Add Collar Ties when adding Dormer?

[cdservices]

Am adding a shed dormer to an 80 y.o. shed roof (long rectangle with short hips on end). Probably a 5:12 pitch. Existing roof (full attic on top of 1 story residence) has no collar ties.

Was surprised there were no ties. When I add the shed dormer is there more reason to need collar ties? Looking to understand some of the forces at work.

Thanks

[Riversong]

Re: Add Collar Ties when adding Dormer?

Is this an existing shed (one-sided slope) roof? Is it sloping from a high wall to a low wall or is the peak of the shed attached to a sidewall of a taller building?

Collar ties (tension ties in the upper third of the rafter span) are meant to resist wind uplift forces on a gable roof. Rafter ties (tension ties in the lower third, and typically at the bottom, of opposing gable rafters) are intended to resist the out-thrust of a triangle that's trying to flatten under snow load.

A shed roof (a right triangle with the rafters as hypotenuse) experiences no outward thrust, since both ends of the roof are supported by walls (like a structural ridge in a gable roof). If the building is in a high wind zone, and particularly if there is a significant overhang on the high side of the roof, then collar ties will help resist uplift forces.

If you're cutting in a dormer with a roof that extends from ridge to eave (wall to wall), then it doesn't change the force dynamics. If the dormer has a shorter span than the primary rafters, you'll be placing concentated loads onto the adjoining common rafters and will have to double them or "ground" the loads to the floor below.

[James Eggert]

Re: Add Collar Ties when adding Dormer?

Nice Explanation!

[cdservices]

Re: Add Collar Ties when adding Dormer?

Riversong,

Thanks for the great explanation. I mistyped my question however, it is an existing
"hip" roof, not a shed roof. The dormer is to be a shed dormer.

[David Meiland]

Re: Add Collar Ties when adding Dormer?

Not enough info. Does the new shed dormer start at the existing ridge and bear on the existing exterior walls? Is the ridge structural and somehow supported below, or are there ceiling joists acting as rafter ties?

[Riversong]

Re: Add Collar Ties when adding Dormer?

I assume there are ceiling joists tying the long load-bearing walls together, otherwise a low-pitch hip roof just loves to flatten out and invert (unless the house is built like a yurt, with a perimeter tension cable).

[cdservices]

Re: Add Collar Ties when adding Dormer?

Existing ridge is not structural, just a ridge board. Design hasn't been finalized yet. I am getting the message that it will be better if new shed bears on existing exterior walls. Will it contribute to strengthening of any of the forces that collar ties are designed to resist?

[Riversong]

Re: Add Collar Ties when adding Dormer?

Will it contribute to strengthening of any of the forces that collar ties are designed to resist?

You still haven't indicated whether there are ceiling joists acting as rafter ties. It's rafter ties which are the primary tension control element in a roof system, but they are often confused with collar ties which are up near the neck and have far less importance.

[Kent Brobeck]

Re: Add Collar Ties when adding Dormer?

If they are putting in a dormer they must be adding living space. Are there ceiling joist/rafter ties in the existing building which will act as floor joist in the new space?

[TSJHD1]

Re: Add Collar Ties when adding Dormer?

Was surprised there were no ties.

Ceiling joists are the ties.

When I add the shed dormer is there more reason to need collar ties?

Absolutely.

Tom

[Riversong]

Re: Add Collar Ties when adding Dormer?

When I add the shed dormer is there more reason to need collar ties? Looking to understand some of the forces at work.

Absolutely.

The OP was wanting to understand the forces and the reason for adding collar ties.

TSJHD1 Would you care to offer an explanation?

[Mlandry]

Re: Add Collar Ties when adding Dormer?

Existing ridge is not structural, just a ridge board. Design hasn't been finalized yet. I am getting the message that it will be better if new shed bears on existing exterior walls. Will it contribute to strengthening of any of the forces that collar ties are designed to resist?

It sounds like you need a competent designer to design this project for you.

[Top Notch]

Re: Add Collar Ties when adding Dormer?

Had this argument many times, recently on CT. Collar ties in the upper third act in compression not tension. Thier sole purpose is to hold the plumb cuts tight to the ridge. When they are eliminated metal straps are to installed in place of them.

Basically they are sway braces as wind forces push against the upper portion of the roof the ties compress against the opposite side to hold back.

Its always a topic of confusion and many people believe they dont do all that much so they get left out or disregarded. As roofed dormers become flatter in pitch collar ties will do less. The ceiling joists or rafter ties will take on both purposes of ties and collars. It becomes a braced floor. And runs 35 ft or more need braced wall returns to resist wind.

[Riversong]

Re: Add Collar Ties when adding Dormer?

Collar ties in the upper third act in compression not tension. Thier sole purpose is to hold the plumb cuts tight to the ridge.

Incorrect. A tie, by definition, is a structural member that acts in tension. A strut is a member that acts to resist compression forces.

It's true that in days gone by, when roof rafters were commonly undersized, collar ties in the middle third of a roof's height served as struts to prevent excessive deflection and possible breakage of rafters.

But code-required collar ties, which are mandated to be placed in the upper third of a rafter's span, are to resist wind uplift at the ridge. On cathedral ceilings, where collar ties are not possible, metal tension straps can be used to connect opposing rafter pairs for the same purpose.

Basically they are sway braces as wind forces push against the upper portion of the roof the ties compress against the opposite side to hold back.

Up to a 9:12 roof slope, wind perpendicular to the ridge will exert negative pressure (uplift). It's not until you get steeper than 10:12 that perpendicular wind pressures are positive (see attachment).

The ceiling joists or rafter ties will take on both purposes of ties and collars.

Ceiling joists are the most common form of rafter tie, which is designed to resist the tension at the bottom of the roof triangle (out-thrust). Ceiling joists, or rafter ties, cannot perform the function of collar ties. Roof thrust increases dramatically as the roof slope decreases (see attachment).

[Top Notch]

Re: Add Collar Ties when adding Dormer?

Robert,
Thats a nice chart you have there, where did that research come from? I Think I going to save it.
You make a good point that under some conditions negative pressure places collar ties in tension. Either way we agree on the purpose.