When I wrote about valleys I mentioned valleys with two sides with different slopes. When the valley makes a transition from a higher pitch to a lower one and the water applies the Theory of Gravity to it's itinerary, the roofer should consider this nexus a completely different valley. The top/cut side might need to be reversed because water quantity is out voted by water speed.
For example, a steep but small cricket behind a chimney should be roofed last. Water, no matter if it is 10X's in volume, will not crawl up a steeper slope. It will immediately turn and follow the low slope. Pronto.
At these pitch changes the underlayment, the valley metal, and the roofing must terminate and the new underlayment, valley metal, and roofing begin anew. If you're not using metal then you need to sign off right now because I, nor anybody else, can help you. One little poke by a stick and you could lose your family. The amount of water that enters a valley poke qualifies for "irreconcilable differences" in every state but Louisiana.
The other problem encounter at this nexus is the workmanship of the carpenter. At minimum, the deck of the low slope is on top of the steeper slope deck because the framing was done on the steep part first. When the low slope is an addition to an older home the problem is exacerbated by the old roofing. You know the carpenter put his new decking on top of the old roof too causing a bulge on the lower slope.
Now the happily moving water that passes the nexus finds a low spot to converge in. The water will move sideways. Nobody knew this when they left because the dip showed up after the hot Dallas sun softened the roofing and it settled.
The leak now continues down the old roof and leaks far away from the "tie in." Look as he may, the carpenter will never find it.
Again, our old friend Mr. Metal Flashing, comes to the rescue. If a width of metal were placed on top of the shingles on the low slope and the roofing continued on the high slope leaving the bottom of the metal exposed, the leak would not occur. The roofing felt should also be under and above the metal respectively.
Recently I sold a roof repair on Amherst in Dallas, near Highland Park but west of the North Dallas Tollway. I mentioned to the homeowner that he might also develop a leak where his roof was added onto. He climbed onto his roof and saw what I was talking about. He asked me to look at a roof, gutter, and soffit leak repair but I surveyed his entire roof.
I drove for twenty minutes, set up my ladder, sweat, and diagnosed what he wanted me to but I went a step further.
We'll do the additional work for very little but it helped secure the repair and a new friend who has already recommended me to a co-worker in Dallas.
Different slopes for different systems too. A lower slope add on might not be able to handle the Timberline or Certainteed Trilaminate Presidential big boy. The slope is not enough and the water comes to visit inside. The cheaper three tab over the International Residential Code requirement of two ply felt for slopes between 2/12 and 4/12 made be the soup du jour. Any less than a 4/14 pitch and the magic disappears. Now you need a low slope or flat roofing system like an SBS, a TPO, an EPDM, or any other alphabet soup at the industry's cupboard.
Sure the customer will howl that the roof don't match. Challenge him to see for himself, from the ground, because the only ones who drink soup on a roof are roofers and a few French weirdos that have no idea how hot it will be up there in thirty minutes. And I've moved the ladder. Their soup won't get cold.
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