I'm redoing my entire roof (roof deck included). The previous owner built a carport parallel to the house. The carport is a permanent structure with a normal roof - trusses and whatnot of equal quality to the house itself.

The two roofs are parallel. They come together at a valley roughly 12 feet long with almost zero slope in the valley itself. I'm worried that water will run down either roof and just stay in the valley. How can I flash this properly so it will not leak?

House meets parallel carport

Responding to Ecnerwal:

Are you talking about something like this? Modified roof peak

  • Re your diagram, it would actually be easier to put a ridge in the middle and slope it both to the front and back. That way you get a better ratio of slope to height and don't have to cover the high end at the end of the roof (top in your diagram).
    – Comintern
    Jul 12, 2014 at 2:15

1 Answer 1


Since you are redoing the roof deck, you are at an ideal time to fix this right, or as right as you can without redoing the major framing.

Depending whether you want the water to all go one direction, or go two ways, you build a flat (but sloping) triangular roof in the valley - either raised at least one foot and sloping all the way to one end, or raised at least 6" in the middle and sloping both ways.Cover with ice and water shield (first) and metal flashing in a wide continuous sheet running up both roof slopes. Essentially, you are giving this valley floor some slope - to do so you have to give it some width as it rises from where it is now.

  • That sounds great! I modified my question to clarify. Is that what you're talking about? Jul 12, 2014 at 1:46
  • Yup, that is the idea! You probaly should use 3 passes of Ice and water shield (if not doing the whole roof, which some folks do, but it's spendy) so you have one on the vally floor and one on either side overlapping the floor peice (I&W is adhesive backed if you haven't met it before)
    – Ecnerwal
    Jul 12, 2014 at 1:49

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