Roof Rafter Bridging

Place a long piece of framing lumber alongside the rafter so that the narrow edge is flush with the underside of the roof deck and the bottom reaches the ceiling joists.

Roof rafter bridging.

Thermal bridging is the movement of heat through a material that s more conductive than the air around it. With steel framing at about r 04 per inch it s thermal bridging on a whole different scale. The main purpose for this type of bracing is to reduce the bucking length of the individual truss members and to assist in erection spacing of the trusses. Steel framing typically reduces the in cavity r value by as much as 50 while wood framing reduces in cavity r value by a bit less than 10.

It can consist of lateral members spanning in between trusses t or i bracing on individual members sheathing or metal strapping. Structures such as sheds or barns can be frames with rafters every 24 again make sure to consult local codes. Most codes allow foil faced foam to be left exposed. In either case seal the foam edges well to the rafters with spray foam.

The blocking or bridging between joists or rafters also contributes to this phenomenon. It is always better to have a large rafter rather than be concerned with a sagging roof. For this size job you would want a large foam canister and professional spray applicator. For example with bridging installed between joists a piano leg sitting over one 2 10 is actually supported by several.

Place a short piece of framing lumber against the rafter above it so that the two boards form a continuous line. The weight of the piano is centered over a single joist but the bridging transfers some of the weight to the adjacent joists. Drive a nail into either end to hold the board in place. Failure to mitigate can account for overall heat loss of up to 30 so it s not a thermodynamics lesson to be taken lightly.

Brush up on thermal bridging 101 here.