Octagonal Roof

Walls are parallel

Draw Crown Rafters

Set Back ½ Rafter Thickness

Draw & Bisect Crown End Rafter

Draw Tangents to the Crown Ends

Draw Top Plate

Clean Up Drawing

Clean Up Drawing

Angle Between Major Crown End & Centring Rafters = 90⁰

Clean Up Drawing

Angle Between Crown End Rafters = 45⁰

Clean Up Drawing

Angle Between Crowns Ends & Hips = 22.5⁰

Angles formed by the Roof

These angles are the same no matter the size of Roof as it is a Geometric Shape

Length Of Octagonal EndsIf the angle between the members is constant 22.5⁰

We can say in all cases, no matter the size of the roof½ End = ½ Span x tan 22.5⁰

= ½ Span x 0.414 (tan 22.5⁰ is always used)

If the angle between the members is constant 22.5⁰

We can say in all cases, no matter the size of the roof

½ End = ½ Span x 0.414

Length Of Octagonal Ends

If the angle between the members is constant 22.5⁰

In all cases, we can say

Full End = ½ Span x 0.414 x 2

Length Of Octagonal Ends

If the angle between the members is constant 22.5⁰

Also in all cases, the full end will be

Full End = ½ Span x 0.828

Length Of Octagonal Ends

These formulas are the same no matter the size of the Roof

½ End = ½ Span x 0.414Full End = ½ Span x 0.828

Length Of Octagonal Ends

These formulas are the same no matter the size of the Roof

Calculating Centring rafters and Crown End Rafter use the same method as a normal hip roof

Major Crown End Rafter

These formulas are the same no matter the size of the Roof

Calculating Centring rafters and Crown End Rafter use the same method as a normal hip roof

Major Crown End Rafter

These formulas are the same no matter the size of the Roof

Minor Crown End Rafters join in a similar manner to a hip rafter

Minor Crown End Rafters

These formulas are the same no matter the size of the Roof

Minor Crown End Rafters join in a similar manner to a hip rafter

Minor Crown End Rafters

The Plan run of the minor crown is reduced by ;

½ Centring Rafter x 1.414

These formulas are the same no matter the size of the Roof

Minor Crown End Rafters join in a similar manner to a hip rafter

Minor Crown End Rafters

Minor Crown = ½ Span x ½ C.R Thickness÷

Edge Bevel Minor Crown End Rafters

Plan Angle = 45⁰

Edge Bevel Minor Crown End RaftersPlan Angle

Plan Angle = 45⁰

Plan Angle = 90⁰

Edge Bevel Minor Crown End RaftersPlan Angle

Plan Angle = 45⁰

Plan Angle = 90⁰

Both Sides MustBe Equal

Edge Bevel Minor Crown End RaftersPlan Angle

Plan Angle = 45⁰

Plan Angle = 90⁰

Both Sides MustBe Equal

When Can You Use The Plan For the Edge Bevel ?

Edge Bevel Minor Crown End RaftersTrue Angle