A butterfly roof, named after its resemblance, to the shape of a butterfly’s wingspan, is simply an inversion of a gable roof. As opposed to a traditional gable roof, the roof surfaces of a butterfly roof slope inward to meet at a central valley, positioning the eaves slightly higher than the central ridge.
Also known as a V-roof, this particular roof system was popularized in the 1950s by the works of architect William Krisel in his designs of several tract houses of Southern California.
Listed below are some of the characteristics and design considerations to be taken into consideration while designing a butterfly roof.
1. Daylighting And Water Collection
One of the most advantageous aspects of a butterfly roof is its ability to provide longer and higher perimeter walls which in turn provide more wall surface area to accommodate larger windows and ventilation openings.
The inward slope of the roof allows for the collection of water at the ridge through a central water collection channel connected to a drainage spout.
2. Aids In The Designing Of Column Free Perimeter Walls
Since in essence a butterfly roof in an inverted gable roof the structural system would include a combination of trusses, rafters, purlins, and roof tiles (or any roofing material alike). Although in an ordinary gable roof, sloped like a pyramid the load would be carried downwards away from the center-ridge as against a butterfly roof where the central valley portion would carry a significant amount of the load of the roof.
This demands stronger if not additional supports at the center for two reasons – firstly to carry heavy loads gained by the accumulation of water or snow on the roof and secondly to allow for a column-free periphery. This way the edges can be free of load-bearing structural elements and accommodate larger windows and ventilators.
3. Maximizing Daylighting In The Interiors
Such a roof system allows light to travel deeper into the habitable spaces of buildings. With higher edge walls and the roof surface sloping inwards and downwards to the center, the ratio of the depth of the room to the height of the edge walls—with an opening for light—needs to be maintained such that light penetrates deep into the room, reaching the farther point possible and lights up a maximum volume of the room, not leaving any corner dark.
4. Suitability And Usage In Cold Vs. Hot Climate
A butterfly roof would work well in a cold climate since it allows for a large amount of daylight and along with it a sufficient quantity of heat into the interiors through the large peripheral walls.
But these very qualities may be problematic for a hot climate that needs to reject heat and glare. In such scenarios, additional heat and glare cutting mechanisms or devices need to be installed either internally or externally to maintain thermal comfort in the interiors.
5. Inclination Of Roof To Allow Water Drainage And Collection
The unique quality of the roof surfaces sloping inwards creates an opportunity for water collection at the valley to be used for various purposes. This type of roof can be designed in all angles but, most commonly an angle of inclination within the range of 15-30 degrees is used. Geographical and climatic conditions may influence the angle of inclination chosen.
For example, in areas that receive heavy rainfall or heavy snowfall a greater angle of inclination would be required whereas in an area that receives less rainfall or snowfall an acute angle of inclination would be sufficient to carry out the functions of drainage and water collection on the roof.
6. Helps To Optimize Daylight
Given that a major advantage of using a butterfly roof in a structure is its capacity to allow perimeter walls to draw in large amounts of natural light it is important to consider an appropriate orientation for the open walls to face.
After considering the major functional and climatic requirements, if suited, orienting the building along the north-south axis would fulfill the larger goal of drawing in big quantities of daylight.
7. A Wide Variety Of Usages
A butterfly roof finds use as a standalone structural system to be used as a roof in typologies of designs ranging from independent houses to bus stop sheds to gas station roofs by creating edges free of large load-bearing elements and increasing the flexibility of usage of spaces.
8. Waterproofing Of The Roof
Since the roof is expected to carry the load and drain off a variety of natural materials it would be necessary to ensure that none of these materials seep in to make their way into the interiors of the house/ building.
Lining the sides and bottom of the water collection channel at the valley with sheets or the appropriate waterproofing material would keep the interiors protected from any leakage or seepage.
9. Inclination Of Central Water Collection Channel
One of the core reasons for the popularity of the butterfly roof was its simple yet unique property of serving as a collection tank for rainwater which would otherwise simply be drained off of the roof as run-off.
The roof surfaces sloping down to a central valley channel the water down to a water collection channel which would then be connected to a drainage spout at the ends of the channel down to a collection tank. This indicates the chances of water or snow getting accumulated in the water channel and clogging and stagnating the roof.
To avoid this, the water collection channel itself should be inclined with a minimum slope measure to ensure complete drainage of water or snow being channeled towards it.
10. Edge Water Drop Line
Unlike in a pitched roof where the eaves of the roof ensure that the water from the roof drains off of the edge of the roof vertically, a butterfly roof does not ensure the same. This can be made sure by providing the edge cover with a water dropline so that water doesn’t seep into the edges of the inner walls.