What causes a lump or dip in the roof?

Friday, July 20, 2018

A sunken or buckled area in the roof is most easily visible when looking up the slope of the roof from the ground. Here’s four common causes:

Roof leak - Leakage of the roofing that has caused rot and partial collapse of the sheathing in the area of the leak is the #1 cause of a roof dip. It often occurs next to a roof penetration, such as a chimney or dormer, or at the edge of the roof, as in the photo above, and  is easily verified by checking the area in the attic. The sheathing will be dark and discolored when viewed from below, with possible mold growth. Got to our blog post Why is my roof leaking? for more info. 


Warped sheathing - Uneven stresses between the plies of a sheet of plywood can cause it to warp at the edges over time. H-clips are installed at the center of the span of unsupported abutting edges to stiffen the them; but, if they are missing, damaged or installed off-center, buckling can occur. It is usually not a structural problem. To learn more, see our blog post What is an H-clip? 


     Here’s an example of a more advanced sheathing deflection below, often referred to as “buckling.”

Roof impact - When a heavy object such as a tree branch drops on the roof, it will cause a depression in the area and likely some structural damage to sheathing and roof trusses or rafters underneath. In the photo below, the impact depression is accompanied by a small hole.

Structural Defects - Undersize roof rafters or trusses that have had their center web members removed to make room for attic storage can cause a roof to sag along its span. Removal of the lateral bracing necessary for a roof rafters that are framed without a ridge beam will make the ridge dip noticeably, as in the photo below. 

    Minor dips or lumps can also occur at hip ridges or valleys due to poor roof truss alignment or other framing problems. They are typically barely noticeable and not a real problem. One of the advantages for builders of the heavier weight and thickness of dimensional/architectural roof shingles is that they can conceal minor imperfections in the roof deck.

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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 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?

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.

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