I suggest tearing it all off and replacing it with a product that is designed for such a low-slope roof.
Asphalt shingles are generally only designed for roof pitches above 3" slope per foot.

Andy.

 

That lower part looks little shallow for shingles, Personally I would metal the whole thing, very least I&W the lower part.

 

What is the pitch of the back section? When they added the back they did not do enough at the join, causing the leak,

 

My gut says I should rip off all of the shingles, lay a course or two of ice and water shield at the valley, and re-shingle the roof.

That will be fine if it is done properly

 

If physically able, that roof is made for diy. Big enough tarp, one side first, ready to cover. Remove all old shingles. The eave edge looks like maybe there are 2 layers. That's about max for architectural shingles.
Low slope, whole back roof looks low enough, cover all deck with ice shield. It closes around nails so still protected from wind blown rain. Roll roofing is what's usually used though rolls have less life. But my roof, rubber modified sbs roll, is now on 25th year. Though nj. Some under tree shade, some full sun. Base sheet then I hand troweled cold adhesive. Hand trowel because I was an amateur. I think the longer than usual life is due to cold adhesive that went on thicker than usual using hand trowel. Even now there is no apparent cracks in the roofing though it must be assumed this roof is well past its service life. As such, I assume the cold adhesive is also serving to water proof. Roll roof, its life can be extended somewhat with roof coatings. Usually acrylic. Search the forum for coatings.
Mine is 2 ply, base and cap sheet. There are 3 ply roofing also that give longer life. Not a roofer and I don't know about these 3 ply roofs. If the cap sheet starts to let water thru, one can assume the layer below protects but what does water/moisture do to the adhesive? Start lots of bubbles heated by sun? Just a guess.

 

I'd strip it down to the wood decking, replace any rotted or soft decking, add self-adhered base sheet (I&W shield) over the whole roof, and then lay a self-adhered top sheet (AKA self adhered rolled roofing).

 

I found one system. Is this about what you are suggesting?
How To Install a DeckSeal Low Slope Self-Adhered Roofing System - YouTube

 

If your budget is good for 100% ice shield cover, you might consider architectural shingles.
Always saying roll roofs have shorter life, got curious why. There are as many low slope/flat roofs as sloped roofs, so why not make roll roofing as durable? Few pages of search and no clear answers. Usual such as it would get too heavy and such. Someone said larger sheets, rolls, move more so more likely to develop cracks.
Grace ice shields. It melts quickly under sun. Even hot temps. Gets dicy to walk on. I put tarpaper over it with cap nails every 12" or so on top edge? then just half inch staples. Holds at least for not too steep roof. Then shingle as usual.

BTW, if decide to start with roll roof then shingles where slope changes, roll roof must go under the shingles for about 36". This is to prevent wind driven rain.

 

My garage looks similar, and after getting a metal roof professionally installed on my house, I talked my son into helping me replace the garage shingles DIY. With no funny gables or hips, it was pretty easy. I ordered all the sheets to length so there was no cutting. The hardest part was tearing the shingles off and patching the rotten deck. The panels were face screwed and went on in half a day. No more shingles for me.

 

This is incredibly helpful everyone, thank you! It took me a while to get to this (plenty of other projects) but I finally got up on the roof to take measurements.


@Nealtw
The ridge to valley (steeper) pitch is 3.4:12 (drops 20.5in/52cm over 72in/183cm).
The valley to eave (flatter) pitch is 1.5:12 (drops 9in/23cm over 72in/183cm).

It sounds like the best method is to:

1. Rip off the shingles
2. Ice and Water either the valley (OK), lower flatter section (better), or whole roof (best)
3. Replace with (A) strapping and metal roofing, (B) roll roof, or (C) EPDM.

I have only ever installed basic metal roofing, so given the options I would prefer to go that route unless the roll or EPDM roof would be significantly better for my low slope. I know that metal roofs are usually for 2:12 or 3:12 or steeper.

@AndyGump is there a particular product you would recommend DIY for a low slope (1.5:12) roof?

@carpdad @HotRodx10 it sounds like you both prefer the roll roofing. Do you think it would be better than metal?

@vandamme I think I could replace this DIY with metal pretty easily. You had no issues with your low slope and regular DIY metal?

Thank you all!

 

@carpdad @HotRodx10 it sounds like you both prefer the roll roofing. Do you think it would be better than metal?

Not necessarily better. I thought easier and probably cheaper. If you've done metal roofing before, that might be easier for you.

 

@vandamme I think I could replace this DIY with metal pretty easily. You had no issues with your low slope and regular DIY metal?
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I had a shed with a 3:12 roof tacked onto the 5:12 garage. I did have a leak where they joined; there's a special flashing that's supposed to go between them, but was a little crooked or something and water backed up under the flashing. I tarred the crap out of the joint and no more leak. So far.