Ask Me Help Desk - Flashing Roof Vent on Flat Roof

Flashing Roof Vent on Flat Roof

I am getting ready to install some ventilation in my flat roof. I've decided to go with this system: Flat Roof Top Attic Ventilation. It looks perfect for my application.

I've done some research on how to install the fan, including on a couple other forums, but have a couple of unanswered questions, and would like to confirm my process with all here. As I am sure it is important, I have a granular surface modified bitumen roof. So, the process I am considering so far is:

Step 1: Cut hole in roof vent location selected.

Step 2: Prime top and bottom surfaces of flange of vent that will be set on roof. Allow primer to dry.

Step 3: Coat bottom of flange with roofing cement.

Step 4: Set vent in place, and mechanically fasten to roof deck with screws.

Step 5: Apply roofing cement to top of flange and cut piece of roofing felt to fit around penetration. Apply roofing cement to one side of felt. Apply felt to flange and rub together.

Step 6: Repeat Step 5 with another piece of felt.

Step 7: Coat top of roofing felt with roofing cement. Coat bottom of a piece of base membrane with roofing cement. Apply membrane to felt and rub.

Step 8: Coat membrane with cement.

Does this process sound correct? Am I missing anything? Anything overkill?

So, I have a couple of questions.
1) What type of primer should I use on the flange?
2) For the first two layers should I use #15 asphalt saturated felt or asphalt saturated glass fabric? Does the glass fabric come in wide rolls?
3) Can I use SBS modified bitumen roofing as the final layer or is there something better?

Anything else?

Thanks!

Hi stubits. The metal flange can be handled just like a boot vent/ Cut hole. Silicone bottom of flange/set flange by screwing Stainless steel screws. Small can of black Jack fibered roof pitch around edges of metal lip.
Done in and out.

To keep building it up over the metal flange really doesn't get you anywhere. The height of the unit is the flashing

As always, thanks so much. The method I described seems a little overkill, for sure. In the method you describe, what keeps water from leaking in around the screws?

Either Daps of silicone sealer on the heads or mark under the flange where the screws are going to go through and when that compresses it squeezes up to the screw. If the screws are at the edge more then fibered roof pitch covers that edge and the screws close to that edge. It's a Neatness factor difference. They get covered one way or Anthe's. A dap of silicone on the plate and screw through that and the silicone acts like a rubber grommet.

Excellent. Thanks.

So, this really isn't all that difficult then, is it? That's great news. I've been so afraid to mess with the roof for fear of trouble.

How long will these connections last?

How long does a modified bitumen roof generally?

Have you ever seen/worked with this system before? Flat Roof Top Attic Ventilation I am planning to use the 18" to vent my 650 sq. ft. roof. Any thoughts?

Thanks!

To help a bit on water shed you can use a piece of 30lb felt on the high side of the flange to lip up over that area.

Stubits is this the roof you have there now?
http://www.tremcoroofing.com/filesha...ly_Systems.pdf

I thought the old roof was a mineral roll roof tar coated or not over the years

Great idea. I have never been sure of the roof type, but recently spent a lot of time looking at photos on the web and going to the roofing supply store. Based on looking at the materials, mod. Bit seems closest. My roof looks exactly like this...

http://www.fcroofing.com/roofing_pic...pp_Bitumen.jpg

And this...

http://www.schindlerroofing.com/_ima...fied_flash.jpg

Which are both mod. Bit. Does that change anything? What do you think? Better? Worse?

Stubits the application of a vent doesn't change anything other then the length of screws and being self tappers. I really think all you have there is a mineral roll roof. I work on thousands of city row homes and haven't seen more then standing seam roof, mineral roll or upgrade to rubber on a residential home. Most of the old commercial is hot mop and stone or up grade to rubber

Hmm, thanks a lot. Just looking at photos of mineral roll and really can't tell the difference. How would I go about figuring out what exactly I have? Could you tell just from photos? Will I be able to tell once I cut it open?

So, have you ever worked with this particular type of vent? Any thoughts? Seems to work better than a turbine?

Would you say an install like this on the vents is good?

http://www.roofvents.com/holem1.jpg

Yes that's fine. A seal is a seal. The more layers the more chance for heat movement and breaking the seal. The way to see what the roof is made of is just cut through it. You can get up in the rafters and see what planking/sheeting you have and nails sticking through.

Great, thanks. I guess I will just wait until I cut the roof open. So, you recommend silicone instead of roofing cement? Any particular type?

Frankly this doesn't sound all that difficult. Why is everyone so afraid of cutting a flat roof open? How long should I expect this flashing to work?

Quote:

Why is everyone so afraid of cutting a flat roof open? How long should I expect this flashing to work?

The fears start with the word "Flat" roof which very few actually are. And none I ran into in residential yet. It's a low pitched roof that's referred to as a "Flat" roof.

It's a Mind bend trick we do.. Flat floor/yard water doesn't drain

I use 100% silicone under the flange and roof pitch at the edges. The seal will last as long as the roof pitch several years or more on the fibered pitch

Stubits you will be fine don't panic. Cut through hundreds of roofs for plumbing vents on flat roofs...

Thanks so much. Not panicing, just want to make sure I get it right. So, why do you prefer silicone to roof cement under the flange? Oh and is roof pitch the same as roof cement?

Thanks!

The metal flange and the asphalt roof flex at different rates. The silicone equalizes the movements. Rubber effect.