How to Add Thickness to Rafters in the Attic

Measure and record the dimensions of an existing rafter, including the top length, the bottom length, the angle of the cut where the rafter meets the ridge beam and the dimensions and angle of the notch at the bottom end of the rafter where it sits on the top wall plate.

Stop your measurements at the outside edge of the wall plate because the portion of the existing rafter that extends past this point will not be included in the new rafter. Imagine a vertical line running from top to bottom of the existing rafter where it meets the outside edge of the plate. This is as far out as you will measure.

Transfer the measurements to a piece of dimensional lumber the same size as the rafter. In many cases, this will be a 2-by-6 board, but it could be a different size, depending upon the pitch and span of the rafters.

Cut the new rafter with a circular saw and position it beside an existing rafter to make sure the dimensions are accurate. If the new rafter fits well, take it down and use it as a template to cut all the new rafters.

Position a new rafter beside an existing rafter and use a pneumatic framing nailer to insert three 16d nails through the top edge of the rafter, at a 45-degree angle into the ridge beam. This is called “toenailing.” An air compressor is necessary to run a pneumatic nailer.

Insert two additional 16d nails, through the side of the new rafter, into the existing rafter at 1-foot intervals. When you reach the lower end of the rafter where the space is too small for the framing nailer, remove the nailer from the air hose.

Attach a pneumatic palm nailer to the air hose and insert three, 16d nails, toenail-fashion, evenly spaced, through the bottom edge of the new rafter where it sits on the top wall plate.

Insert two additional nails through the side of the new rafter, just above the first three nails, evenly spaced, attaching it to the existing rafter.

Continue attaching new rafters to the existing rafters in the same manner.