The 30-second version
- The curb is the wooden frame the skylight sits on — transitions the roof plane to the skylight opening.
- Prefab curbs: factory-built kits sized to Velux openings. Fast install, standardized detailing.
- Custom wood curbs: site-built on top of your roof deck. Slower install, but often required for non-standard skylight sizes or complex geometry.
- Warranty implications: Velux's leak warranty requires specific curb detailing. Improperly detailed custom curbs void warranty.
Why the curb matters
Every curb-mounted skylight sits on a wooden curb — a 2x lumber frame that transitions from the roof deck to the skylight frame. On LA flat roofs, that curb is doing three critical jobs:
- Structurally supporting the skylight above the roof plane
- Providing a clean transition for waterproofing (base flashing wraps around the curb)
- Providing thermal separation between the exterior and the interior skylight assembly
Get the curb wrong and the skylight leaks, sweats, or transmits excessive heat. Get it right and the skylight performs to manufacturer spec for its full lifecycle.
Prefab curbs
Velux and its distributors sell factory-built curb kits sized to standard Velux skylight openings. A prefab curb comes pre-assembled with proper corner joinery, factory-applied waterproofing membrane, and matched flashing pieces.
- Pros: fast install (1-2 hours vs 4-8 for custom), factory-controlled dimensions, matches Velux warranty requirements automatically, pre-integrated with Velux flashing kits.
- Cons: only available for standard Velux sizes, less design flexibility, cost premium of $150-$400 per curb vs custom lumber.
Custom wood curbs
Site-built curbs constructed by the roofing crew from 2x lumber (usually pressure-treated Douglas fir or cedar) on top of the roof deck. Common approach for larger skylights, non-standard sizes, or when integrating a skylight into an existing roof assembly.
- Pros: any size, any geometry, lower material cost, easier to integrate into complex roof shapes.
- Cons: slower install, quality depends heavily on carpenter skill, must match Velux warranty spec exactly (corner joinery, waterproofing prep, flashing integration), higher risk of installation error.
What Velux warranty specifically requires
Velux publishes detailed curb-mounting specifications that must be followed for the manufacturer's leak warranty to remain valid. Key requirements:
- Curb height minimum 4 inches above finished roof surface
- Corner joinery — mitered or lapped, no butt joints
- Waterproofing membrane wrapped up and over the curb top before skylight installation
- Correct base flashing detail — factory Velux flashing kits are engineered for specific curb dimensions
- Proper counter-flashing at any tie-in to adjacent roofing materials
Custom curbs can meet all of these requirements — but only if the installer knows to detail them correctly. This is why we often specify prefab for straightforward projects and custom only when project geometry requires it.
When to use which on LA flat roofs
Practical decision tree:
- Standard Velux size, straightforward roof: prefab curb. Faster install, warranty automatically met, minimal risk.
- Standard Velux size, complex roof geometry (multiple planes meeting near the skylight): custom curb — allows integration of drainage cricket and specific counter-flashing.
- Non-standard skylight size or custom fabrication: custom curb — no prefab option available.
- Multiple skylights in a row: custom curb assembly with continuous flashing — cheaper per unit than multiple prefabs and better waterproofing.
- Historic or heritage restoration: custom curb — allows matching original detailing and materials.
Cost comparison
- Prefab Velux curb (single M04 size): $180-$350 material + 1-2 hours labor
- Custom wood curb (single equivalent size): $60-$150 material + 4-8 hours labor
- Break-even: for a straightforward single-skylight install, prefab is often cheaper total once labor is counted. Custom pulls ahead on multi-skylight installs or non-standard sizes.
What Halo Build Co typically specifies
On LA flat-roof projects, we default to prefab Velux curbs for single-skylight installations at standard Velux sizes. We use custom curbs when project geometry, non-standard sizes, or multi-skylight arrays make prefab impractical. Every custom curb we build follows Velux warranty specifications and includes flashing integration with the surrounding roof assembly.
See our Velux flat-roof skylights guide or FCM fixed skylight guide for product specifics.
Frequently asked questions
What's the difference between prefab and custom skylight curbs?
Prefab curbs are factory-built kits sized to standard Velux skylight openings — pre-assembled with proper corner joinery, factory waterproofing membrane, and matched flashing pieces. Custom wood curbs are site-built by the roofing crew from 2x lumber. Prefab is faster and lower-risk for standard Velux sizes; custom is required for non-standard sizes, complex roof geometry, or multi-skylight arrays.
Does the curb type affect the Velux warranty?
Yes — Velux's leak warranty requires specific curb detailing (4" minimum height, mitered or lapped corner joinery, waterproofing membrane wrapped up and over the curb top, correct base flashing integration). Prefab curbs meet these requirements automatically. Custom curbs can meet them but only if the installer knows to detail them correctly — this is why installer skill matters more with custom.
Which is cheaper — prefab or custom curbs?
For a straightforward single-skylight install, prefab is often cheaper total once labor is counted (higher material cost, but much lower labor). Custom curbs pull ahead on multi-skylight arrays (single continuous flashing serves multiple skylights) or non-standard sizes. Prefab: $180-$350 material + 1-2 hours labor per skylight. Custom: $60-$150 material + 4-8 hours labor per skylight.