Low Slope Roof Truss Design

There are three factors that most affect the strength of low pitch trusses.

Low slope roof truss design.

According to low slope roof definitions low slope roofing can be classified into five main categories. A roof that slopes in only one direction is called a shed or pent style. Classification of low slope roofing. On low slope roofs in the snow zone epdm will be work better than standing seam since it can t leak even under high winds whereas metal roofs will.

This can be cantilevered at one or both ends. The run or the base number is 12 inches. It also is often used on dormers extensions built on a basic roof to provide more light or space inside. However ponding water is often observed in these low slope roofs.

Just as there are many types of roofs with many roof parts there are many different types of roof trusses this extensive article explains through a series of custom truss diagrams the different truss configurations you can use for various roofs. It is a common roof on sheds and other outbuildings and is frequently used on carports patios porches and small additions to a house. Pitch slope the pitch or slope is the incline of the roof expressed as a fraction describing the rise over the run. The flat truss can be designed as bottom or top chord bearing or for multiple and simple spans.

Epdm won t last as long as metal but there s a reason it s the most common roofing for very low slope roofs. A low slope roof design typically has a pitch of between 2 12 and 4 12. Combine these three at their most extreme i e. With limited slope to drain well it can take forever to be rid of any leakage moisture.

Another approach is to build a cupola or ventilated doghouse in the middle of the low slope roof an approach championed by joe lstiburek. Specifically the code text typically reads roofs shall have a design slope of not less than one fourth unit vertical in 12 units horizontal 2 percent slope for drainage the stated and intended purpose of the code specified 1 4 in 12 slope is to provide drainage. For example a slope of 6 12 means for every 12 inches of horizontal distance the slope of the roof will rise 6 inches vertically. While this article focuses on configurations we also have a very cool set of illustrations showcasing the different parts anatomy of roof trusses.

Building offsets are common and create intersecting planes that contribute to drainage issues for low slope applications. Design professionals frequently specify the minimum code permitted slope with little if any consideration of the resultant valley slope created by the intersecting planes figure 1 ponding water is commonly observed at valley intersections for low slope roof and deck. Understanding low pitch roof truss design. If the upper edge of a low slope shed roof intersects a vertical wall the most logical approach in my opinion is to detail the roof as an unvented roof.

The rise will be the elevation change over the 12 inches. It is out of these categories. Pitch span and wind speed.