This solution shown on the right will help make construction of these roofs easier and faster, as shown here in this old gazebo roof I covered with copper. I was converting it to add a cupola.

The real reason these roofs are not vented is only because of the Framer may not have known this alternative design to work this into the construction with all those rafter beams conjoining in the center. Even if they were not thinking of this venting issue this logical solution is a far better design and construction with less board feet used.

A roof vent requires a solution that will not leak even in stormy weather. Carpenters know nail-guns and saws. Most Roofers also come from a mindset of hammers and nails, and are not sheet-metal workers, so this sort of solution escapes their logic, so it's a lot easier to just pretend venting is not needed, knowing that a decade later the homeowner cannot legally come back on them for this negligence to not have vented this part of the roof.

A Solution, even if it is already built:
Few Carpenters realize how much easier it would be to construct these roof; join the stringers over this strong octagonal channel made with common 2x6 boards they probably have left over in there rubbish burn pile. They can simply angle cut the long sides at 22.5 degrees. Then assemble this hollow core with glue to allow for around 100 square inches of vent through the center. If they use 2x8 or 2x10 lumber they can run the grain horizontally for even more strength, but it is best to predrill the screw holes, so there is a lot less chance these boards crack.

If they don't care about venting the plywood can be still run up to the center point and not need any other support underneath, since it is only spanning another 6" to 12" depending on the roof angle.

This center hub is made with just 2x6 boards. It's not wide enough inside to swing a hammer or use a nail gun, which is why most Contractors would veto this idea, but you can see here how easy it would be to attach the roof joists from inside this with deck screws using this 18V impact driver, as shown in the right photo.

Nails are for Neanderthals!

Here is a hexagon hub sent to me by a client, with the side edges trimmed off at a 30 degree angle. Note: he had trimmed the top edge to fit the roof angle and over each seam, so he had flat sides to attach the rater beams onto, but this may be a little trickier to do:

If at the time you are reading this it is not already too late, I can make you one of these in a simple kit or pre-assembled: glued and screwed together, then shipped to you. They run just $100 for a kit, or $175 assembled delivered. These can be made in different sizes to suit your needs.

Below is a much wider Key, which is 36" wide ISD support ring, that I made from a 10' long 2x10. I made it this larger for under a 4' wide cupola to allow easy access and shower down the light from the tinted windows above. It is plenty strong to support over a ton if needed.

These channels can be simply covered with a copper roof cap for making a low profile cupola mounted up off the shingles on some wood blocks to allow for the hot air to escape under them, yet still cover the center hole from wind driven rain, as shown below.

Size Does Matter:
As long as the roof cap is large enough to cover the vent holes by a good 3" to 6" margin (depending on the roof angle), this is a good time to rectify this oversight and add roof venting at the top. You may also need to see what can be done about fresh air entering your attic space at the bottom, so that fresh air will heat up and rise out the top to draw in more fresh air from below for continuous flow of air and minimize moisture build-up, which can cause mold issues. It is easy to just mount a set of spacer blocks under any of my new copper roof cap designs (I wrote this out in detail on our Installation Instruction web page).

Appearances:
I feel it also makes the roof caps look better set up off the shingles to give the roof cap a more solid attachment. Especially if the roof cap has a fancy set of tails, but that is certainly a matter of opinion. When the copper tarnishes to a brown this graduation up off the roofing shingles gives the bottom edges a 3 dimensional distinction that is more easily seen from a distance. Some people might be concerned that bees will build a hive up under this gap below where the screen is. That may be true, but bees will always find some place to build their hive on your property. At least this is way it's up high out of harm from children or guests.

This underside shot shows the wood blocks w/copper screen I mount inside the copper roof cap base (optional) to suspend the cap up 1.5" up over the roofing shingles. The screen is to seal out pests that may want to nest up inside these caps or inside the turret roof it's self. 

This shows the stainless steel screen I now use (5/08) attached with stainless steel screws instead of staples. I switched to using the stainless steel screen since it is stronger than the copper gutter screen I was using, and has a little smaller hole spacing as well: 6 gaps per inch. Any smaller would become plugged with dust over time. Window screen would be too tight of a weave to last w/o cleaning for the 2 to 3 decades it will be in service uninterrupted.

The Jessum Turret Caps
(installed photos)

Most roofing shingles are pretty soft. These wood blocks also help to give the roof cap a more solid mounting surface, so when the stainless steel screws are tightened down it is much less likely to dimple the copper and make sure these SS screws have a good solid wood to attach to. With the pair of longer deck screws holding these block in place it doubles the hold.

The Andreson Turret Cap
(installed photos)

The blocks I use are a knot free wood that is primed and painted with a high quality black paint, so they would not be visible from ground level. Each block has a pair of holes drilled and counter sunk for the 3" deck screws (included) to lock them to the roof. The holes for the Stainless steel screws are pre-drilled as well, so they will not split  or crack the blocks.

The Pray Turret  Roof Caps
(more photos)

This client inspired me to begin pre-mounting these blocks inside the new roof caps (3/07). It is very simple; just put a dab of the expanding glue on the bottom of the blocks just before setting it on the roof, making sure it is level and the riveted seam is to the rear. Then after waiting an hour or so for the glue to set remove the SS screws to remove the roof cap off the blocks/screen and screw the blocks to the roof. Peal off the shingles and remove the plywood above the screen, or use a 3" to 4" hole saw to cut vent holes in the roof. Then just re-attach the roof cap again. Problem solved!

The Bogaards Turret Caps
(some install photos)

I'd recommend at least 1.5 sq" of venting hole for each foot width of turret or gazebo roof. If you use a 4" hole saw this would be about one hole per 4' width, so a 12' wide turret roof should have at least 4 holes cut that size.

The Ober Turret Caps
(for more photos)

The Allison Turret/Gazebo Caps

The Szakacsy Turret Cap

The Masco Gazebo Cap

The Sean Gazebo Cap