Rafter ties (metal straps)

[hogheavenfarm]

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.

[RicodJour]

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.

R

[Matt W. Barrow]

Pardon,

How could a smaller dormer experience MORE turbulence?

Thanks!

[jloomis]

[RicodJour]

[RicodJour]

More turbulence than a roof without a dormer? Easy. You're putting an object in the way of the air stream => turbulence and quite possibly increased localized wind speed.

R

[Matt W. Barrow]

Okay...maybe I'm missing something here: the point was not a dormer versus NONE, it was a smaller one versus a BIGGER ONE.

Okay...you've totally lost me.

[hogheavenfarm]

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 be compromised. 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 whistles necessary?

[RicodJour]

Code does not differentiate so you are required to conform to the roof assembly requirements on all parts.

R

[RicodJour]

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.

R

[hogheavenfarm]

Ok, thanks all, that makes some sense to me . Just wondering what the rest of the world did.

[Matt W. Barrow]

Gotcha!!

Thanks.

[RicodJour]

I can usually explain something in three or four attempts...unfortunately people wander off after two or three. ;)

R

[Matt W. Barrow]

Hmmm...Hmmmmmm..hmmmm.....what?

:~)