The Rooflight Centre Guide to Measuring Curb Sizes

Measuring curbs for rooflights can be quite confusing for the layman.

It can be hard to understand the difference between phrases such as ‘structural opening’ and ‘overall finished curb’.

Both are key during the fitting process, but confusing the two means that your new rooflight won’t fit.

The Finished Curb

The finished curb is the typical dimension that most manufacturers will require to manufacture a tailor-made rooflight, as this is the ledge onto which the rooflight frame will lay on. The overall finished curb is the complete width and length, including any finish or waterproof membrane, as explained in the image below:

a visual example of the finished curb

It’s also a good idea to advise as to the width of the curb as this varies from rooflight to rooflight. For example, our Quik2Fit rooflight will require, as standard, a 75mm wide curb, but our EcoGard Opening Rooflights requires 100mm.

Similarly, knowing where to measure from and to can vary depending on whether the rooflight is ‘stand-alone’ as above or abutting a wall at the top or side.

an abutted window against a brick wall

Our Guide

We’ve put together some diagrams below to display the dimensions required for fitting rooflights in a variety of scenarios.

It’s essential to refer to the manufacturer’s installation details before constructing a curb in order to ensure the best appearance and fit.

 

Type A: No Wall Abutment

type a: no wall abutment

(View From Outside)

 

A = O/A Finished Curb Length in mm

B = O/A Finished Curb Width in mm

C = Width of Curb (Incl. Roof Finish) in mm

↓ = Direction of Fall (Min 5° Recommended) Over Shortest Dimension

 

Type B: Wall Abutment Top

Type B: Wall Abutment Top

(View From Outside)

 

A = O/A Finished Curb Length in mm

B = O/A Width from Wall to Outside Finished Curb in mm

C = Width of Curb (Incl. Roof Finish) in mm

↓  = Direction of Fall (Min 5° Recommended) Over Shortest Dimension

 

Type C: Wall Abutment R/H Side

type c: Wall Abutment R/H Side

(View From Outside)

 

A = O/A Length from Wall to Outside Finished Curb in mm

B = O/A Width from Wall to Outside Finished Curb in mm

C = Width of Curb (Incl. Roof Finish) in mm

↓ = Direction of Fall (Min 5° Recommended) Over Shortest Dimension

 

Type D: Wall Abutment L/H Side

Type D: Wall Abutment L/H Side

(View From Outside) 

 

A = O/A Length from Wall to Outside Finished Curb in mm

B = O/A Width from Wall to Outside Finished Curb in mm

C = Width of Curb (Incl. Roof Finish) in mm

↓ = Direction of Fall (Min 5° Recommended) Over Shortest Dimension

 

Type E: Wall Abutment Top & One Side R/H

(View From Outside)

 

A = O/A Length from Wall to Outside Finished Curb in mm

B = O/A Width from Wall to Outside Finished Curb in mm

C = Width of Curb (Incl. Roof Finish) in mm

↓ = Direction of Fall (Min 5° Recommended) Over Shortest Dimension

 

Type F: Wall Abutment Top & One Side L/H

Type F - Wall Abutment Top & One Side L/H

(View From Outside)

 

A = O/A Length from Wall to Outside Finished Curb in mm

B = O/A Width from Wall to Outside Finished Curb in mm

C = Width of Curb (Incl. Roof Finish) in mm

↓ = Direction of Fall (Min 5° Recommended) Over Shortest Dimension

 

Type G: Wall Abutment Top & Both Sides

(View From Outside)

 

A = O/A Length Between Walls in mm

B = O/A Width from Wall to Outside Finished Curb in mm

C = Width of Curb (Incl. Roof Finish) in mm

↓ = Direction of Fall (Min 5° Recommended) Over Shortest Dimension

 

Type H: Wall Abutment Two Sides

Type H - Wall Abutment Two Sides

(View From Outside)

 

A = O/A Length Between Walls (mm)

B = O/A Finished Curb Width (mm)

C = Width of Curb (Incl. Roof Finish) (mm)

↓ = Direction of Fall (Min 5° Recommended) Over Shortest Dimension

 

 

For more information about what we’ve covered in this blog, or any other product in our range call on 020 3695 2308 or drop us an email at rooflights@lonsdalemetal.co.uk.