Limiting Height of Chimneys
Where a chimney is not adequately supported by ties or securely restrained in any way, its height H when measured from the highest point of intersection with the roof surface, gutter etc. should not exceed 4.5W, provided the density of the masonry is greater than 1500 kg/m3 (i.e. brickwork, heavyweight concrete block etc.), where:
W = the least horizontal dimension of the chimney measured at the same point of intersection, and
H = measured to the top of any chimney pot or other flue terminal.
A parapet is a continuation of an exterior wall above the roof level. The portion of a wall rising above a roof is the "parapet." Parapet walls are unstable because they are free standing. A freestanding wall is considered by engineers to be the least stable of three basic types of walls (free standing, nonbearing, and bearing wall) because it has fewer connections to a structure.
The wall is exposed more to the elements than other masonry wall sections. The top of the parapet wall is especially vulnerable to water penetration. Choosing an appropriate cap is important to eliminate this from happening. The cap of the wall can be made of stone, pre-cast concrete or metal.
Limiting Height of Parapet walls
The minimum thickness and maximum height of parapet walls should be as indicated in the diagram below.
If the criteria indicated below cannot be attained design calculations should be carried out to indicate suitability.
 |
Thickness
(mm) |
Parapet height H
to be not more than
(mm) |
t1 + t2
equal or less than 200 |
600 |
t1 + t2
greater than 200
equal to or less than 250 |
860 | |
|
Thickness
(mm) |
Parapet height H
to be not more than
(mm) |
| t = 150 |
600 |
| t = 190 |
760 |
| t = 215 |
860 | |
| Note: t should be less than or equal to T |
Note, the above guidance relates only to structural aspects of parapet wall design. Further advice should be obtained with regard to moisture penetration.
Please check with your local building control department before using information contained in this page to ensure that they comply with your local building codes.