Sloped Roof Drainage Design Residential

Sloped roofs on this modern house match the rest of the lines used on the exterior to create a futuristic looking home.

Sloped roof drainage design residential.

Lucid architecturedesigned this house in western michigan. The drainage requirements for flat roofs on buildings present some special problems that we describe and illustrate here. The sloped roofs on this wood clad modern home promote excellent drainage and open up the house to allow to take advantage of the greenery around it. Historically roof drainage was ac complished by simply sloping the roof downward.

There are three types of drain systems commonly used on flat roofs. First there is drainage. Water could then pour off overhanging roof eaves onto the ground below or in more sophisti cated assemblies flow toward the perimeter and into a clay wood or metal gutter system or through a drainage outlet in the exterior walls. Roofs of this design will have one or more roof drains or scuppers installed.

Healthy drainage occurs when the slope is between 2 percent and 5 percent which equates to 2 to 5 inches of drop off for every 10 feet you measure as you move away from your home. These will drain to somewhere near the center of the roof and down through the building to underground storm mains or a detention pond. Watch for clogs blockage at the roof drain or scupper. For example if a practice will receive runoff from a single downspout that drains of a rooftop you would calculate the drainage area by dividing the entire roof area by 4 see figure 1.

At first glance that s a good thing but not so much when you realize that it would be poor design to have all sides of a roof pouring water on you. To design a sloped or flat roof drainage system we need to know several things. This form of roof drainage is preferred if a clean exterior appearance is desired. You will need to determine the drainage area or the area of impervious surface that drains to each practice.

Of course the best designed flat roof has the correct amount of pitch to drain the water off naturally. Each of these systems has its own pros and cons. The roof is sloped towards the drain through either tapered insulation or through joist modifications. The maximum rainfall rate anticipated in the area where the building is located the flow in liters per minute per square meter for some x number of minutes.

According to the chart for 1 8 slope per 12 of pipe for 5 000 square feet of roof area with a 6 per hour of rainfall the required pipe size is 8. To size the vertical roof drain for the same parameters 5 000 square feet of room area and 6 per hour of rain consult table 1106 2 figure 3. At page top and at left our photos show a low slope roof surrounded by a parapet.