At the supplier, the low-profile skylight looks sleeker than the unit on a raised box. "Does that make it better?" the owner asks. Only if it fits the roof. Curb-mounted and deck-mounted units suit different slopes, openings and flashing systems; the mounting type is not a quality league table.
For replacement, fitting the visible hole is only the start. A unit can be the right-looking size and still be wrong for the roof pitch or existing curb.
Measure the Roof Before Choosing the Unit
Roof slope is a product-selection input, not a detail to resolve after delivery. Manufacturers give approved pitch ranges and flashing requirements for specific models. A model intended for a pitched roof should not be assumed suitable for a low-slope addition merely because it fits the hole.
Use the VELUX Canada installation instructions for the actual product under consideration. The relevant documents distinguish mounting types and flashing systems. Ask the installer to identify the combination in writing.
Dave puts the attractive brochure aside long enough to name the roof covering. Shingles, metal and low-slope membrane need different connections. The logo can match perfectly while the generic flashing kit is wrong for the roof underneath.
The Hole You See Is Not Necessarily the Size You Buy
The owner has measured the drywall opening carefully, which is useful information but may be the wrong ordering dimension. Trim can hide the structural opening; a curb's outside dimension differs from the daylight opening below. Mike checks the selected model's required measurements before anyone converts that careful indoor measurement into an expensive outdoor mismatch.
The installer needs the dimensions required by the selected model: the rough opening for a deck-mounted arrangement, or the relevant curb dimensions and construction for a curb-mounted unit.
This affects the finished price. A replacement that fits the existing arrangement may involve little interior disturbance. A changed opening can add framing, shaft insulation, trim and painting. Those are different projects even when both quotes say skylight replacement.
Both mounting styles need a complete water route
This plan view shows the flashing relationship around the opening. Curb-mounted and deck-mounted units use different assemblies, so their parts and installation details are not interchangeable.
Schematic view. Shows relationships, not installation dimensions.
The Curb Is Part of the Roof
The old curb gets its own inspection. Sound timber, correct dimensions and suitability for the new installation matter; added roof insulation also changes its height above the finished surface. A new glass unit does not grant the box beneath it a fresh start.
The curb-to-roof flashing and the skylight-to-curb connection perform different jobs. Both need attention. Replacing the glass unit on a failing curb does not rebuild the underlying roof detail.
Choose the Roof Fit First, Then the Room Features
Once roof fit is settled, bring back the brochure. Opening operation, blinds, cleaning access and daylight are worth choosing carefully. Pat still asks how someone will operate a venting unit out of reach, and how the shaft will be insulated and air sealed. The sensible selection should work from both sides of the ceiling.
The reroofing timing guide explains when addressing both at once makes sense.
Confirm the complete overhead-glazing construction, including the room-side pane, as well as the mounting style.