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:
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