Portland, Oregon 850 sq ft 1927 double wythe brick house with flat roof (1:30 actual pitch) and parapet ranging from about 10" height to 27" height.  New membrane roof & plywood decking going on after seismic retrofit.  2x8 ceiling joists and 2x8 roof rafters (both are 16" centers), r38 fiberglass batt insulation, and 5 roof vents on roof, but nothing at sides (through the brick walls). 
My question: in one corner, due to slope of roof, the roof & ceiling joists overlap (the opposite corner at high point of roof they are about 18" apart).  So in the area they overlap, I have been told would be good to drill holes in the roof joists to allow air flow between bays, but I have not had any luck tracking down hole size or spacing.  Any suggestions?  Thanks!

Roofs have rafters.

Floors and ceilings have joists.

Are you really talking about drilling laterally through the joists? (Bad idea- nothing good to come of it.)



Or are you talking about drilling holes in the facia boards perpendicular to the rafters & joists that keeps the squirrels from stealing the exposed fiberglass for nesting material (or setting up shop the attic)?

You claim R38, but with on 7-1/4" of vertical height between the 2x8 joist and the roof deck where it meets the wall means you don't have that much. To meet code you need a minimum of 1" of clearance between fiber insulation and the roof deck, which means you're limited to 6-1/4" of insulation (about R22-R23) near the soffits, increasing to R38 in the middle where you have more room to work with.

Or maybe I'm not seeing the-picture of what your framing really looks like(?).

Ah, it is a flat roof, with double wythe (two rows) of brick walls with a parapet surrounding the roof (no soffits, facia, et al). So the slight slope of the flat roof results in the 2x8 joists and 2x8 rafters (as you prefer to call 'em ) overlapping in one corner. So across most of the roof, there is a good sized gap betwixt them and evidently allows for good (enough) air flow. At the area in question there is 12" space between the ceiling below, and roof deck above. At the opposite corner of the roof the space is a little over 30". Anyway, an architect friend suggested the RAFTERS need to have some vent holes drilled through them to allow better air flow in the one corner. We are using a high density fiberglass batt insulation in that corner so I have legal clearance from it to roof deck (1 3/4" is minimum at far corner)... I've seen in some data that people put down furring strips across the rafters for air flow enhancement, but don't really want to do across entire roof to address a problem in one corner. I've also thought I might core holes through bricks at ends of the bays and install some round (screened) soffit vents but kind of a pain to get the core drill up there... So there ya have it! The fun never stops!

I don't understand what you expect to gain by horizontal flow between bays - 1 3/4" is enough. But perhaps you are asking if the attic ventilation is adequate.

There isn't anything to be gained by drilling the rafters, and some structure to lose.

Even high density rock wool batts are only R25 @ 6-1/4" (1' clearance with 2x8 framing), and if you're really have 1-3/4" clearance that's only 5-1/2" of batt thickness. R23 rock wool is the highest performance batt solution that fits. HD fiberglass would come in at R21, barely more than half the stated R38.

If the membrane roof isn't installed yet, putting a few inches of rigid foam between the membrane & roof deck would eliminate moisture issues in the thin section, eliminating the need for venting the attic and allows you to put the batts in contact with the roof deck. Per IRC 2012 chapter 8 you would need a minimum of R10 above the roof deck (you are in climate zone 4C), at a total center-cavity R of R49:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

To get there most cheaply with something that is still remotely walkable would be 2.5" of Type-II EPS (1.5lbs per cubic foot density, R4.2/inch). For R10 that would run about a buck a square foot for virgin-stock, but about 25-30cents/ft^2 if using reclaimed foam from commercial roof demolition. Using 2 layers with staggered seams is better than a single layer, due to seasonal & temperature related dimensional changes opening/closing gaps where the sheet foam edges meet.

With 3" of exterior foam (R12.6, if EPS) you could then drop to standard density R25s between the rafters and still end up at ~R38, but since the rafters would be thermally broken by the foam above, it would outperform R38 between 2 x 12 joists. If you want to take it up to IRC 2012 levels you could put 4-5" of polyiso above the roof deck, with R25s between the rafters. Kraft facers on the batts would be fine.

Reclaimed foam is really cheap, if you have a local source. I have several reclaimed foam vendors large & small in my area (eg: http://www.greeninsulationgroup.com/ ) These folks ship anywhere in the US (for a price) if the quantity warrants it:  http://www.nationwidefoam.com/

Ultimately, yes, the concern is with sufficient attic ventilation to prevent condensation on bottom side of decking.  According to Building Science Corporation website (lots of great info there!), even with best of attempts, there will always be a low level of warm moist air leakage into attic space, and is why (for a cold attic), proper ventilation is very important.   In the corner in question, since the rafters & joists overlap, those areas are kind of dead ends for air flow, and I believe the architect I spoke with was saying that to ensure there is no rot in decking at that corner, it would be a good idea to add some ventilation holes in rafters above the level of insulation.  I'm going to speak with the building inspector and see what his/her take on it is...  thanks for your interest- 

For about $300 (if reclaimed) to $1000 (if virgin-stock) worth of foam you can skip the ventilation and have a higher thermal performance than what you're planning, and a more resiliant & reliable solution overall.

And that's without calculating the savings of going to mid-density batts from high density, and the cost of the vents, etc. It could even be cheaper to do it that way.

You typically need a 1:300 or 1:150 vent ratio with 50% of it located in the soffit area (I assume some in each bay) and 50% in high/centrally located vents. But your code may vary and your attic doesn't have the usual 3+' of height. On the other hand, very little moisture comes through a well sealed attic floor and it doesn't take much to remove it. And you don't have ice dam issues on a flat roof.

Given that the roof terminates into a brick wall (that extends higher in a parapet), there's absolutely no way to do soffit-ridge venting correctly without coring large vents through a double-wythe brick wall.

Everything about this situation screams "unvented roof", with a decent fraction of the R-value above the roof deck.

Vent holes can be done any type of meta roofs. As you described in your question I think you should make holes through drills in the roof joists to allow air flow between bays as it help in the ventilation as well as it protects your house.