Question about flat roof with tie-in to shed dormer

Hey folks,

Trying to make sense of this roofing project I’ve got approaching. It’s above a mudroom, has been leaking for a while as you can see, very low pitch. I’m thinking this probably should have had a rubber roof on there but it seems like they just shingled in layers of asphalt to create a slight pitch or something? My plan was to add OSB sheathing and then apply a rubber roof but then I realized it doesn’t really look easy to tie-in the shingles from the shed dormer and the rest of the roof, the small valley portion.

This will be my first official DIY on the roof. Last summer I spent some time fixing leaks on the other side of the house, removing shingles, laying ice and water shield, felt paper, and replacement shingles, but this is less straightforward and getting into more building science that I’m not as familiar with.

I may be totally wrong about the “flatness” under those shingles and end up finding a structure built completely different, but that I won’t know until I tear off the shingles. Received advice to tear off the drywall in the room below and get a look at the pitch of the ceiling joists-Probably do that first. The rest of the roof has shiplap sheathing so I’m guessing I should expect it here as well.

If anyone can offer some suggestions it would be much appreciated. If it’s a better idea just to go with shingles again I’ll just make sure I’ve got plenty of ice/water shield under there etc, I’m open to that as well but just wondering if that’s what is called for here.

What I keep contemplating is whether the decking should look something like he photo with the lines, with a metal channel between red and blue running off the corner and into the gutter? If that’s the case, how do I tie-in the rubber portion of the roof to the shingles? Or does that mean stick with shingles?

On another forum it was advised that the window sill is probably too low (will have to be replaced because of rot.) Is it possible removing the shingles by the window will reveal a space that will put the edge of the flat roof in line with the rest of the roof? Any flattening of the roof would definitely call for rubber wouldn’t it? Water has to be directed away from that window and too those corners I think.

That’s what I’m not clear on. Does any of this make sense the way I’m asking it?

Thanks in advance.

Here’s a better shot of the windows:

Can’t really see the pitch from those pics

Phone pressed against window framing:

I’ve seen millions of shingles. I need a big picture to get an idea of how to proceed.

Thanks. Can the photo be from the ground or should I get the ladder out and get level with the roof?

Want to see how the flat ties into the steep sides and window. I realize it might be a hard shot to get, but the big picture is initial size-up. Tells you what went wrong and if it is correctable/needs tweaked or what needs to be done to make it last.

I would install self adhered Modified bitumen
On all that low slope.
And it would match the shingles.

Edit… specifically “Certainteed” “Flintlastic”
One roll equals 100 sq feet or one square.
You will need to buy its underlayment.
The nail base underlayment covers 2 squares.
You’ll need some simplex also.

Thanks for the name of that system. I had often wondered what it was called, my neighbor down below me, she had put that on a flat area and I knew it wasn’t rubber because of the granules. The thing is, as far as tying it into the rest of the roofline, would that look more like this? Self-adhered modified bitumen below the line, shingles above?

Watching a GAF Liberty product installation video, seems membranes like these (and even ice water & shield?) shouldn’t be applied directly to sheathing? I wasn’t aware of that. Didn’t realize it was a nightmare if you ever needed to remove it. Thanks for the info. I’ll look it up.

This project is beyond the he skill level of most diyers. Flat roofs (most types) in the field are mostly pretty simple to install, it’s where it meets walls and penetrations that are difficult. And with the conditions of your window and siding it’s really gonna be tricky to do properly. Removing siding and parts of your window will be necessary.

I know this isn’t what you wanna hear but you may cost yourself more in the long run by attempting this project.


You would be peel and sticking the “cap sheet”
You would be nailing the underlayment.
Yeah, as simple as this system is,
Doing it on this structure isnt DIY.
Oh well.
I was hoping you were Just looking for the correct material to use.
We dont have the years it would take to explain it all.

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Also, this entire roof needs redone. Not just the area you are talking about. It’s a bad idea to try to tie into shingles in such poor condition. If you wait, in the near future when there are issues with the shingles above the flat you will also have to worry about not damaging the newly installed flat roof in the process.

Maybe your plan all along was to redo the entire house but from my understanding of your original post it seems like you just want to stop the leaks under the flat roof.

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Oh hell no, he wasnt planning on tying into those shingles, was he???

This entire roof should have been replaced 10 years ago.

You cant Tie into these shingles!!!
No way!!!
It needs all new roof for sure!!!
10 years ago.

Do the whole porch roof with S mod bit, shingle the rest.