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.
Width
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.
Ridge
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.
Sheathing
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).