How to Measure a Commercial Skylight Curb: COD, CID & NFID

Logistics Lighting is a national commercial/industrial distributor and specifier - not a manufacturer - under Skylight Sherpa(SM). To order a replacement commercial skylight or smoke vent that fits, measure the existing Curb Outside Dimension (COD) - including any flashing or membrane on the curb - then record Curb Inside Dimension (CID) if safety screens attach to the top of the curb. Units are typically fabricated about 1" larger than COD, so COD maps into the unit's Net Frame Inside Dimension (NFID). Wrong COD is the most common fit failure on recover and replacement jobs.

Last updated: 2026-09-17

What COD, CID, and NFID mean

COD is the finished outside of the curb the unit sits on (the "shoe box"). NFID is the inside of the skylight or smoke-vent frame that fits over that curb (the "lid"); NFID is typically about 1" larger than COD. CID is the curb inside opening and is required when safety screens attach to the top of the curb. Definitions live on Terminology. The measuring PDF is linked from Skylight Specification Sheets (Curb Measurement Guide).

Term	What to measure	Why it matters
COD	Outside of curb including flashing/membrane	Critical fit dimension for skylight or smoke vent
NFID	Inside of unit frame (factory)	Typically ~1" larger than COD for clearance
CID	Inside of curb opening	Required when screens attach to curb top
Curb height	Roof surface to curb top	Needed if curbs are replaced or added

Nuance: typical curb wall thickness is about 1.5" unless otherwise specified (Terminology). Always verify every curb on the roof shares the same dimensions before you freeze a size code.

Step-by-step measuring method

Measure from the roof, preferably with the existing skylight removed so you are measuring the curb only - not the skylight frame. Include flashing or membrane in COD; collect CID when screens are in scope; collect curb height when curbs will be replaced or added (Ask Our Experts; Curb Measurement Guide).

Step	Action	Common mistake
1	Remove or lift unit; measure curb, not frame OD	Measuring the old skylight frame instead of the curb
2	Record COD with flashing/membrane	Omitting membrane thickness after a TPO recover
3	Record CID if screens attach to curb	Ordering screens without CID
4	Confirm all curbs match	Assuming every opening is identical
5	Note curb height if replacing/adding curbs	Guessing height from the old unit

Nuance: custom skylights can be made to fit existing curbs; smoke vents are standard sizes only, with a curb adapter used when a standard vent must fit an existing curb (Ask Our Experts).

Typical curb height

Typical minimum curb height is about 8" above the roof, but final height is set by the architect/engineer, geography, building type, and roofing membrane provider (Ask Our Experts). Treat exact height for a given membrane system as design-dependent / UNKNOWN until the roofing manufacturer detail and AHJ path are set.

Item	Verified LL guidance	Status
Typical minimum curb height	~8" above the roof	Starting point
Who sets final height	Architect / building engineer + geography, building type, membrane provider	Design-dependent
Integral curb	Curb + skylight fabricated as one unit	Different measuring path

How size codes relate to the curb you measured

Inch codes such as 4896 mean a nominal 48" x 96" curb ID family; Sunoptics 4080 means 4'-0" x 8'-0" for the same nominal opening. After you have COD, match the storefront family on the opening-size hub - do not substitute brands by nominal size alone when COD differs by ~1". Full chart: How to spec by opening size.

Nominal	Inch code	Sunoptics ft	COD fit note (typical LL fields)
4x8	4896	4080	Often COD fit ~51x99
5x6	6072	5060	Inch ~63x75; Sunoptics ~62.375x74.375
4x4	4848	4040	Inch ~51x51; Sunoptics ~50x50
3x10	36120	-	VELUX model 3612; COD fit 39x123

Nuance: specialty families such as Kingspan Alumilite 4689 or melt-out 5596 / 55102 are not interchangeable with 4896 - see the hub tables.

Fall protection when measuring and replacing

OSHA does not test or approve skylights. For openings 4+ feet above a lower level, fall protection (safety screens supporting 400 lb or twice the intended load, guardrails, covers, or personal fall arrest systems) is summarized under OSHA 1910.28 on Ask Our Experts. Confirm jobsite means of fall protection before removing units for measurement.

Proof that correct curb fit matters on recover jobs

At Fairburn, GA, Sta-Dry Roofing modernized skylights during a TPO overlay using Sunoptics Signature Series Model 5060 with Curbs Plus 5x6 curbs and safety screens - products coordinated through Logistics Lighting as distributor (case study). Wrong COD/CID would have broken that curb-mount + screen package.

Next step - Skylight Sherpa(SM)

Need a curb fit check (COD/CID), cut sheets, smoke-vent pairing, or a quote package? Logistics Lighting coordinates quote to order to delivery as a distributor/specifier - we do not manufacture.

Call 844-455-7283 or 307-633-9696 | sales@logisticslighting.com

Last updated: 2026-09-17