What H-Clips Do
H-clips provide additional stiffness between bearing points for the unsupported edges of wood panel (plywood or OSB) roof sheathing, along with the necessary 1/8” gap for expansion, as shown below illustrated on a bag of Simpson H-clips. Some older H-clips do not provide a built-in gap.
Not Always Required But A Good Idea
Most builders install them, but they are not automatically required for all applications. The International Residential Code (IRC) and the Residential Edition of the Florida Building Code (FBC) both refer to Table R503.2.1.1(1) of the APA Engineered Wood Construction Guide for specs. H-clips are referred to in the APA Guide as “edge support."
For the most common roof sheathing in our area, which is 1/2” nominal (7/16” actual) sheathing over trusses or rafters at 24” on center, there is no requirement for H-clips. The APA table has a span rating for the sheathing installed with, or without, H-clips, and the span rating is the same both ways. A low slope roof (below 2/12 pitch) at the same span is an exception, however, and an H-clip is specified. H-clips are also necessary for other wood panel thickness and span combinations. One or two H-clips will often allow a longer span between roof trusses/rafters.
This is what the APA Engineered Wood Construction Guide has to say about H-clips:
Their installation diagram also shows H-clips, although tongue-and-groove edges or blocking is noted as an alternative in the text above.
If The H-Clips Are Missing
If H-clips are missing when you are walking on a roof, it is often discernible by the slight flexing when you step on an unsupported plywood edge. Also, H-clips reduce any buckling of those edges over time, so they are a good idea even if not always required. But, if your roof surface is sagging between trusses, lack of H-clips is only one potential culprit. Visit our blog post Why is my roof sheathing sagging between the trusses? for other possible causes.
Diagnosing Other Roof Problems
Also, see our blog posts What causes a sagging roof ridge line? and What causes a lump or dip in the roof? and What's the average lifespan of a roof?
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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?
• 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 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.