How To Look At A House
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What would cause a roof to sag?
Monday, January 6, 2020
There are several ways a roof can sag, and each one has its own possible causes:
1) Ridge Line Sag - A sloped roof exerts lateral pressure on the end walls that is bears on. In other words, it will push the walls outward unless the pressure is contained. When a roof is framed with rafters, it is kept in check by collar ties or ceiling rafters; but, if they are disconnected, the roof will splay outward and cause the ridge to drop. Undersize or termite-damaged rafters that sag over time will also cause ridge sag. To learn more, see our blog post What causes a sagging roof ridge line?
2) Sag Between Trusses or Rafters - This can be caused by a roof impact, such as from a tree branch, warped sheathing, rotted sheathing due to a roof leak. Also, in some cases, lack of an H-clip at unsupported edges of the sheathing between truss will cause a sagged area. Go to our blog post What causes a lump or dip in the roof? for more on these problems.
3) Sag at Drip Line - Backed up gutters, corroded or missing flashing, or loss of tab adhesion at drip line are three causes of a roof sage at the drip line. There are others. See our blog post Why do roof edges start leaking?
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To learn more about roofs and attics, see these other blog posts:
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