My contractor has just finished installing a new beam and columns on my flat roof and now I am going to install a new ceiling on the flat roof. Currently, there are exposed 2x6 joists and the ceiling is about 16' x 34'. It is open to the attic. I plan to install HB&G permaboard planks (5 1/2" x 12'). I will first install new recessed lights in the ceiling. I then plan to add kraft faced insulation between the joists to keep some of the heat out during our Florida summers. I was talking to an inpsector and they told me I need to ventilate the ceiling. This I was not aware of and am unsure how to go about it. Would grill vents at specific intervals near the attic be appropriate? I've also considered the 4 1/2" wide continuous soffit vent, which I could install where the flat roof joists go into the attic. Not sure which is best.

Any adice would be greatly appreciated.

I don't understand how you can have a flat roof and still have an attic. So the picture of your construction is very unclear, and that makes it difficult to comment. Also, being from the northeast, I don't understand what a lanai is. If it is a screened enclosure, there is little point in insulating, because the temperature inside will be the same, essentially, as the temperature outside.

I'm sorry for not being more clear. I was originally from the north as well. A lanai is basically a deck or porch. There is a flat roof over it, but the rest of the porch is open. There is a pool and a screen enclosure over it that attaches to the flat roof. The contractors here suggested insulating the flat roof, between the 2 x 6 joists. Their reasoning was that it would dissipate some of the heat emanating from the flat roof. There is no attic on it but the flat roof is attached to the house and you can look between the flat roof joists and see the attic of the house. The inspector was here and stated that when we cover the ceiling of the flat roof, we have to have proper ventilation. I am assuming this would work just as it does with the soffits around the house in that I should provide air into the attic.

Thank you for your comments.

It seems to me that insulating the roof of an open porch is pointless. However, ventilating the roof structure when it is enclosed with a ceiling is necessary. I do not feel that the roof should be connected to the attic. I think it should be entirely independent, if for no ther reason than to help prevent the spread of fire, so I would suggest firestopping the joist spaces at the attic wall.

Your porch (lanai) roof is 16 x 34, or 544 square feet. It needs to have 1/150th of that area in ventilation clear area. That is 3.6 square feet of vent clear area. Assuming the joists span the 16 foot dimension, you should have 34 feet of soffit at the outboard side, so if there were a soffit vent the full length, with a net free area of 50%, almost anything you would install there would be sufficient.

However, if the roof were to be insulated, an air space above the insulation would be required for ventilation purposes. Still, I see no advantage to insulating. The roof would remain cooler if it is above an uninsulated but vented space, which is good for the life-span of the roof. If possible (unless it's too late) use a light-colored roof, to reflect rather than absorb heat.

Thanks for your advice Richard. If I was to insulate, the 2x6 joists, I would do so with 4.5 inch insulation but your point on insulating is well taken. At the very least, I will use the 4.5" soffit vent along the 34' outside edge. FYI. The joists run the 16' length and are firestopped. I will speak with a few other contractors down here to get their opinion on insulating or not. You bring up good points; if it's pointless, why do it?

Thanks for your help.