How to Measure Rooflights Correctly: A Practical Guide

How to Measure Rooflights Correctly: A Practical Guide

If you’re planning to install a new or replacement rooflight, taking accurate measurements is essential for achieving a perfect fit and a smooth installation. Whether you’re upgrading an old rooflight or adding more natural light to your home, proper measuring will save you time and effort. SM Glastech offers a variety of fitting guides and expert advice to simplify the process. You can also call 07974205492/02087400040 to speak directly with a professional installer.

Complete guide of measuring for a rooflight

To measure for a rooflight, accurately measure the structural opening (the timber hole) in millimeters at multiple points to find the smallest dimensions, ensuring you include the thickness of any drywall. Most suppliers require the external kerb/upstand size (often 50–100mm larger than the opening). Always factor in a 5mm tolerance for the final, installed size.

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.

Include Upstands: For flat roofs, ensure the upstand is at least 150mm high for water runoff.

Account for Overlap: The rooflight glass typically extends 50-100mm beyond the structural opening for a weatherproof seal.

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?

To determine the right size skylight, calculate 15–20% of your room’s floor area for general, balanced lighting, or 25–30% for rooms needing more light. The skylight should be sized based on room dimensions, roof structure (rafters), and orientation, with north-facing rooms often requiring larger units for consistent, soft, and balanced lighting.

Key Considerations for Sizing

Floor Area Rule of Thumb: For optimal light, the total skylight area should be roughly 15–20% of the total room floor area (e.g., a 20𝑚2 room could use 3m2 to 4m2 of glass).

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.

Roof Structure: The skylight size is often constrained by the distance between roof rafters (commonly 400 to 600mm spacing). While wider units are possible, they require more structural modification (cutting rafters).

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

1. Measure the floor space: Calculate the square footage/meterage of the room.
2. Calculate the 15-20%: Aim for this to get the total glazed area needed.
3. Check Roof Framing: Determine if you can fit the unit between existing rafters or if you need to cut them.
4. Consider Purpose: A study may need less light (5–10% rule) than a kitchen or living room.

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**

1. Deck-Mounted Skylights: Installed directly on the roof deck, parallel to shingles. Common in newer homes.
2. Curb-Mounted Skylights: Placed on a raised frame that diverts water, sitting above the roofline.
3. Tubular Skylights: Small, cylindrical units that channel light through a tube, ideal for small spaces.

**Replacing a Skylight: Measurement Steps**

1. Find Identification Labels: Look for labels on the skylight’s perimeter or spacer bars for details.
2. Measure Exterior Dimensions:

– Deck-Mounted: Measure the length and width of the glass pane.

– Curb-Mounted: Measure outside curb and glass dimensions. Record accurately.

3. Measure Internally: If accessible, measure the drywall opening, width, length, and the distance from the ceiling to the skylight base.
4. Check for Squareness: Measure diagonals from corner to corner. Equal measurements indicate a square opening.

**Installing a New Skylight: Measurement Steps**

1. Determine Size and Placement: The skylight should be 5% to 10% of the room’s area. Position south-facing skylights for maximum light.
2. Identify Structural Elements: Mark rafters or joists that may affect placement. Standard spacing is usually 40 cm or 60 cm.
3. Plan Opening Size: Choose a skylight that fits between rafters. Prefabricated options are generally less expensive.
4. Assess Roof Pitch: Use a manometer to measure vertical distance for selecting the right flashing kit.

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.
• Upstand Requirement: The upstand must be built to the correct size (usually 150mm high) to allow proper drainage.
• Tolerance: Ensure the opening allows for a 5mm tolerance around the unit for a proper fit.

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:

1. 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.

2. 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.
• Allow for Tolerance: It is common to subtract 10mm from the total opening to leave room for fitting and adjustments

3. Curb and Structure Requirements

• Minimum Pitch/Slope: Even on a “flat” roof, the skylight must sit at a minimum pitch (often 3-5 degrees) to allow water to drain and prevent pooling, which can cause leaks.
• Upstand Height: The curb (upstand) should be at least 150mm high to prevent water from pouring over the top during heavy rain.
• Width: The timber upstand should typically be around 70mm to 85mm wide.

