Sloped roofs with a ridge depend on the inherent strength of a triangle to create a sturdy roof. The unique property of a triangle is that it cannot be deformed as long as all three sides remain intact and connected. But a rectangle can be deformed into a parallelogram by any load that causes rotation at the connections, and shapes with more than four sides can similarly be deformed at their connecting points.

  The most common cause of sag at the roof ridge is loss of triangulation due to failure of the bottom leg of the triangle, which makes the peak of the roof sag and top of the walls spread outward in the area of failure. 

    Most houses that have a roof framed with rafters use ceiling joists that connect across the house between the bottoms of each pair of rafters to form the necessary triangulation. Although not shown in the diagram below, they are often actually two pieces of lumber joined together over an interior wall or beam. 

    A ridge board or ridge beam at the peak of the roof is also an important component. The ridge board provides a stable nailing surface and adds some lateral stability, whereas a ridge beam is a structural support member that eliminates the need for a connection between the end walls to avoid spread. 

    If the ceiling rafters are removed to “open up the ceiling” in a renovation, ridge sag will occur unless some other form of triangulation is added—such as rafter ties added within the bottom third of the vertical height (rise) of the roof. The house shown below is an example of this problem: ceiling joists were removed to create a higher ceiling and add a shed dormer window, without additional reinforcement.

    A tie in the upper two-thirds of the rise is called a collar tie which, although it adds some rigidity, is too small to create a strong enough triangle to resist spread. 

    Ridge sag is called “deflection” by construction professionals and is easy to spot from the ground when in an advanced stage, but minor deflection is best caught by sighting down the ridge of the roof. The first photo below shows a roof with an area of ridge deflection viewed from the side that is barely noticeable, followed by the same area sighted down the ridge line.

    Another cause of ridge deflection is impact damage to rafters. When the damage occurs on only one side of the roof, the ridge will drop and move sideways, as shown below.

    Rafters that sag because they are overloaded with snow or multiple layers of roofing, undersized for the span, weakened by removal of truss web members or purlin braces in rafter roof framing, or deteriorated due to termite damage or rot will also make the ridge drop as they deform.

    A roof framed with manufactured trusses is less likely to have ridge deflection, unless there is a direct impact on the ridge—which would cause easily visible damage—because of the multiple triangles created by the web members that connect the three sides of the truss. 

    Unfortunately, most roof structural defects don’t respond well to homeowner fixes and require professional evaluation and repair.

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  To learn more about roofs and attics, see these other blog posts:

• Why is my roof sheathing sagging between the trusses?

• 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?

• What causes leaks at a fake roof dormer? 

• 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 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?

• 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.