The height of the rough opening would be the height of the garage door plus the thickness of whatever you are putting underneath the header in addition to the thickness of.
Garage door framing opening size 8 x 6 6.
Cut the header from 2 x 12 lumber and to a length of the rough opening plus 6 inches.
For instance if you have a 16 door and are using river stones that are approximately 2 to 2 1 2 you will want the rough opening to be roughly pun intended 16 6.
So if you have a bedroom door that is 30 wide which is considered a 2 6 or 2 6 door just add 2 to the width and frame it 32 wide.
If you are installing a 9 wide door the opening will be 9 3 wide.
Install the header on top of the trimmers and nail.
Now all you need to know is how to properly frame your door rough opening.
Door height plus 18 is required.
Use 2 inch lumber for the jambs as well and ensure they are appropriately nailed to the door opening with the help of a nail gun or hammer.
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Frame the rough opening to 3 inches wider than the garage door size you have selected.
The rough opening is equal to the size of the door but you need to figure back head and side room before choosing and installing the new garage door.
Ideally 2 x6 is the most used header size.
The height is 80 which is considered 6 8 or 6 8 add 2 1 2 to the actual door height and frame it 82 1 2 high.
Additional back room may be required for automatic garage door.
The width of the lumber should be between 6 and 8.
Nail or screw 2 x 6 jambs to the framing on the top first and then the sides of the rough opening.
Framing nails are highly recommended.
After installing the jambs your garage door opening should now be the same size as the garage door.
Step 1 measure the old door if you are replacing an existing door with one of the same size or the new door of choice if you are building a new garage.
If you are installing a 9 wide door the opening will be 9 3 wide.