Proper Ventilation for Flat Roof

I have a flat roof on an 75 year old rowhouse. It appears to be a rolled asphalt roof in relatively good shape. During the winter I insulated my attic(it had never had insulation). It was great for the winter, but now that the summer has come around it is much hotter than before.

There is no ventilation for the roof whatsoever given the way the house is constructed. Any suggestions for how to properly vent the roof? The only solution I’ve found is an attic fan, but it seems like that would just extract air from the attic without drawing any air in. How are flat roofs usually vented?


We would use a turbine vent or a spinner vent.

painting it wht or silver will help.


Great. I’ve been looking at the turbine fans as well. They seem a great option for this.

How do I go about actually installing them? Is it necessary to build a curb? The roof is currently rolled asphalt. After I cut the current roof and install the turbine, how do I seal it all up?

[quote=“Stubs”]Great. I’ve been looking at the turbine fans as well. They seem a great option for this.

How do I go about actually installing them? Is it necessary to build a curb? The roof is currently rolled asphalt. After I cut the current roof and install the turbine, how do I seal it all up?[/quote]

First of all, what do you mean by a “rolled asphalt” roof? Do you mean an asphalt built-up roof, 90# installed in mastic, or a modified bitumen roof system?

Assuming you have a smooth-surfaced asphalt built-up roof system, you will need to purchase turbine ventilators designed for low-sloped roofs.

Step 1: Cut hole in roof at each turbine vent location selected. Hole should be the same size as the turbine vent. A Sawzall or similar type saw will work nicely for cutting the circle. Also, make sure you center your hole between joists before cutting.

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

Step 3: Coat bottom of flange with roofing cement. A nice thin coat is best. Not too much that is oozes out around the flange all over the place, but thick enough not to see the metal.

Step 4: Set turbine in place, and mechanically fasten to roof deck with screws rather than nails to reduce the chance of fastener back-out.

Step 5: Flash the flange into the existing roof. This can be done with felt and hot asphalt, a piece of modified bitumen can also be used to flash flange. If you are not a roofer, you can create a composition flashing system with roofing cement and a heavy, reinforced, flashing membrane. BTW, Step 5 is the hard part!

Wow, this is really excellent. I have spent a lot of time looking at pictures and I think my roof is a modified bitumen roof.** But to be certain, how can I tell the difference?**

Does the method change much if it is a modified bitumen roof vs. a built up? Step 5 is what totally scares me. Can it be done successfully DIY? I’ve got lots of experience, just none with a flat roof.

Do you think the turbine fan is a good idea or should I go with a powered fan or a solar fan? Or something completely different?

Finally, the way the house is built, there’s no intake air at all. Admittedly, the house wasn’t built air tight 75 years ago, but still. Would a system like this work?


I’m not familiar with that vent, but it looks like it would work. In reality though, your ventilation will be passive and the turbine vents should work to achieve what you are trying to do. I wouldn’t spend the money and effort on an electric fan, but that is just me.

As for your questions about roof type. If it is a modified bitumen membrane it will either be smooth-surfaced or granule-surfaced like a shingle. Either way, the rolls are approximately 39 5/8 inches wide with an exposure of approximately 36 inches once adjacent membrane is overlapped onto the sheet. If it is a built-up roof you laps will likely be every 9 or 12 inches (3- or 4-ply). Of course, if there is gravel set in asphalt or coal-tar, you have a built-up roof.

Yes, the method is the same pretty much. However, some modified bitumen manufacturer’s don’t want their membrane in contact with roofing cement, where others only allow you to use modified roofing cement.

Can a DIY successfully flash in a turbine vent? Yes, but it would probably be best if you had a roofer or someone who knows what they are doing oversee your work. Also, if you have a modified bitumen membrane, it is best to flash penetrations with a torch-applied APP modified bitumen membrane. You will need a special torch for this, as a brazing torch would be like using a knife to cut down a tree.

To Cerberus,

What do you think of turbines in general? Do they actually work. Do they help remove damp air on a flat roof?

I was always under impression that they don’t really work and are just a gimmick.

How will an 8" turbine compare to an 8" static mushroom vent? on a flat roof installed at the same location, in terms of venting out moist air?

To stubs,you need your soffits openedto attic you could use styrofoam funnels that lay on top of your insulation and below your roof deck to open up…

Excellent. Thanks so much. I agree, I don’t think I will bother with a powered vent, but will plan on installing a turbine to suck the air out and then some form of a static vent to allow air to enter in. Does that make sense? I assume I’d install the turbine higher up on the roof with the static vent installed farther down. Right?

Ok, so based on your description and looking at photos I am pretty confident that I have a modified bitumen roof. Unfortunately though, I have no idea who the manufacturer is.

Is it necessary to install the turbine on a curb or can it be attached directly to the roof? How can I tell, or how could a professional tell, whether or not cement can be used?

What are my options if I don’t want to use the torch flashing method? Are there other options?

Do it as Cerberus described, use neoprene flashing cement.

Neoprene cement is uv resistant and cures to a hard rubbery consistency.

Thanks everyone for the great help.

