Blind Valley Roof

A Blind Valley aka California Valley or Reverse Gable is ideal for situations where a new roof must tie into an existing roof. It can also be used for such things as decorative false gables or chimney crickets. In this type of valley the main roof is framed and sheathed as usual and the intersecting roof is framed on top of the main roof.

To Calculate a Blind Valley Roof

Select Blind Valley from the Roof Type dropdown list or select the corresponding button on the toolbar. The preview window displays an isometric view that illustrates the proportions of the blind valley roof.

Design Tab

Over Slope

Enter or select the slope of the intersecting roof.

Under Slope

Enter or select the slope of the main roof.


Enter the width of the blind valley. If the blind valley is to meet up with an intersecting roof then this measurement is the width at the point the intersecting roof meets the main roof and should take into account differences in wall heights, sheathing thickness on the main roof, and any other variables that could affect the dimensions of the blind valley.

Rafter Spacing

Select an on center spacing from the dropdown list. There is no layout drawing for the Blind Valley, but this measurement is used to to calculate the common difference which is displayed on the Longest Jack drawing tab.

Lumber Sizes

Enter the lumber sizes for the jack rafters, valley plates, and ridge board. If no ridge board is used enter zero for the ridge thickness or select "No Ridge/Purlin" from the dropdown list.

Drawing Tabs

Valley Plates

The Valley Plates drawing tab displays a dimensioned view of the left and right valley plates viewed from above at an angle perpendicular to the slope of the under roof. The plates lay flat on top of the under roof sheathing and provide a nailing base for the jack rafters. They are designed to butt together at the peak and have a beveled miter cut at the bottom to frame against the end rafters of the over roof.

Note: If the slope of the addition roof is shallow and the seat cut of the jack rafters is very long, you may need to use very wide valley plates or insert filler strips to ensure that the jack rafters are sufficiently supported along their seat cuts.


The Ridge drawing tab displays a side view of the blind valley showing the ridge board, nearest valley plate, and nearest jack rafter illustrating how they all fit together.

When assembling the blind valley the valley plates are butted together at the peak and the ridge board is placed on top of the valley plates, centered over the joint between them with the shoulders of the ridge board peak cut flush with the valley plates (see below).

Longest Jack

The Longest Jack drawing tab displays a side view of the longest jack rafter and a top view of the rafter illustrating the bevel angle for the seat cut of the jack rafter. The jack rafter common difference is calculated base on the spacing selected on the Design tab and is also displayed in this drawing.


The Sheathing drawing tab displays a view of one side of the blind valley looking from above at an angle perpedicular to the plane of the roof.

The drawing displays the overall dimensions and angles of the roof and if the roof is taller that the width of a single piece of sheathing it also displays dimensions for a section equal to the width of a standard piece of sheathing (48" imperial, 1220 mm metric).

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