1.) What would be the relative costs of each option? i.e. is Mod Bit more or less than EPDM, and same for a Liquid coating?
**Mod bit would cost the most, then EPDM and Hydro-stop would be close with Hydro-stop most likely being a little cheaper. Just remember, you will get what you pay for. **
2.) What’s the relative durability and maintenance level? I don’t mind some fairly regular upkeep, since I have to get on this roof once or twice a year to clear leaves from the gutter anyway.
The mod. bit. will be the most durable. I noticed you have trees near your house, and you should keep the branches trimmed so they don’t come into contact with the roof, but this is especially true with the EPDM.
3.) is the current roof modified bitumen? From what I’ve seen it looks like it is, but I’m no expert.
**It is hard to tell from the photos, but since the laps appear to have been nailed down I’m going to guess that it is 90# rolled roofing. However, it could also be a poorly installed mod. bit. **
4.) Are any of these options DIY-friendly? I’d think not, but I have done quite a bit of non-roof related remodeling work (2 kitchens, 1 basement), and I have a coworker who spent a summer helping a commercial roofing crew do rubber roofs who could help.
**The only one that I would attempt as a DIY’er would be the Hydro-Stop. The other ones definitely need to be done by professionals. **
5.) Assuming I need to find a contractor, am I better off dealing with a residential roofer who usually does normal slope shingles, or trying to get a commercial contractor to do a small residential job?
** You would be well advised to find a commercial contractor. You will want to call around to find a small-to-mid-sized contractor, but be careful since some of these contractors are “big” for a reason. I wouldn’t want a shingle roofer touching a low-sloped roof if it isn’t something they do on a regular basis. **
Now lets talk about some other issues.
I noticed it doesn't appear the chimney is properly counterflashed. You will want the roofer to saw-cut a reglet into the chimney and install metal counterflashing that is secured with lead plugs and sealed with a polyurethane sealant.
I couldn't tell from the photos, but it looked like the masonry chimney cap may also be in poor repair. You may want to set up a ladder adjacent to the chimney and check it out before you start roofing. While you are at it, check all your exposed mortar joints and make sure there are no voids. Even small openings in the mortar joints can leak, so examine the chimney carefully.
When replacing the roof, make sure the roofers extend the new membrane at least 12-18 inches up the sloped shingle roof adjacent to the low-sloped roof, and make sure the membrane extends beneath the felt underlayment for the shingles. The roofer is going to need to remove a couple of rows of shingles to properly flash the transition.
Looking at the first photo, it appears there is a peel-and-stick membrane or something that covers the back leg of the gutter and extends up and possibly beneath the existing roofing membrane. I don't see any perimeter metal flashing, so these are issues you will want to address with your new roof.
Hydro-Stop was suggested by one poster, and that wouldn't be a bad temporary repair, but I think you would be better off spending that money on having a good new roof installed. If you have any nails backing out of the laps, the Hydro-Stop isn't going to stop them from puncturing the roof and cauing a leak. Futhermore, the Hydro-Stop is only going to be as good as the substrate you apply it to, and your existing roof doesn't look all that great.
Where is it that you live that you are wondering if you can find a qualified roofing contractor?
Anyway, if you have any more questions, feel free to ask.