4. 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).
• Safety Glazing: If the skylight is located close to a floor (e.g., in a loft) or is walk-on, it must have laminated safety glass, which is usually mandated by building regulations.
• 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.

O-lite round rooflight

Measuring a Round Rooflight: To measure a round rooflight like the O-lite, follow these key steps:

Key Points

1. Diameter: Measure the diameter of the opening, not width or length.
2. Dimensions: Confirm if you need external (outer edge) or internal (structural opening) dimensions.
3. Upstand Height: Ensure the upstand is at least 150mm high for waterproofing. It should also have a slope of 3 to 5 degrees to prevent water pooling.
4. Replacement: Measure the flange-to-flange width of the existing dome, not just the roof opening.
5. Check for Obstructions: Make sure nearby vents or pipes won’t restrict dome size.

Measurement Steps

1. **Diameter**: Measure the outer edge of the upstand across the center at several points.
2. **Upstand Height**: Measure from the roof surface to the top of the upstand.
3. **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:

1. Technical Factors

   Pitch Compatibility: Ensure the window matches your roof pitch—most fit 15° to 90°.

Waterproofing: Use appropriate flashing kits to prevent leaks.

Height: Position the bottom edge 90–120 cm from the floor and the top edge around 200 cm.

   Support: Cutting rafters may require roof reinforcement.

   Insulation: Opt for low U-values and consider triple glazing.

2. Design Choices

Opening Mechanism:

• Centre-Pivot: Rotates 180 degrees for easy cleaning.
• Top-Hung: Suitable for escape routes.

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.

3. Safety and Regulations

Planning Permission: Allowed if it doesn’t protrude more than 150 mm.

Conservation Areas: Use specific windows to preserve historical aesthetics.

Glazing: Choose laminated or toughened glass for safety.

   Escape Routes: Ensure compliance with size requirements.

4. Comfort and Maintenance

• Solar Control: Use blinds or heat-reducing glass for south-facing windows.
• Positioning: Aim for windows that cover 15-20% of the room’s floor area.

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:

1. Installation & Structure:

• Upstand: Build a timber or concrete kerb at least 150mm high (some at 115mm) for waterproofing.
• Roof Pitch: Minimum of 3°–5° pitch to prevent water pooling; 10° for spans over 200cm.
• 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.

2. Operation & Mechanics

• Motorization: Use concealed 24V electric motors for operation.
• Safety Features: Include automated closing with rain sensors and manual overrides.
• Maintenance: Annual servicing to check motors, seals, and clean tracks.

3. 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.

4. Planning & Security:

• Permitted Development (UK): Allowed if not projecting more than 150mm above the roof plane.
• Security: Must conform to standards like LPS 2081 or PAS 24 if easily accessible.
• Acoustics: Use laminated glass for sound reduction in urban settings.

5. 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

1. Accurate Measurements: Know the structural opening, external kerb size, and internal dimensions.
2. Upstand Construction: Use an insulated timber upstand at least 150 mm above the roof covering.
3. Optimal Pitch: Maintain a pitch of 3 to 5 degrees to prevent water pooling.
4. Professional Installation: Consider hiring a professional to ensure proper installation.

Installation Steps

1. Prepare the Opening: Make it 30 mm to 50 mm larger than internal dimensions and ensure it’s secure.
2. Build the Upstand: Construct it with a 5-degree pitch and cover with a waterproof membrane.
3. Apply Sealant: Use high-quality, weather-resistant sealant.
4. Position and Secure: Center the rooflight and secure it without over-tightening.
5. Install Flashing: Add flashing kits and seal the edges.
6. Test for Leaks: Conduct a water test to check for leaks.

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.