Dormer windows are typically the most complex element on a dwelling regarding envelope area calculations and as such they can be a source of error in envelope calculations. I will be showing how I would work though an example calculation that will hopefully help testers with their own calculations.
Obtaining Dimensions
The dimensions needed to calculate the areas of the dormer windows are normally formed from a combination of the section drawings and the floor plans. On the floor plans the changes between flat and pitched roof are normally denoted by dashed lines. The dimensions for these may not be present on the drawings and as such you either need to scale them or you can measure them when you are on site.
Envelope Area Calculation for the Wall Area of the First Floor
In the example there are basically two shapes to calculate for the wall area. These are the two gable ends and then the dwarf wall and front dormer wall at the front and back of the building. The gables ends and the cheeks of the dormer window can be calculated as a single rectangle as shown in the image below. These can then be multiplied by 2 to calculate half of the first floor wall area.
The front and back walls are basically an upside down T shape with the dwarf wall running the full width and then the addition of the face of the dormer window. The width of these elements can be obtained from the drawings by either using the provide dimensions from the floor plans or by scaling them. The dimensions can also be measured on site.
The wall areas are calculated as follows: (First Floor Wall Areas)
First Floor Wall Areas
Gable = 0.484 x 2.65 = 27.783 m2
Dwarf Wall = 6.188 x 1.567 = 9.697 m2
Dormer Face = 2.19 x 1.083 = 2.372 m2
Total = 2 x (27.783 + 9.697 + 2.372) = 2 x 39.852 = 79.704 m2
Calculating the Roof Areas
The roof area is a combination of both flat roof area and the pitched roof area. I will work these out individually so that the breakdown of the areas can be shown.
The flat roof area is calculated as follows:
First Floor Flat Roof Area
Main Roof = 7.377 x 6.188 = 45.649 m2
Dormer Window Roof = 1.554 x 2.19 = 3.403 m2
Flat Roof Area = 45.649 + 3.403 + 3.403 = 52.455 m2
The final area to calculate is the pitched roof and as we have both the height and depth of the dormer window this allows us to calculate the hypotenuse of this triangle using Pythogian Theorem. Once we have the hypotenuse we can then calculate the pitched roof area each side of the dormer window.
