Vent for screen porch with flat roof?

2383 Views 8 Replies 5 Participants Last post by Bud9051, Mar 20, 2016

LarryJ-nova

Discussion Starter · Mar 20, 2016 (Edited)

I'm having an EPDM roof installed on my screen porch Tuesday. It has a flat roof (only 4" drop over 14 ft.). Will ask the contractor about the need for venting, but wanted to get thoughts here as well. It's not included in their estimate. I suspect the roof will be black. Porch faces east. Not really shaded. Live in Virginia Beach.

The ceiling was removed to replace some rafters and all roof sheathing, but I plan to re-install the ceiling. The 14' rafters (2x6s) will basically be sandwiched between ceiling and roof. The rafters overhang the far end of the porch by 5 inches. The underside of the rafter overhand is finished, so I've cut three vents on the underside. However, air flow will be limited to the far end due to the 2x6 spacers placed midway bewteen each rafter for structural support.

Is there a need for venting? If so, do I cut holes in rafter spacers to allow for air flow? Or, is it best to drop the ceiling four inches on the house end to allow air flow from underhang to a roof vent. How many vents for a 14'x14' roof.

Thanks.

Save

1 - 9 of 9 Posts

Bud9051

· #2 · Mar 20, 2016

Horizontal natural venting doesn't work very well as there is little height difference. If the eave is exposed to some wind, that can get the air moving if you provide a path. Not clear what your path is, but needs either vents for each rafter channel or a common cavity where a few vents feed air into all. Then a common space at the other end where the air can exit.

Bud

Save

joecaption

· #3 · Mar 20, 2016

No location mentioned, we have no idea how the ceiling was insulated, no pictures, so with out all the info it's had to say, I do have to say I agree unless there was enough space between the rafters and the ceiling joist any roof venting is useless.

Save

craig11152

· #4 · Mar 20, 2016

I know your planning to reinstall the ceiling but if it were me I would leave it open and just paint the underside of the deck and rafters some color I...errrr.....my wife found pleasing

Save

SeniorSitizen

· #5 · Mar 20, 2016

The spacers that were installed mid-way are of no value structurally and to have any venting success they will need to be removed and each rafter bay will need to be vented.

Save

LarryJ-nova

Discussion Starter · #6 · Mar 20, 2016

Bud9051 said:
Horizontal natural venting doesn't work very well as there is little height difference. If the eave is exposed to some wind, that can get the air moving if you provide a path. Not clear what your path is, but needs either vents for each rafter channel or a common cavity where a few vents feed air into all. Then a common space at the other end where the air can exit.

Bud

craig11152 said:
I know your planning to reinstall the ceiling but if it were me I would leave it open and just paint the underside of the deck and rafters some color I...errrr.....my wife found pleasing

SeniorSitizen said:
The spacers that were installed mid-way are of no value structurally and to have any venting success they will need to be removed and each rafter bay will need to be vented.

Thanks for the replies. Good points to consider in all of them. Just to take a step back, and for my education, what would be the point of ventilation in this case where the inside space isn't heated. I'm assuming that condensation prevention isn't the goal, but overheating is. What will "overheating" the rafter space result in.

I should also add that the contractor will be installing 1/2" insulation board under the membrane. Will that keep interior temps down enough to eliminate the need for ventilation.

Save

Bud9051

· #7 · Mar 20, 2016

Larry "I'm assuming that condensation prevention isn't the goal, but overheating is."
Correct, this would be for heat only and would have minimal effect, IMO.

I would need to see pictures to speculate as to where vents should go, if used.

Why were the rafters and sheathing being replaced?

Bud

Save

LarryJ-nova

Discussion Starter · #8 · Mar 20, 2016

Bud9051 said:
Correct, this would be for heat only and would have minimal effect, IMO.

Why were the rafters and sheathing being replaced?

Bud

So if "overheating" would have minimal impact or effect on the roof structure, then why ventilate? If the OSB sheathing isn't going to warp or deform with overheating, then why ventilate?

I know where to put rooftop vents, just wondering/questioning if it's necessary.

Wood was replaced due to rot. The old roof was a rolled asphalt material that had lots of leaks.

Save

Bud9051

· #9 · Mar 20, 2016

One of the research facilities in Florida tested unvented roof assemblies to see if they would overheat and shorten the life of the shingles. Their conclusion was the temp rise was a max of 8 degrees (if memory serves me) and would not shorten the shingle life. And, unvented roof assemblies where they spray foam on the bottom of the roof have become very common. I don't like them but for other reasons. I'm a long ways from FL and way up here I have seen shingles with poor or no ventilation curling. But, your roof will still transfer some of that heat to the ceiling and dissipate it to the open air below. I wouldn't expect a problem.

Leave any soffit venting you have as it is and that alone will help.

Bud

Save

Reactions: 1

1 - 9 of 9 Posts

This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.

Top Contributors this Month