I’m trying to figure out “step 5” from above. Do these photos look about right?

What I am not seeing in these images and what I am getting hung up on, is how to seal where the flashing meets the base of the vent (the cylindrical part). Any thoughts?

If you do it like the first photo, you will be fine.

As for the bond around the pipe penetration, if you go with the torch modified bitumen you want to apply pressure to the membrane while the bitumen is still hot.

Here is a photo of a roll of mod. bit. being installed. The person on the right is torching the membrane side lap, the person in the middle is rolling the laps to ensure they are mated together, and the person on the left is casting additional granules into the hot bitumen bleed-out.

Here is a close-up view of a flue stack penetration with a void between the metal base flashing and the flashing membrane. In this particular case, the membrane manufacturer wants a bead of SBS modified bitumen cement applied around the base to fill the void. In the case of most torch-applied mod. bits., however, the bitumen bleed-out will fill the void if you make a good cut. Sorry I don’t have better photos, but most of the photos I take are of people doing things incorrectly, defects and deficiencies. I tried to find some good photos, but I couldn’t! LOL! I know I have some, but just couldn’t locate them quickly.

Thanks for the photos. So, the second photo I posted is not good? Why specifically?

Are there any acceptable options other than the torch method? I don’t have the appropriate tools and for such a small project, just one roof vent, it doesn’t seem to make much sense to acquire them. Also, I am pretty sure I’d burn my house down!

[quote=“Stubs”]Thanks for the photos. So, the second photo I posted is not good? Why specifically?

Are there any acceptable options other than the torch method? I don’t have the appropriate tools and for such a small project, just one roof vent, it doesn’t seem to make much sense to acquire them. Also, I am pretty sure I’d burn my house down![/quote]

First of all, you will need and want to install at least two vents, unless you have soffit vents or some other means to provide an intake and exhaust.

The second photo isn’t bad, but it is smooth-surfaced and does not cover the entire flange. While it would be acceptable, your first photo is preferred.

Since you don’t have the means or methods to torch down mod. bit., and you asked if there is another method, the answer is yes. You can create composition flashing by alternating courses of roofing cement and felt or a heavy, reinforced flashing membrane. In a pinch, you could apply roofing cement around the penetration and coat the back side of a piece of roofing felt already cut to fit around the pipe penetration. Rub the two pieces together, and yes you are going to make a mess the first time you “play” with roofing cement. Be sure to have some hand cleaner and plenty of rags available ON THE ROOF.

After you have applied the first ply of felt, you can either install another ply the same way, or you can move on an install the heavy base membrane. Something like a 40# base sheet could work if needed. Coat the top of the roofing felt that is in place, coat the underside of the base membrane (which should be cut at least 2-inches wider in all four directions than the underlying felt) and rub it into place. In your situation, I would probably then coat the base sheet with roofing cement, and then in about a week or two I’d coat the cement with an aluminum or elastomeric coating. Though you could get away without coating the flashing, but it won’t last as long.

Here are some photos that don’t show a pipe flashing, but show a core being patched with roof cement and reinforcement fabric. And no, this is not sufficient for flashing in a turbine vent, as this patch is only meant to be temporary. Besides, for those with a sharp eye, the patch is an asphalt based roofing cement on a coal-tar pitch roof.

Though you have to do it a little differently because you will be flashing in a pipe rather than patching a hole, the principle of building up with membrane and mastic is the same:

Photo 1 - Roof core take to inspect roof sandwich and roof deck.

Photo 2 - Core set back in place, but set in roof cement.

Photo 3 - Roof cement applied over patch area after core was set back in place and sealed.

I’ll continue in another post.

to continue:

Photo 4 - Spread roofing cement over the patch area with a trowel.

Photo 5 - Reinforcement fabric set in cement. You will want to use felt and coat the felt so you mate the surfaces like you would if using contact cement.

Photo 6 - After first layer of fabric was troweled into place and re-coated with more cement, and second piece is installed. I like to turn it at a 45 degree angle from the first piece. This won’t be an issue for you.

Photo 7 - Set fabric into roof cement

Photo 8 - Coat fabric

Photo 9 - Apply loose aggregate over patch area

Photo 10 - Mark repair area, just in case!

That is a close to being able to hold your hand as I can get.

Oh, one last thing. I’ve been doing this for a long time so I can keep everything clean. So, even though things look clean in my photos, for you this will not be the case. You will end up wearing the roofing cement and getting it everywhere. In fact, you will get it on places that you won’t know how it got there. We used to always kid that roofing cement has legs!

Also, don’t buy your roof cement from Home Depot or Lowes. Find a roofing/builders supply house, and buy something like Karnak or Tamko. Tubes might cost you more, but they will be cleaner for you to use, as opposed to you sticking your hand in a bucket of cement!

Good luck! :smiley:

Nice tutorial Cerb.

Thanks Axiom!

I hope it can help out others besides Stubs.

I still think that he should use neoprene flashing cement as opposed to plastic asphalt cement.

They are two totally different things, all that they have in common is color and consistency.
Application is the same for both.