Here is a question I get asked alot, and I am not sure of the answer.
We build houses in High Wind Zones often, largely on the Jersey shore.
We frequently need to strap the peaks and the rafters to the top
plates and the studs. The question always comes up with the smaller
parts. What do you do with a 5ft dormer? Rafter ties? Peak straps?
Full blocking for all plywood seams? We currently do all of the above,
but I wonder if it is truly necessary for such a small roof area that
is at right angles to the wind plane.
The smaller dormer could easily experience more turbulence and be
subjected to greater forces, and since code requires that certain
areas (end zones) be designed with a greater wind load for a given
wind speed, you're right in not skimping on the dormer.
I was thinking more of uplift which is generated by the square footage
of the roof area (and the pitch). A large uninterrupted area would
actually have a greater uplift than if a dormer or two created some
turbulence, which by its nature, disrupts the uplift forces. The
dormers are also at right angles to the wind direction (facing the
wind) so the uplift forces don't act on the dormer roof, only the main
roof. The gable end of the dormer would be subject to a direct wind
load, but not an uplift force.
Remember also, that our dormer units are completely preassembeld, so
they are not part of the roof structure, (no studs continue through to
the roof space to the floor). If a wind was to completely sever a
dormer from the roof, no structural component of the roof system would
Back to the original question - I was asked whether the square
footage of the dormer roof creates enough uplift to justify full 'wind
treatment' . Not being an engineer, I can only surmise that a smaller
footprint creates less uplift. At what point are all the bells and
Roofs are designed with loads that are attributed to zones. Eaves,
edges and ridges are subjected to greater stress and the design loads
and nailing pattern requirements are in acknowledgment of the
increased stress on those areas. Since a smaller dormer is
essentially all end zone the requirements are actually more stringent
than in the middle of the roof field or on a larger dormer.
A certain volume of air is moving at a certain speed across a roof,
that creates a certain pressure and a certain load. When the wind
encounters an obstacle it doesn't just stop or change direction with
no change in pressure. Whether you approach it as a Venturi effect,
an airfoil or simply compressing the air, the net effect is the same -
the wind pressure/load is increased locally.
I would treat the dormer similiar to the construction of the main frame....
Straps are fine, A35......H1........MSTA.......Blocking on plywood seams is
Earthquake country this is usually the norm. High wind areas is just a
I have had wind gust 120 mph here in Ca. during some storms.......the roof
Anyway, it does seem like a lot to throw at a construction, but "the chain
is only as strong as the weakest link"
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