Design Considerations

Tapered Insulation provides lots of flexibility in installation, regardless of insulation type the product is generally considered easy to handle and cut on site where necessary.

Options for the scheme configuration (the layout in plan view) are numerous. It could be said that there is no right or wrong layout, only the end result of positive drainage being the desired outcome… But consideration should be made to the theme of the tapered scheme!

This decision can be influenced by adjoining or abutment details, the use of the roof or the roofing contractor’s preference. The required U.Value and the distance of fall will also impact the decision on scheme configuration, often numerous scheme layouts need to be presented to fully consider the best option.

Design consideration & Impact of Scheme Design

There are several parameters to consider when designing a tapered insulation scheme that could have a significant impact on the rest of the build.

  • Design Fall of Insulation
  • Direction of Fall
  • Quantity of drainage points
  • Position and distance between drainage points
  • Upstand Heights
  • Intended use of Roof Area, i.e. a Terrace.
The table below shows just some of the variants that could be designed for this example roof area. There are 4 different scheme designs for an identical roof area which utilize different gradients of fall and fall directions. Each design achieves a 0.13W/m²k U Value calculated in accordance with BS EN ISO:6946:2017 Annex E.

The table shows the variance in Insulation thickness, area of flat board zones and the heat loss at the minimum thickness of insulation (the flat gutter/ sump zones).

As you can see, there can be huge difference on each parameter compared to other design variations; some of which may be viable, others not.

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Falls:

1:60

Gutter Falls:

1:60

Min Thickness (mm):

50

Max Thickness (mm):

250

Heat Loss at Min thickness (W/m2K):

0.73

Area of gutter/sump (m2):

2.88

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SELECT A DESIGN TO SEE THE DIFFERENCES IN PERFORMANCE
Design Fall Gutter Falls Min Thickness (mm) Max Thickness (mm) Heat Loss at Min Thickness (W/m2K) Area of gutter/sump (m2) Notes
1 1:60 Flat 170 230 0.19 23.1 Short Fall Distance
2 1:40 Flat 150 240 0.19 23.1 Short Fall Distance
3 1:60 1:60 50 250 0.73 2.88
4 1:40 1:40 30 330 0.73 2.88
5 1:60 1:80 60 230 0.4 2.88 U Value achieved 0.11W/m2K
6 1:40 1:80 30 270 0.73 2.88
7 1:60 Flat Sump 130 270 0.24 2.88 Mitred
8 1:40 Flat Sump 120 330 0.4 2.88 Mitred
9 1:60 Flat 130 290 0.24 10.1 Longer Fall Distance
10 1:40 Flat 90 330 0.35 10.1 Longer Fall Distance

Based upon the roof achieving 0.13 W/m2K (BSENISO6946:2017 Annex E)

150mm Medium density concrete deck

No void/fixed ceiling

PIR Insulation, suitable for adhered single ply/bitminous felt system

BS6229:2018 states U Value of 0.35W/m2K or better should be achieved by the minimum thickness of insulation. On the examples, where 0.35W/m2k is not achieved the minimum thickness of insulation will increase further

Summary

Differences
Thinnest Min Thickness 30mm
Thickest Min Thickness 170mm 140mm
Thinnest Max Thickness 230mm
Thickest Max Thickness 330mm 100mm
Smallest Area of Gutter 2.88m2
Largest Area of Gutter 23.1m2 20.22m2
Worst heat loss at min thickness 0.73W/m2K
Best heat loss at min thickness 0.19W/m2K 0.54W/m2k

Oatking Resource Hub

Our resource hub contains useful product data and technical information for a wide range of insulation types, along with industry standards and guidance from roofing affiliates.

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Falls:

1:60

Gutter Falls:

1:60

Min Thickness (mm):

50

Max Thickness (mm):

250

Heat Loss at Min thickness (W/m2K):

0.73

Area of gutter/sump (m2):

2.88

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SELECT A DESIGN TO SEE THE DIFFERENCES IN PERFORMANCE
Design Fall Gutter Falls Min Thickness (mm) Max Thickness (mm) Heat Loss at Min Thickness (W/m2K) Area of gutter/sump (m2) Notes
1 1:60 Flat 170 230 0.19 23.1 Short Fall Distance
2 1:40 Flat 150 240 0.19 23.1 Short Fall Distance
3 1:60 1:60 50 250 0.73 2.88
4 1:40 1:40 30 330 0.73 2.88
5 1:60 1:80 60 230 0.4 2.88 U Value achieved 0.11W/m2K
6 1:40 1:80 30 270 0.73 2.88
7 1:60 Flat Sump 130 270 0.24 2.88 Mitred
8 1:40 Flat Sump 120 330 0.4 2.88 Mitred
9 1:60 Flat 130 290 0.24 10.1 Longer Fall Distance
10 1:40 Flat 90 330 0.35 10.1 Longer Fall Distance