In Design Economics and Cost Planning, the general rule of thumb is that the simpler the shape of the building, the lower will be its unit cost ($/SF).
One of the key design ratios that every Quantity Surveyor and Cost Manager should understand is the “Wall to Floor” ratio.
What is Wall to Floor ratio?
1. Wall to floor ratio is one of the best indicators to understand whether a building’s envelope is economically designed or not, through examining the ratio of enclosing walls to gross floor area.
2. Let’s assume two buildings have the same overall floor area of 300,000 SF, but Building A has a wall to floor ratio of 1.5, while Building B has a wall to floor ratio of 0.5, then this means that Building A is very uneconomical since it requires a much greater area of enclosing walls than Building B.
What impacts the Wall to Floor ratio?
1. Floor shape (circular, square, rectangular, irregular, etc.)
2. Floor size / area
3. Story Height
4. Total building height
How is Wall to Floor ratio calculated?
1. Ratio is calculated by dividing the external wall area (including windows and doors) by the overall gross floor area.
2. Circular buildings have the best wall/floor ratio, however, any saving in the wall quantity is usually offset by higher cost of circular vs straight construction
3. Square buildings are next in efficiency, followed by rectangular.
What makes a building’s envelope economically inefficient?
1. Long and narrow buildings: tend to have higher wall to floor ratio
2. Irregular and complicated shapes: tend to have high wall to floor ratio, in addition to increasing other costs such as surveying & setting out, drainage, siteworks, etc.
In summary, Quantity Surveyors and Cost Managers should make their Clients and the Design team aware of the cost consequences resulting from changes in a building’s shape, size, story height, and total height.