Flat roof venting question

I’ve got a similar issue with the posts

I’ve got flat roofs too and live in a hot area. I’ve painted the roof with a white Elastomeric to reduce the heat. But the house still gets heat trapped in it. The house has 3 roof levels, the heat gets trapped mostly in the highest level.

I’m interested in knowing if putting in roof vents in the upper most roof will reduce the heat inside the house.

The key issues are:

  1. Want to reduce the heat the gets trapped in the house’s upper floor during the hot months.
  2. But don’t want to lose the heat in the upper floor of the house during the cold months.

Tried to put on a link for pictures I have, but it wouldn’t let me as a new user


If your roofs are really flat, vents will do little to help you get rid of the heat. Air just doesn’t move along a flat plain. The best answer would be to increase the insulation on the roof or in the attic. The elastomeric coating helps.

Flat roofs are hard to ventilate, but it can be done. Turbines or ext fans work best for any roof. Though they can often be an eye sore, they help tremendously. In Texas I have been putting a pvc membrane down on flat roofs for the past several months. These membranes help to reduce 30-40% of the heat, and they are incredibly durable. We order them straight from the manufacture and they have to be customized to each roof. Having said that, they can be pricy.

The best way to prevent trapping heat is to make sure the R-value of your insulation is at R-60. After that, you can put fans in to help pull the air out. The best thing is to make sure the air can circulate. Just by making the air move you cool down the attic. It is also important to understand, that in the summer, when the temperature out side is 100 you should expect the attic space to be 120+ degrees. Roof decking is not meant to protect from heat, but a properly insulated attic, and fans are your best bet.

Hope this helps!

The roof was re-roof just 2 years ago. Had I known then that I needed more instulation I woud have done it then. But now, I cannot remove the roof to add the insulation.

The roof is flat. It appears that the roof is made of 2" X 12"s, there is about 2" of insulation at the bottom of the space, then the 2" X 12"s are covered by plywood, then the asphalt roofing (with tar).
stevevarma [dot] com/roof/roof.htm if this link is accepted by the forum, you can see a couple shots of the roof.

C4Roofing Specialist - can you give me the manufactuer information for the PVC membrane?

Can ext fans/turbines be put in to let the hot air out of this limited space?
Can I put in a vent from the 3rd floor ceiling in to get the heat out of the upstairs room into the limited roof space to be taken out by the exhaust fans? Can this be set to they are turned off for the colder months so I don’t lose the heat from the upper floor?


Next time you roof it, or if you’re considering re roofing it soon look into an Iso board called vented R board. I’ve ran into it a lot on fed & state jobs the past 10 or 15 years. Architechs & engineers swear by it.