How to Make Cross Ties to Prevent Sagging

Gain access to your attic and use a tape measure to measure the distance from the roof joist beams (attic floor) to the top inside of the roof at its highest point. Plan to install cross ties one-third of the total distance from the top. For instance, if your floor-to-peak distance measures 9 feet, then you will install cross ties 6 feet up from the floor, from one rafter to another. Make a pencil mark on both rafters, 6 feet up.

Measure the distance from one rafter span to the other at the same horizontal locations. Move down the length of the attic and take several more measurements, 6 feet up from the floor. Make sure the measurements all fall within the same acceptable lengths. Count the number of rafters you have from one end of the roof to the other and divide that number in half. You need only install cross ties to every other rafter for adequate support.

Use a hand saw or power saw to cut the cross tie boards to length, either using 2-by-4-inch or 2-by-6-inch boards. Start from one end of the attic and place your cross tie board against the rafter at your pencil mark, having an assistant support the other end. Have a third assistant hold a carpenter's level on top of the board and level it. While the second assistant holds the other end of the board, drive three nails into the board on your end. Move to the other side and drive three nails into the board.

Install the cross ties every other rafter, leveling each one. You may butt the corner of the cross tie up into the angled part of the roof if you wish, as long as it remains level. For a more professional touch, draw a pattern off the angled rafter, by using a scrap piece of board. Then transfer the angle to the cross ties -- this will allow the cross tie to sit flush with the underside of the roof angle. Take your time and allow frequent breaks.