Topic 14 – Intersecting Roof Components and Calculating Valley Rafters and Ridge
Topic 14 – Intersecting Roof Components and Calculating Valley Rafters and Ridges
Intersecting Roof Components
Intersecting roofs are roof which contain a projection such as an L or T shape. They have multiple ridges that intersect at a point and can be either gable or hip roofs. It is important to be familiar with the terminology to describe the components of an intersecting roof, which are outlined below:
Many of these terms are shared with both gable and hip roofs. These serve the same function as before and the math has not changed. Some new terms are as follows:
It is important to differentiate between the terms “jack” and “cripple jack”. Jacks are supported at one end by either the exterior wall plate or ridge and supported at the other by either a hip or valley rafter. The rafter that supports the one end gives it its name. Cripples are cut short and do not extend to the ridge nor exterior wall.
Major and Minor Roof
There are two or more roof sections in intersecting roofs. The major (which contains the major ridge) runs the entire length of the building and is the larger of the two. The minor roof (which contains the minor ridge) is the smaller of the two. The intersection between the major and minor roofs is constructed depending on the span of the minor roof in relation to the major roof. When the minor roof span is smaller than the major roof, the minor ridge does not intersect the major ridge (when the slopes are equal). These types of roofs have both supporting and supported valley rafters, so called as the supporting valley rafter supports the supported (shortened) valley rafter:
When the span and slope of both roofs are equal, the major and minor ridges directly intersect and there is no supporting valley rafter:
Valley Rafters
In terms of geometry, valley rafters are very similar to hip rafters. The unit run is 16.97” (17” when completing layout). As a result, the unit line length is also calculated the same way. The amount of units of run is dependent on whether it is a supporting or supported valley rafter.
Look at figure 18 above. The supporting valley rafter (B) spans from the exterior wall up to the major ridge. The same can be said for the valley rafter (A). By appearance alone, we can see that both of these are the same length as the hip rafters on the major roof. For supporting valley rafters, the line length is calculated the same way. We have a rafter span of 13’-0” and a 4/12 slope:
The line length for the supported valley rafter is the calculated the same way, only with shortened run. The units of run are taken from the end of the minor roof. The supported valley rafter runs from the exterior wall to the minor ridge. As a result, its span is equal to the common rafters on the minor roof, which is half the span of the minor roof; in this case, 8’-0”. The ULL remains the same as the valley and hip rafters:
The rafter tails are calculated using the same methods as hip rafters.
Deductions on Valley Rafters
At the rough fas