Roof pitch and roof light installation

Pitched roofs introduce geometry that flat decks avoid: the covering runs at an angle, but the roof light glass is almost always horizontal for drainage, standard build-ups and interior appearance. Bridging that difference is the job of the curb (an upstand built to the roof angle) and the roofing flashings that dress the junction. This guide walks through measurement, build sequence and the coordination between roofer, carpenter and glazier.

Step 1 — Confirm roof pitch and product suitability

Measure the roof pitch in degrees or as a ratio (for example 40° or 35°). Standard frameless roof lights suit low to medium pitches when mounted on a correctly detailed curb with horizontal glazing. Very steep roofs may need longer cheek faces, additional flashing steps or a bespoke approach.

Decide early:

• Framed or frameless — affects curb height and visible edge at ceiling level.
• Thermal requirement — insulated build-up depth sets curb width.
• Ceiling finish — internal lining returns must meet the curb inside the room.

Order the glazed unit to the bearing opening at the top of the curb, not the rough structural hole in the rafters.

Step 2 — Set out the structural opening

Mark the roof opening allowing for trimmers, ridge and eaves layout. The structural aperture is larger than the glazed unit — the curb reduces it to order size. Check:

• Rafter trimming and headers carry loads around the opening.
• No clash with ridge, valley or hip within minimum distances required by the covering manufacturer.
• Internal ceiling aperture aligns with where the curb will finish.

Produce a simple section drawing: pitch angle, curb height, covering thickness, glass position. Share with the glass supplier before manufacture.

Step 3 — Build the curb

The curb is typically timber, sheathed and lined, fixed to trimmed rafters. It has:

• Upslope and downslope faces following the roof plane.
• Side cheeks vertical or angled per detail.
• Top bearing plane level in both directions — this is where the unit sits.

Critical checks:

• Bearing plane level — not parallel to the ground if the roof is twisted.
• Square plan at the bearing level.
• Height sufficient for cover flashings to tuck under the surrounding tiles or slates on the upslope side.

Allow the curb structure to dry before dressing membranes. Damp timber shrinks and moves sealant bonds.

Step 4 — Roof covering sequence

Coordination with the roofer prevents leaks and rework:

1. Install underlay overall; cut and dress around the curb footprint.
2. Set battens — stop or step at the curb per flashing detail; do not assume standard batten courses pass through unchanged.
3. Dress base flashing and side aprons up the curb before completing adjacent slates or tiles where the detail requires it.
4. Complete covering on the downslope and sides; leave upslope courses short until top flashing is ready.
5. Install top flashing or soaker arrangement tying into the upslope covering.
6. Fit counter-flashings or cover pieces supplied with framed kits.

Membrane-only or single-ply pitched roofs use welded dressings instead of tiles — same principle: curb first, dressings continuous, glass last.

Step 5 — Install the roof light unit

When the curb is weathertight and dimensions verified:

1. Position setting blocks per the manufacturer’s layout.
2. Lower the unit onto the curb — use suction lifters; never drag glass across the bearing surface.
3. Fix clips, cappings or frame screws to torque guidance — uneven clamping twists the unit.
4. Apply perimeter sealant at external throats and internal ventilation-compliant details.
5. Complete any remaining tile cuts and ridge proximity work.

Inspect glass drainage after install — an out-of-level curb causes ponding and seal stress.

Step 6 — Internal finish

Return linings to the curb and maintain the manufacturer’s ventilation gap. Insulate curb cheeks to match the roof build-up.

Pitched roof pitfalls

• Ordering glass before the curb is built — site variances force remakes.
• Batten layout ignoring curb width — courses do not align at aprons.
• Skipping the upslope top flashing — the obvious leak path in driven rain.
• Twisted curb — glass rocks on setting blocks; seals fail asymmetrically.
• Walking on surrounding tiles before flashings are complete — disturbs sealant and cracks covering.

When to call the manufacturer

Contact your supplier when pitch exceeds their standard detail, when the unit is near a ridge or valley, or when the covering is unusual. Manufacturers can advise on curb dimensions before you commit the structure.

Every Vant Glass roof light is made to order in Britain, backed by a 20-year guarantee and free UK mainland delivery. Configure frameless or framed sizes in the online calculators or call 03330 902 592.