Curbs Plus CPC-3 vs CPC-4 vs CPM-3: Which Roof Curb to Use

Of the three Curbs Plus roof curb families we carry, the CPC-3 is the insulated curb for conventional roof decks, the CPC-4 is the same style of curb without insulation, and the CPM-3 is the insulated curb built to sit over the ribs of a metal building roof. Once you know the family, the rest of the model number tells you the inside dimension, the height, the steel gauge and the safety screen.

CPC-3, CPC-4 and CPM-3 compared

	CPC-3	CPC-4	CPM-3
Roof type	Conventional roof deck	Conventional roof deck	Ribbed metal building roof
Insulation	1-1/2 inch rigid fiberglass with white facing	None	Insulated
Interior finish	White faced insulation, not paintable	Walls primed white	Walls primed white
Height as listed	12 inches (10 and 14 available)	14 inches (10 and 12 available)	9 inches (12 and 14 available)
Steel	18 gauge galvanized	18 gauge galvanized	18 gauge
Special features	Metal liner available if the interior must be painted	1 x 4 wood nailer shown on the shop drawing	Water diverter on the 39 inch side and R-panel rib covers

When to use the CPC-3

Choose the CPC-3 over conditioned space. The manufacturer describes insulated curbs as the way to limit condensation and heat loss at the curb wall. The white facing on the insulation is the finished interior surface and cannot be painted. If the interior must be painted, ask for the metal liner option. We list the CPC-3 in 4848, 4896, 6060, 6072 and 36120.

When to use the CPC-4

Curbs Plus describes the CPC-4 as the same welded, one-piece construction without insulation, suited to unconditioned spaces. It is also the usual pick when the roofer will insulate the curb wall in the field. We list it in 4848, 4896, 6060, 6072 and 36120.

When to use the CPM-3

The CPM-3 36120 is for pre-engineered metal buildings. The curb flanges fit over the roof panel, rib covers close off the R-panel ribs, and a diverter on the 39 inch side sends water around the curb. The installation sheet calls for structural support framing under the curb walls and flanges, tape caulk under the flanges, at least four fasteners per side, and polyurethane caulk along the flange edge. Fasteners must go into the support framing, never through the roof panel alone. On sliding standing seam roofs, do not tie the support framing to fixed structural members.

How to read a Curbs Plus model number

Take CPC-3 36120 12H 18GA 4OC as the example:

Segment	Meaning
CPC-3	Curb family: CPC-3 insulated, CPC-4 non-insulated, CPM-3 metal building
36120	Curb inside dimension in inches, 36 x 120. Nominal 3 x 10 feet
12H	Curb height, 12 inches
18GA	18 gauge steel
4OC	Safety screen with 4 inch on center wire spacing

Inside and outside dimensions

The size in the model number is the inside dimension. On every curb we list, the outside dimension is 3 inches larger each way, and the outside dimension is what the skylight or smoke vent frame has to fit over.

Size code	Inside dimension	Outside dimension	Nominal
4848	48 x 48 in	51 x 51 in	4 x 4 ft
4896	48 x 96 in	51 x 99 in	4 x 8 ft
6060	60 x 60 in	63 x 63 in	5 x 5 ft
6072	60 x 72 in	63 x 75 in	5 x 6 ft
36120	36 x 120 in	39 x 123 in	3 x 10 ft

Screen options and paint notes

The standard 4OC screen is 3/16 inch (6 gauge) welded wire on a 4 x 4 inch grid. Each curb can also be ordered with a 6 x 6 inch screen, with 5 inch on center burglar bars, or with no screen. The shop drawings carry two paint notes worth passing to the painter: do not paint the screens on a CPC-3, and paint only the welds on the curb exterior rather than the whole curb.

Spec sheets and other curbs

Curbs from every fabricator we carry are on the roof curbs page, and manufacturer drawings are on the roof curb spec sheets page.

FAQ

Can I paint the inside of a CPC-3 curb?

No. The white faced insulation is the interior surface and is not paintable. Order the metal liner option if the interior has to be painted, or use the CPC-4, which comes primed white.

Is the size in the model number the inside or the outside of the curb?

Inside. A 4896 curb is 48 x 96 inches inside and 51 x 99 inches outside.

Which curb goes under a 3 x 10 skylight on a metal building?

The CPM-3 36120. It has the diverter and R-panel rib covers needed to seal to a ribbed roof, which the CPC curbs do not have.

Next steps

See the full line on the Curbs Plus brand page. Send the quantity, family, inside dimension, height and screen option through the curbs request for quote page and we will confirm the configuration and lead time.