Low Slope Roof Truss Design

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.

Low slope roof truss design.

Pitch slope the pitch or slope is the incline of the roof expressed as a fraction describing the rise over the run. It is a common roof on sheds and other outbuildings and is frequently used on carports patios porches and small additions to a house. 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.

Pitch span and wind speed. Low pitch large span and high wind speeds and this is where you ll find a truss working the hardest. 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. It also is often used on dormers extensions built on a basic roof to provide more light or space inside.

With limited slope to drain well it can take forever to be rid of any leakage moisture. Epdm won t last as long as metal but there s a reason it s the most common roofing for very 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. Building offsets are common and create intersecting planes that contribute to drainage issues for low slope applications.

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. Another approach is to build a cupola or ventilated doghouse in the middle of the low slope roof an approach championed by joe lstiburek. It is out of these categories.

Understanding low pitch roof truss design. Just like any other roofing system low slope roof designs will have several effects on the overall construction of the building including the interior volume the style and design of the house the drainage system and the material used for covering. A flat truss is not necessarily flat as it slightly slopes to prevent the pooling of water on the roof and prevents the damaging or collapsing of the entire structure. However ponding water is often observed in these low slope roofs.

The flat truss can be designed as bottom or top chord bearing or for multiple and simple spans. According to low slope roof definitions low slope roofing can be classified into five main categories. Classification of low slope roofing. 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.

This can be cantilevered at one or both ends. There are three factors that most affect the strength of low pitch trusses. A roof that slopes in only one direction is called a shed or pent style.