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:
• Why is granule loss a problem for an asphalt shingle roof?
• What are the mistakes to avoid when doing attic improvements?
• What causes roof shingles to curl up at corners?
• What causes shingles to buckle along a line on the roof?
• Why is my roof leaking?
• What causes leaks at a fake roof dormer?
• What causes a sagging roof ridge line?
• What causes bubble-like blisters in a built-up and gravel roof?
• Why does it cost so much more to replace a steep roof than a low slope roof?
• What is "ponding" on a flat roof?
• Is an attic required to have a light by the building code?
• How can I inspect my roof for hurricane damage?
• Why is premature curl of roof shingles a problem?
• How can I tell if a roof has more than one layer of shingles?
• What are the common problems with attic insulation?
• What is the life expectancy of an asbestos cement shingle roof?
• What's the average lifespan of a roof?
• Why is it a mistake to replace a roof and not replace its flashings?
• Why is there no attic access hatch in the house?
• What is the building code requirement for an attic access hatch, scuttle, or door?
• Does a roof with multiple layers of shingles last longer?
• What can I do to prevent roof leaks?
• Are roof trusses better than roof rafters (stick framing)?
• Why is a popped nail in a shingle roof a problem? How do I fix it?
• What are the most common problems with wood roof trusses?
• What causes a lump or dip in the roof?
• If my roof is not leaking, why does it need to be replaced?
• How can I be sure my roofing contractor got a permit?
• How many layers of roofing are allowed on a home?
• What are the dark lines running parallel to shingles on my roof?
• Can metal roofing be used on a low slope/pitch roof?
• How can I make my roof last longer?
• What are the warning signs of a dangerous attic pull-down ladder?
• How can I find out the age of a roof?
• Should I buy a house that needs a new roof?
• Should I buy a house with an old roof?
• What are those metal boxes on the roof?
• What does "lack of tab adhesion" in an asphalt shingle roof mean?
• Why do roof edges start leaking?
• Why do my dormer windows leak?
• Do home inspectors go on the roof? Do they get in the attic?
• Should I put gutters on the house?
• How much of a roof truss can I cut out to make a storage platform in the attic?
• What's the difference between an "architectural" and a regular shingle roof?
• What does a home inspector look for when examining a roof?
• Do stains on the ceiling mean the roof is leaking?
• How can I tell if the house needs a new roof?
• Why does my homeowner's insurance want a roof inspection?
• What are the hazards to avoid when going into an attic?
Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.