Measuring for Roof Lanterns: Get the Aperture Right First Time

Measuring for roof lanterns correctly is the single most important step before placing any order. Unlike a standard window that can be shimmed, packed, or trimmed on site, a bespoke aluminium roof lantern is manufactured to your stated dimensions and delivered ready to install. If those dimensions are wrong, the unit does not fit and there is no return or refund on a custom-made product.

This guide tells you exactly what to measure, from where, and what tolerances apply.

1. The Four Reference Points You Must Understand

Before you take a single measurement, you need to understand what you are measuring - because "the size of the roof lantern" can mean four entirely different things depending on which part of the system you are referencing.

Reference Point	What It Describes	Used For
Structural opening	The cut hole through the roof deck (joists to joists)	Structural engineer calculation
Internal kerb dimension	The inside face of the upstand kerb	What you see from inside the room
External kerb dimension	The outside face of the upstand kerb	What you order from the supplier
Overall lantern footprint	The outer edge of the base frame including any overhang	Checking roof membrane clearance

2. Step-by-Step: How to Measure the External Kerb

What you need: Steel tape measure (not a cloth tape), pencil, notepad, a second person to hold the tape on larger openings.

Step 1 - Measure the external kerb width

Place the tape measure on the outside face of the kerb on one side. Pull it across to the outside face of the opposite kerb wall. Record this measurement in millimetres. Take it at three points: front edge, middle, and rear edge. If all three readings match, your kerb is square. If they differ by more than 5mm, your kerb may not be square — note this before ordering.

Step 2 - Measure the external kerb length

Repeat across the other axis. Again take three readings along the length. Record the shortest dimension if readings vary — you always order to the smallest measurement to ensure the lantern base frame fits without forcing.

Step 3 - Check the kerb is level

Place a spirit level across the top face of the kerb in both directions. A roof lantern base frame requires a level bearing surface. If the kerb is out of level by more than 5mm across its full dimension, have your builder pack or plane it level before the lantern arrives on site.

Step 4 - Check the kerb is square

Measure both diagonals of the kerb from corner to corner in both directions. If both diagonal measurements are equal, the kerb is square. If they differ by more than 10mm, the kerb is out of square and the lantern base frame will not sit correctly. Correct the kerb before ordering.

Step 5 - Measure and record the kerb upstand height

The upstand height - the vertical dimension of the kerb above the finished roof surface - must be a minimum of 150mm. Measure this at all four sides. If any side is below 150mm, the kerb must be built up before installation proceeds.

3. The Tolerances That Protect You

Roof lantern manufacturers build a tolerance into their base frame dimensions to allow for minor site variations. For the Brett Martin Roof Lantern, the tolerance is:

Lantern base frame = ordered size +20mm / -10mm

This means a lantern ordered to a 2,000mm x 1,500mm external kerb dimension will fit a kerb that measures anywhere between 1,990mm x 1,490mm and 2,020mm x 1,520mm. It will not fit a kerb that is 2,050mm x 1,550mm - the frame will be too small and the overlap will be insufficient for a weathertight seal.

The practical rule is: always order to the smallest dimension you measure. If your kerb reads 1,995mm on one axis and 2,010mm on the other axis due to minor variation, order 1,995mm. The tolerance absorbs the difference; ordering to the larger number risks the frame falling short of the kerb edge.

For the Wendland Roof Lantern, confirm the specific tolerance with our team before ordering, as this varies by product line.

4. The 25% Glazing Rule: Sizing Within Building Regulations

Before finalising your dimensions, confirm your proposed roof lantern size complies with the 25% glazing rule under Approved Document L of the 2026 Building Regulations. The total area of all windows, doors, and rooflights in an extension cannot exceed 25% of the total floor area without SAP calculations to demonstrate overall energy compliance.

Example calculation:

Extension floor area: 4,000mm x 3,000mm = 12 m²
Maximum glazed area: 25% of 12 m² = 3 m²
Proposed roof lantern: 2,000mm x 1,400mm = 2.8 m² ✓ (compliant)
Proposed roof lantern: 2,500mm x 1,800mm = 4.5 m² ✗ (requires SAP calculation)

If your proposed lantern size exceeds the 25% threshold, your architect or SAP assessor will need to demonstrate through energy calculations that the extension fabric compensates elsewhere — typically through enhanced wall or floor insulation values. Choosing a more thermally efficient glazing unit, such as one meeting a U-value of 1.0 W/m²K rather than the 1.2 W/m²K minimum, also assists this calculation.

Conclusion:

A wrongly measured roof lantern is an expensive lesson. At Skylights Roof Lanterns, our team checks every order against the information provided and will flag any dimension that looks inconsistent before manufacturing begins. Browse our roof lanterns collection or call 0204 538 3079 and email sales@skylights-rooflanterns.co.uk if you want a second opinion on your measurements before ordering.

Frequently Asked Questions:

How do I measure a roof opening for a new roof lantern?

Measure the external face of the upstand kerb—not the structural hole, not the internal ceiling opening. Take three readings on each axis (front, middle, rear) and order to the smallest figure. Both diagonal measurements must be equal to confirm the kerb is square. The kerb upstand must be a minimum of 150mm above the finished roof surface before a roof lantern can be fitted.

What is the difference between internal and external roof lantern dimensions?

The external dimension is the measurement across the outside face of the upstand kerb—this is the number you order. The internal dimension is the visible opening seen from inside the room, which is always smaller because the kerb wall has physical thickness on each side. Ordering to internal dimensions rather than external is the most common measuring error on roof lantern projects and results in a unit that is too large to fit the kerb.

What tolerance do roof lanterns have on their base frame dimensions?

The Brett Martin Roof Lantern has a base frame tolerance of +20mm / -10mm on the ordered external kerb dimension. This means the lantern will fit a kerb that is up to 20mm larger or 10mm smaller than the stated order size. Always order to the smallest measurement you take across the kerb to ensure the frame seats correctly with adequate overlap for a weathertight seal.

Does the size of a roof lantern need Building Regulation approval?

Yes. The size of a roof lantern affects compliance with Approved Document L's 25% glazing rule, which limits total window, door, and rooflight area to 25% of the extension floor area. Oversized roof lanterns require SAP calculations to demonstrate overall energy compliance. All roof lantern installations also require Building Control approval regardless of size.

Can I order a roof lantern before the kerb is built?

Yes—and on new build extensions this is the recommended approach. Choose your preferred standard lantern size first, then instruct your builder to construct the kerb to that exact external dimension. This eliminates the risk of the kerb being built to a non-standard size that does not match any available product, which would force a bespoke order at higher cost and longer lead time.