not a PG. FGIA explains that NAFS-08 added this clarification because some manufacturers marketed products solely on a DP rating. PG is the number to write into a specification, because it carries the water penetration, air leakage, operating force and forced-entry results with it.
| Item | DP (design pressure) | PG (performance grade) |
|---|---|---|
| What it states | The wind load the structural test was based on | The DP plus every other NAFS requirement passed at that pressure |
| Structural test | 150% of DP for windows and doors (200% for skylights) | Same test, as one of several |
| Water penetration test | Not implied | 15% of DP for classes R, LC and CW and 20% for AW, with a 2.90 psf (140 Pa) minimum for R and a cap of 12 psf (580 Pa) in the US and 15 psf (720 Pa) in Canada |
| How it is written | "DP 50" | "CW-PG50" (class, then grade) |
What are the NAFS performance classes?
NAFS sorts products into four performance classes, R, LC, CW and AW, from least to most stringent, each with a gateway grade a product must pass to enter the class. The 2008 edition reduced the earlier five classes to these four. The gateway values below are from RDH Building Science's performance class explainer (accessed 2026-11-02).
| Class | Typical use | Minimum PG | Minimum DP | Structural test pressure |
|---|---|---|---|---|
| R | Residential | PG15 | 15 psf (720 Pa) | 22.5 psf (1,080 Pa) |
| LC | Light commercial | PG25 | 25 psf (1,200 Pa) | 37.5 psf (1,800 Pa) |
| CW | Commercial | PG30 | 30 psf (1,440 Pa) | 45 psf (2,160 Pa) |
| AW | Architectural | PG40 | 40 psf (1,920 Pa) | 60 psf (2,880 Pa) |
A product can carry a grade above its class gateway, so a specification names the grade the project needs and lets the class follow. One psf is about 48 Pa.
How do I read a DP rating?
A full rating reads class, grade and tested size, for example "CW-PG50, 1,500 x 2,500 mm", and each part limits what the number covers. The class says which gateway tests the product passed, the grade says the design pressure it was tested at, and the size says the largest unit the rating applies to. A unit larger than the tested size is not covered; for sizes that were not tested, an engineering analysis is an accepted route. A bare "DP 50" with no class and no size is the weakest form of the claim, and the first thing I ask for is the report behind it.
Do European windows have a DP rating?
Yes, when the specific assembly has a NAFS test report or an engineering analysis; otherwise European units are rated under a different system. EN standards classify wind resistance in classes 1 to 5, from 400 Pa for class 1 up to 2,000 Pa for class 5. An EN class is not a NAFS DP: the tests and pass criteria differ, so I never convert one into the other.
The catalog data I work from shows the difference. The systems I source list EN wind classes only, except two that list a US class for a tested variant: PG55 on one folding system and CW50 on the MB-79N CO casement. Even then, the figure for your size and glazing comes from the report. The paperwork side is covered in NFRC and NAFS documentation, and the frame side, hardware and fasteners that must not bridge the thermal break while carrying the wind load, in the thermally broken aluminum guide.
How do I confirm the DP for the unit I am buying?
Ask for the document that names your exact assembly, and check it against this list:
- Request the NAFS test report or the engineering analysis, naming the product, the tested size, the class and the grade.
- Check that your unit is not larger than the tested size.
- Check that the glazing build matches the tested one; see triple glazing for how the glass build changes an assembly.
- Ask the engineer or building department which DP the project needs.
It matters most on large lift-and-slide doors, wide bifold openings and tall units. I ask for the report before a unit goes into a schedule. When you are ready, start in the estimator or request a quote, and I will ask for the documentation for your size.
Common questions
- What is a good DP rating for windows?
- There is no single good number. The required DP comes from the project's design wind pressure, set by the engineer or the adopted code, so the right rating is the one at or above that requirement for the actual size. The NAFS gateway grades give the scale: PG15 (720 Pa) for class R, PG25 (1,200 Pa) for LC, PG30 (1,440 Pa) for CW and PG40 (1,920 Pa) for AW.
- Is DP the same as PG?
- No. DP is the structural pressure a product is rated for. A performance grade (PG) means the product was tested to that design pressure and also met every other requirement of NAFS, including water penetration, so a product marketed on DP alone has not shown the full grade. FGIA states this distinction explicitly.
- Do European tilt-and-turn windows have a DP rating?
- They can, for a tested configuration. European units are rated under EN wind classes (400 to 2,000 Pa for classes 1 to 5), while a US DP needs a NAFS test report or an engineering analysis for the supplied size and glass. Ask for it per assembly.
- What does DP 50 mean on a window?
- DP 50 means the window was rated for a design pressure of 50 psf (about 2,400 Pa) and withstood a structural test at 150% of that, 75 psf. It holds for the tested size and configuration only. If the rating is written as a performance grade, for example CW-PG50, the unit also passed the water, air, operating force and forced-entry requirements at that grade.
- Is a higher DP rating always better?
- A higher DP means a stronger tested assembly, but it applies only to the tested size and configuration. A product should be specified on the DP the project requires, so there is no benefit in asking for more than that.
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