How to Measure Rooflights Correctly: A Practical Guide

Complete guide of measuring for a rooflight

Key Steps for Measuring Rooflights

• Identify the Measurement Type:
• Structural Opening (Most Common): Measure the raw hole between timber joists/trimmers.
• External Kerb Dimension: Measure the outside width and length of the upstand, including waterproofing/membrane.
• Replacement: Measure the aperture, not the old, removed unit.
• Measure Precisely: Measure in millimeters (mm) at several points (width and length) to account for any sagging or unevenness, and use the smallest measurement.

Common Pitfalls

• Confusing Sizes: Do not confuse the “hole in the roof” size with the “glass” size or the “external kerb” size.
• Neglecting Thickness: Remember to include the thickness of internal plasterboard when measuring from inside.
• Not Checking Squareness: Measure diagonally to ensure the opening is square.
• Note: For specialized, non-standard rooflights, always consult the manufacturer’s specific measurement guide.

How do I know what size skylight I need?

Key Considerations for Sizing

• Room Size & Shape: Large or L-shaped rooms may benefit from multiple smaller, strategically placed skylights to distribute light evenly rather than one giant, intense light source.
• Direction (Orientation):
• North-facing: Receives consistent, soft light; larger, more numerous windows can be used.
• South-facing: Provides intense, direct light, requiring smaller, fewer, or specially glazed windows to avoid overheating.
• East/West-facing: Captures strong morning or afternoon sun, respectively.
• Existing Window Replacement: If replacing, look for a serial number/code on the frame (often hidden on the inner sash) to identify the exact, manufacturer-matched size.

Steps to Take

• Measure the floor space: Calculate the square footage/meterage of the room.
• Check Roof Framing: Determine if you can fit the unit between existing rafters or if you need to cut them.

Always consult a professional if structural modifications to the roof are required.

Step-by-Step Guide to Measuring a Skylight

**Essential Tools for Roof Work**

Before going to the roof, make sure you have the following:

• A sturdy ladder that safely reaches the roof. Ensure it’s stable and on level ground.
• A metal tape measure for accurate measurements—avoid stretchy tapes that can be misleading.
• A notebook for taking notes instead of relying on memory.
• A spirit level to check roof pitch and ensure surfaces are level.
• Safety gear, including non-slip shoes and protective eyewear.
• Your phone for taking photos to back up measurements and communicate with suppliers.

**Special Considerations for measuring different Types of Skylights**

• Deck-Mounted Skylights: Installed directly on the roof deck, parallel to shingles. Common in newer homes.
• Curb-Mounted Skylights: Placed on a raised frame that diverts water, sitting above the roofline.
• Tubular Skylights: Small, cylindrical units that channel light through a tube, ideal for small spaces.
• Replacing a Skylight: Measurement Steps**
• Find Identification Labels: Look for labels on the skylight’s perimeter or spacer bars for details.
• Measure Exterior Dimensions:
• Deck-Mounted: Measure the length and width of the glass pane.
• Curb-Mounted: Measure outside curb and glass dimensions. Record accurately.
• Measure Internally: If accessible, measure the drywall opening, width, length, and the distance from the ceiling to the skylight base.
• Check for Squareness: Measure diagonals from corner to corner. Equal measurements indicate a square opening.
• Installing a New Skylight: Measurement Steps**
• Plan Opening Size: Choose a skylight that fits between rafters. Prefabricated options are generally less expensive.

Measuring With an Upstand (Kerb)

• External Dimension: Measure the length and width of the outside edge of the existing upstand, including any roof membrane or flashing.
• Existing Kerb: If replacing, the new skylight size usually matches the external dimensions of the current kerb.
• Pre-formed Upstand: If purchasing a unit with its own insulated upstand, measure the structural opening in the roof.

Measuring Without an Upstand (New Opening)

• Structural Opening: Measure the exact size of the hole (aperture) in the roof, from timber to timber.

Key Considerations

• Squareness: Ensure the opening is square by measuring diagonally from corner to corner.
• Roof Pitch: Use a spirit level to check the roof pitch, as a minimum slope is needed for drainage.
• Positioning: For new installations, measure the roof aperture precisely to prevent leakage and ensure structural support.

Special Considerations for Different Skylight Types

Fixed Flat Skylight

Measuring a fixed flat skylight requires extreme precision to ensure a watertight seal, as flat roofs are prone to water pooling. The most critical measurements are typically the structural opening (timber-to-timber) or the outside curb dimensions, depending on the manufacturer’s requirements.

• Here are the key considerations for measuring:

Identify the Type of Measurement

• Structural Opening (Timber-to-Timber): This is the actual hole cut into the roof structure. Measure this if you are building a new curb or if the manufacturer asks for it.
• Finished Curb/Upstand Size: This is the measurement of the raised frame that the skylight rests on, after insulation and waterproofing membranes have been applied. Many manufacturers require this to be exact.
• Replacing Existing: If replacing, measure the existing, finished curb, not the old glass. Measure both the inner and outer edge of the curb to ensure the new frame will cover it.

