Can Water Flow Up hill?

Many times when a sloped porch roof meets the house roof we see leaks from this area, also in most scenarios there is shingles under the new porch or addition deck and no way for air to get from the eaves to the ridge. In the larger circle of the photo below we are demonstrating a channel cut to allow air flow to the ridge. Two 2"x6" are run horizontal to the roof to carry the weight the rafters. At the smaller upper circle, a slot is cut into the existing roof deck to allow the porch decking to nest in and flow smoothly with the existing deck. The lower sheeting is left to give extra support to the lower slope deck. If the new slope is laid on top with no slot, there is almost always a ponding/flat area there. I have seen leaks in places like that when transitioning from a 6/12 pitch to an 11/12, so no transition is exempt from ponding failure.

In the photo below we demonstrate the best case scenario for a 4/12 Pitch to 6/12 transition. We DO NOT have any shingles under the new deck, this is not what we find to be the case in many applications. Many times a new section is built onto a house and the old shingles are left under the new framing. This means compromised attachments and a more difficult to blend in sheeting point. I did not install underlayment/Ice Guard so that you can see what actually takes place. I am including a photo of a pitch gauge that shows the slope of the area over the void being reduced to 1.9/12 pitch, which is less then the allowed slope for asphalt shingles or residential or agricultural panel metal roofing.

In the following photo we can see the negative impact of putting that new 5/8" deck on top of the shingle, the gap to bridge gets bigger, so the slope is reduced even more.

I will now show you what we have seen far too many times, a 2/12 or even far less slope going into a typical 4/12 pitch Ranch house roof. The shingles have been left under the new framing to demonstrate the worst case situation.

It would appear that many licensed Contractors and DIYers think water can be asked to run UP a slope, at their convenience. The simple act of putting a nail gun tip down on that top shingle makes the lower one flex and sag, as shown, you don't need to push it into the gap to make this happen. This is a worst case scenario, But like I mentioned earlier I have seen leaks on 6/12 to 11/12 transitions that were not blended and supported as we do them.