The May Practical Engineering column
looked at the issue of lateral
thrust in roofs — the tendency for
rafters to spread apart under load.
Conventionally framed roofs (see
Figure 1) are often strong enough to
resist this lateral thrust as long as the
carpenters use enough nails or bolts
where the rafter laps the attic joist
(though this can be a challenge in
shallow-pitched roofs).
A problem with this framing detail,
especially in cold climates, is that the
attic insulation tends to get compressed
above the wall plate. Even
with ventilation chutes, the increased
heat loss at the plates may contribute
to ice dam troubles. So many energyconscious
builders use a "raised rafter
plate" detail (Figure 2) which lifts