Understanding low pitch roof truss design.
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.
The rise will be the elevation change over the 12 inches.
However ponding water is often observed in these low slope roofs.
Combine these three at their most extreme i e.
Low pitch large span and high wind speeds and this is where you ll find a truss working the hardest.
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.
With limited slope to drain well it can take forever to be rid of any leakage moisture.
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.
A low slope roof design typically has a pitch of between 2 12 and 4 12.
The run or the base number is 12 inches.
Building offsets are common and create intersecting planes that contribute to drainage issues for low slope applications.
Another approach is to build a cupola or ventilated doghouse in the middle of the low slope roof an approach championed by joe lstiburek.
This can be cantilevered at one or both ends.
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.
It is out of these categories.
Classification of low slope roofing.
There are three factors that most affect the strength of low pitch trusses.
Pitch span and wind speed.
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.
According to low slope roof definitions low slope roofing can be classified into five main categories.
While this article focuses on configurations we also have a very cool set of illustrations showcasing the different parts anatomy of roof trusses.
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.
It also is often used on dormers extensions built on a basic roof to provide more light or space inside.
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.
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 roof that slopes in only one direction is called a shed or pent style.
