structural
I have a detached workshop (30'L x 25'W) with two levels. The second level is mainly (only?) for storage (floor is 3/4" plywood and can be seen in the picture). The floor joists of the second floor are 2×8's separated by 24" and are made of 3 pieces: 4' section at each end with a 16' section in the middle. Each end comes off the wall as shown in the picture.
I want to run 1/2" EMT down the middle of the shop to power my lights. Can I put a 1" hole in the center of every 16' 2×8?
OK, lets start by correcting some terms. The structural members you want to raise are not rafters, they are ceiling joists. You are right that because you have balloon framed walls, that has no effect on the top layer of ceiling joists. Changing these joists have no effect on the balloon framed walls. The thing you have to determine is if the ceiling joists are tied to a load bearing partition somewhere near the center of the house. If you have full roof rafters with a ridge board, it is very unlikely that the ceiling joists are tied into any down supporting bearing wall. Raising them should not be a problem if you plan for support of these joists if there is any center resting point, via a knee wall on the center resting support. If these joists span the entire width of the structure, then you can safely shorten them and reconnect them to the roof rafters. They serve two purposes. First a ceiling joist, and second, a collar tie for the roof rafter. I think you will be fine, however, I'd love to see some pics of the attic area with the joists. Good luck.
I think they're there for stabilizing the brown beams/joists. Typically, a loft is unused space with no floor. Between the joists, there is insulation supported by drywall or other material depending on the age of the house (as in pic 2 and 3).
Typically one would put in blocking between joists to stop horizontal flexing and torsion.
Since an attic is usually unused space, I think someone took a short cut and just screwed the 2x4s to the tops of the joist. (Significant labour savings!). Your flooring probably makes it unnecessary from a flexing point of view, but you still have a torsion problem (especially since you're now loading the joists).
When you go to make improvements, block the joists properly, but I wouldn't worry in the short term.
Best Answer
Drilling a hole in a 2x8 joist would not be a problem, the building codes have guidelines and you could drill holes big enough for 1/2" conduit within those guidelines. However, your roof is not regular joist and rafter construction, you have trusses which are engineered and manufactured and there are no general rules for drilling and notching them. You'd have to consult the manufacturer or engineer. (In fact I think you'd have to check about whether it's suitable for storage, those mending plates in the bottom chord make me think not - but that's a different question.)
Even if it was joists - did you think about how you'd thread sections of EMT through the holes you drilled? It's pretty tricky, it has some flex and you can oversize the holes some to gain some wiggle room, but still. You have 10' sticks of EMT and trusses 2' apart on centers ... think about that a minute. Believe me, you don't want to do that, it would be the hardest possible way to do the job.
There are a number of better alternatives for routing the EMT. You could run it on top of the bottom chord of the trusses, or hang it from the vertical or angled members. I think my preference would be to mount the boxes on the bottom face of the angled members of the trusses, that would give you easy access to the wiring inside and with hangers, it would be easy to support and little to no conduit bending.