Mastering Roof Inspections: Metal Roofs, Part 8

There are two types of fastening systems for metal panel roofs:  through-fastening and concealed clips.

Through-Fastening 

Through-fastening systems use fasteners that penetrate the metal and have gaskets designed to keep the penetration from leaking. Large-head screws with neoprene washers are the most common type.

Fasteners are often exposed and are typically driven through areas where panels overlap and/or through other locations specified by the manufacturer.

Thermal expansion and contraction of the metal roofing, and possibly movement of the substrate, can cause through-fasteners to loosen or back out over time. Metal roofing with loose or missing fasteners will suffer reduced wind resistance and possible moisture intrusion.  These should be mentioned in your report.

Because through-fasteners don’t allow metal to freely expand and contract, they may cause oil-canning, especially in roofs with larger panels.

Clip Fastening

The illustrations below show a variety of metal seams used in the roofing industry.

A number of fastener-related problems may cause leaks or corrosion. Fasteners may be:

• over-driven;
• under-driven;
• driven in crooked or at an angle;
• missing;
• protruding;
• poorly anchored.  Fasteners that are driven into gaps between panels or plywood panels with voids may have inadequate withdrawal strength; and
• the wrong type.  Fasteners not designed for use with metal roofing can be a problem due to galvanic corrosion, inadequate withdrawal strength, or failure to allow for thermal expansion and contraction of metal panels.
  
  

Walking Metal Panel Roofs

Metal roofs can be slippery when they’re dusty or wet. Stay off of them if these conditions exist. Soft-soled shoes or roof boots can help you maintain traction.