Sloped to flat roof TPO Tie in Questions

Thanks to everybody who contributes to make this forum such a great resource!

I am planning to re-roof a low-slope house addition that ties into a 4/12 sloped roof. The existing sloped roof is asphalt shingle and the existing low-sloped addition is BUR. My plan is to overlay the BUR with polyiso insulation boards and fully-adhered TPO.

I’m planning to continue the TPO up the sloped roof two to three feet. My questions about this are as follows:

  1. The sloped roof deck is nominal one inch boards. I’m assuming the TPO can’t be adhered directly to the boards. What is the best choice for a thin underlayment to fasten to the deck boards first?
  2. I’ve read various takes on TPO’s resistance to asphalt. Is is necessary to have a layer of aluminum between the shingles where they overlap the TPO? Would the aluminum be simple roll flashing or something else.
  3. How far down over the TPO should the shingles be nailed. Is there anything needed to seal the lowest shingles to the aluminum and/or TPO to prevent uplift?

Now for the trickiest part: I could use some suggestions on how to tie-in a corner where the addition ends in the middle of the sloped roof. Please see photo (sorry only allowed one photo as I am new user). It’s hard to see from the photo, but the flat roof overhangs the sloped roof here about six inches. There is then a 1x trim board coming down from the edge. The BUR has a lead flashing liberally blobbed with tar. Do I have the TPO come out a foot or so to the left? If so, what keeps it fastened down over the shingles. Or do I use metal? And how to fasten to the TPO? Assistance appreciated.!

In Northern Indiana, we always went 2’ upslope due to snow. You don’t need to leave the tpo exposed, just work it into the step flashing.

Why TPO? The only reason I know that contractors ever used it was based on cost. Such a small area why not use a PVC roof that may last 20 years versus TPO that will most likely fail after 10? Or better yet a 3 ply peel and stick?

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They must do some awesome marketing.

Thanks, Tileman and Darkthirty. I’ll consider PVC for this project.

Darkthirty, I’m unclear of what you mean by working the PVC under the step flashing. It’s not clear from the picture, but the flat roof overhangs the sloped roof by six inches or so. I’m attaching a view from the underside.

When you reach the level of the flat roof with the step flashing, you use the membrane as your last step flashing and shingle over the rest. Keep your nails at least 18" above the transition. Use water-cutoff as an adhesive/sealant under the 1st course of shingles just above the exposure, so you don’t create a dam.

To use the membrane as the last step flashing it would have to wrap around the fascia board, be adhered somehow to the underside of the six inch overhang before coming down the wall. I’m thinking the way this was constructed in the first place is very questionable. Maybe I’m misunderstanding something?

Yup. I always step flash the wall under the overhang and step flash the fascia where it meets the roof. Look up EPDM transition details.

Got it - step flash the fascia. Something the previous licensed roofer didn’t bother to do. Thank you so much for your helpful insights on this project.

Make your last flashing long and slit the top down to the transition and lay both sides of the split flat, shingle, run the drip edge on the flat, then next flashing will be the membrane. A lot easier to do than it is to explain.

Shingle asphalt won’t really hurt TPO.

TPO adheres to wood just fine.

There shouldn’t anything necessary but if you are worried you can run a sheet metal flashing under the last row of shingles.