hey a little help would be appreciated

i got a call today from a big client,they asked me if i could install a curb around a 24by 24 exhaust vent.i have never heard of a curb and ive been in buisiness for 13 years.this is on a flat roof.my guess is that they want me to in install the whole vent from the roof because it is a flat roof.but i thought he was talking about like a rain diverter but you dont need that on a flat roof!

elcid Said: Lanny - you brought up another issue. Your explanation covered a roof ventilator whereby the base incorporates a flange that overlaps the curb. However, centrifugal fans anchored to the roof creates a whole host of problems. We would isolate these fans by installing a wooden platform over a curb, and waterproof same w/ suitable roofings. Thus, in the event of leakage in that area, examination would be basically limited to 4 mounting screws. Oftimes, pitch pans are used, which IMHO are a pain in the arse.

Also seen where the top screws to the fan shroud will loosen over time due to vibration,I seal them with elastomer

Lanny - you brought up another issue. Your explanation covered a roof ventilator whereby the base incorporates a flange that overlaps the curb. However, centrifugal fans anchored to the roof creates a whole host of problems. We would isolate these fans by installing a wooden platform over a curb, and waterproof same w/ suitable roofings. Thus, in the event of leakage in that area, examination would be basically limited to 4 mounting screws. Oftimes, pitch pans are used, which IMHO are a pain in the arse.

---Adding to the good comments already given...They are no doubt adding a fan. It will be mounted on the roof. A hole will be cut into the existing roof for the ductwork. ---The fan needs to be mounted on the roof and that requires a curb. The curb is like a skylight box. The fan itself usually has some kind of flashing that will extend down over the curb. ---We would nail in the curb just like adding a skylight curb. Then we would torch each side wrapping the corners like a skylight. The torch membrane would run to the top of the curb and down onto the existing roofing. there would be plenty of seal to the existing roof. ---When finished the curb would be water tight. The fan would then be mounted on top with the some type of flashing extending down the curb. Just like a fixed skylight sits on a curb and covers the step flashings. Lanny

You mentioned it was an exhaust vent. Even more important - how hot is it. If it is a flue of some sort, the curb should be isolated from touching it. It may be necessary to contact clients insurance company, if they require class "A" or "B' construction. These vents also require counter flashings, which are just as important as the curbs.

flat roof-curb would be 2x 10`s on their side forming a box,fans,skylights are supported on top of the box/curb-roofing needs to be done over 2x 10 to top of curb,then fan or skylight would be mounted with flange of same extending down onto,and is secured to the sides of the curb-typically the curb is flashed with flashing compatitible to the roof system in use.---Do get someone with experience with flat roofing to help you through this-I`m not gonna tell you to pass on it,because theses situations are how we learn,but not learning what needs to be done to do the job properly,can be a fatal mistake in business

No offense intended, but, Holy Moly..... 13 years and never heard of a curb? Then it seems doubtful that you would know how to tie one in either. Do the client a favor and decline.

Ever hear of a skylight?

This may be helpful http://www.roofcurb.com/