Crucial Measurement Steps

• Multiple Measurements: Measure width and length at three different points (top, middle, bottom) to account for an un-square, settled roof.
• Use the Smallest Dimension: If there is any variation, use the smallest measurement to ensure the unit fits in the opening.
• Check for Squareness: Measure diagonally from corner to corner. If the two diagonal measurements are not equal, the opening is not square, which may require shims or structural adjustments.

Curb and Structure Requirements

Other Key Considerations

External vs. Internal Size: Confirm if the manufacturer provides the “daylight size” (visible glass) or the “outside frame size” (total unit footprint).

• Accessibility: Use a sturdy, stable ladder and ensure you have secure footing on the roof.
• Identify Labels: Look for a metal tag on the existing frame (often in the bottom-right corner) for model and size details, allowing you to order an exact replacement.
• Common Mistake: Measuring the old unit’s glass instead of the curb it sits on. The curb is the true, structural dimension required.
• Measuring a Round Rooflight: To measure a round rooflight like the O-lite, follow these key steps:

Key Points

• Diameter: Measure the diameter of the opening, not width or length.
• Dimensions: Confirm if you need external (outer edge) or internal (structural opening) dimensions.
• Replacement: Measure the flange-to-flange width of the existing dome, not just the roof opening.
• Check for Obstructions: Make sure nearby vents or pipes won’t restrict dome size.

Measurement Steps

• Diameter**: Measure the outer edge of the upstand across the center at several points.
• Upstand Height**: Measure from the roof surface to the top of the upstand.
• Confirm Measurements**: Take a photo of the diameter measurement for accuracy when ordering.

Common Mistakes

Overlooking roof membrane thickness.

Confusing “daylight size” with “external kerb size.”

Measuring only one point on potentially warped domes.

• Pitched Roof Windows: Key Considerations**

Pitched roof windows improve light and ventilation in lofts, fitting pitches from 15° to 90°. Here are the essentials for installation:

Technical Factors

• Pitch Compatibility: Ensure the window matches your roof pitch—most fit 15° to 90°.
• Waterproofing: Use appropriate flashing kits to prevent leaks.
• Support: Cutting rafters may require roof reinforcement.
• Insulation: Opt for low U-values and consider triple glazing.

Design Choices

• Opening Mechanism:
• Centre-Pivot: Rotates 180 degrees for easy cleaning.

Materials

• Moisture-Resistant (PVC): Best for kitchens and bathrooms.
• Natural Pine: Ideal for living areas.
• Automation: Electric or solar options work well for hard-to-reach spaces.

Safety and Regulations

• Glazing: Choose laminated or toughened glass for safety.

Comfort and Maintenance

• Solar Control: Use blinds or heat-reducing glass for south-facing windows.
• Sliding rooflights: Key Considerations

Sliding rooflights, including motorized and sliding-over-roof designs, require specialized planning for structural, mechanical, and safety needs. These rooflights enhance natural light, ventilation, and roof terrace access.

• Key Considerations:
• Installation & Structure:
• Weight Management: Ensure the roof can support heavy glass panels, especially for sliding-over-roof models.
• Sliding Space: Require a flat, clear roof area for retraction.

Operation & Mechanics

• Safety Features: Include automated closing with rain sensors and manual overrides.

Glass Safety & Specification

• Safety Glass: Use toughened or laminated glass; inner pane should be laminated to prevent falling debris.
• Heat Soak Testing: Use heat-soak-tested toughened glass to reduce breakage risk.
• Thermal Performance: Employ thermally broken frames with high-performance double or triple glazing.
• Planning & Security:
• Acoustics: Use laminated glass for sound reduction in urban settings.
• Maintenance:
• Cleaning: Wash frames with soapy water and remove debris from tracks regularly.
• Coatings: Consider low-maintenance options for easier cleaning.

Tips for Ensuring a Perfect Fit

To ensure a perfect fit for a rooflight, follow these steps:

Key Factors

• Accurate Measurements: Know the structural opening, external kerb size, and internal dimensions.
• Optimal Pitch: Maintain a pitch of 3 to 5 degrees to prevent water pooling.
• Professional Installation: Consider hiring a professional to ensure proper installation.

Installation Steps

• Build the Upstand: Construct it with a 5-degree pitch and cover with a waterproof membrane.
• Apply Sealant: Use high-quality, weather-resistant sealant.
• Position and Secure: Center the rooflight and secure it without over-tightening.
• Install Flashing: Add flashing kits and seal the edges.

Common Mistakes

Ignoring pitch can cause water accumulation.

Incorrect measurements may lead to gaps.

Low-quality sealant can lead to leaks.

Lack of insulation can cause condensation.

Tips

Get the upstand ready according to the size needed and then buy the rooflight. Alternatively you can buy the rooflight and accordingly prepare the upstand to the required sizes.

Use safety equipment for handling.

Review installation guides for specifics.