So your roof is looking a little shabby and you are wondering when it's time to replace your roof. There are some places on your roof you can check, they tend to be the weakest points on the roof due to high volume of water or trees covering a section of a roof.
Asphalt shingles are a large blanket covering an area of a lot of moisture. Over the years it is quite common especially with bad ventilation to notice that the shingles curl upwards. Over the years the moisture need to find a way out, when the heat softens the asphalt the shingles actually go against gravity and curl upwards, cedar shake has a tendency to do this as well. over time the curling cause the shingles and shakes to split and crack. It generally takes three in a row to split vertically and trouble will begin.
Chimney and skylights over time have the ability to accumulate debris and moss growth around the base where they meet the roof. generally behind the chimney or skylight enough debris accumulates for the water to dam up and push the flow against gravity up a shingle and into the attic. you could stop this leak by cleaning around the flashing and making sure that water has a clear path towards the eaves trough.
Valleys are the area on a roof where two angled roofs meet and is also a point on the roof that has the most water running along it. Using steel in valleys is a common but new practice with the majority of the roofs have knitted shingles in the valley. Shingles are strong against the elements but are not designed to deal with that kind of flow of water. Valleys are probably the biggest reason roofs leak.
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