Don't get SCREWED by your lag screw! Strategies here.

How to PROPERLY use lag screws for solar mounting to rafters.

What’s all the fuss about lag screws?

A lot!  The purpose of lag screws is to secure roofing and mounting systems directly onto the structure of the roof.  However, if these screws are not used properly they can damage the building’s structural integrity.  Though the systems are previously engineered, installers do hold a lot of responsibility for failure.  The Unirac Solar Mount installation manual places responsibility onto the installer to make sure there is enough “pullout strength and shear capacities” for lag screws.  Furthermore, sources concerning details on pullout strength and shear capacity are not commonly available.  At Sun Source Solar Brokers, we have written this article to help inform installers of the correct usage of lag screws for residential installments and how to avoid costly mistakes and liability.

Lag Screw Properties

Lag screws, made of carbon steel are often plated with either zinc or zinc galvanized steel; stainless steel lag screws are commonly classed as either grade 18-8 or 316.  There is no standard definition for the mechanical components of lag screws, though commonly used dimensions and guides for design can be found in the American Wood Council’s ANSI/AF&PA NDS-2005 National Design Specifications (NDS) for Wood Construction.

These days’ lag screws contain rolled threads, which makes the outer surface harder as well as more bendable than one with just the base metal.  Because of this, lag screws made of stainless steel are magnetic.  Lag screw quality can range greatly from ones that are poorly threaded, compared to ones made of galvanized zinc, which can harm the threading that’s cut into the wood.

The most common variation with lag screws is that of the length of the shank.  A lag screw has a hexagon blot head and a threaded shaft, with a space of un-threaded shaft called the shank.  Though the longer the shank the stronger the screw is in the shear, an un-threaded shank gives no resistance to removal.

Limitations of Use

  • The quality of lag screws ranges greatly because there’s no material requirements for performance throughout the process of manufacturing. The removal value per inch of thread usage derived from the gravity of the wood is usually included in the instructions.
  • The base metal does not affect the strength of the thread usage per inch, the strength at the root of the screw limits the total strength of withdrawal and, subsequently, the possible load of the withdrawal. With 55,00 and 70,00 as the maximum strength, ¼ inch lag screws made of steel and stainless steel should be limited to 431 and 548 withdrawal load, and 5/16 screws at 742 and 944.
  • During testing, the carbon steel lags were much more effective and successful than stainless steel lags. When you consider the spacing and shingles in plywood sheathing as a gap, the reference loads for ¼ inch and 5/16 inch lag screws should be limited to 34 and 58 pounds for a carbon steel lag with a short shank, and 86 and 130 pounds for a stainless steel lag with long shanks.  This should be adjusted for bend-ability because of the mounting or testing.

Notes on Wood

A small imperfection in wood causes it to vary greatly in effectiveness and quality, and this results a wider deviation in Sun Source Solar Broker’s final conclusion.  The bearing strength is affected by the fastener’s diameter, and larger fasteners can split or crack wood.  Furthermore, moisture does damage to wood, which is why it’s crucial that moisture does not reach beyond the penetration of the lag screw.

Rafters and trusses are common in residential roofing, with a rafter being a big piece of wood that goes from the top plate to the ridge board, the truss created with smaller wood and secured with jagged metal, and the top cord which supports the sheathing of the roof.   A truss will have webs to more evenly distribute the weight from top to bottom.
These top cords and rafters are often carrying compressed loads from all the materials on the roof.   The gaps in bearing points and webs can be forced to carry bending loads due to further weight on the roof; this further increases compression stress on trusses and rafters.  The force of compression can move down a truss through the lag screws. The load of a roof is often limited not just by the stress, but the deflection that can be allowed, which makes stress calculations an indication of acceptable roof mounting.

Sheathings made of plywood with fasteners must be shear-load resistant for things like high winds and to ensure the PV remains attached.   Also, code requires certain longitudinal gaps in the panels of sheathing to make sure the plywood doesn’t buckle under hot temperatures.   To note; for every eight feet there is a gap around every fourth truss in residential application.   This means that as many as half the lag screws used for a PV installation are around the gaps of the sheets of plywood.   Because of this, it’s un-advisable to add the amount of threads in the plywood for the calculation of shear or withdrawal, but instead to use the amount of threads embedded in the rafter or the truss.  However plywood is best for the value of shear bearing resistance.

Best Practices

Screw Material

The best kind is stainless steel.  This is because, during testing, the shank of many of the carbon steel screws would break when turned past a snug fit.  The stainless steel screws never broke during testing.  That’s because it’s stronger, harder, and deflects stress better than carbon steel.  18-8 screws are the best fit for humid weather or areas of industry, whereas 316 are best for areas with chlorine or saline.  Despite the potential of galvanic voltage, they are each compatible with aluminum.  For more shear strength or better deflections, go with stainless steel.  It’s only in certain situations like under flashing or in rural areas that or zinc plated screws are best.

Shank length

Long shank lengths are more effective in reducing stress on the first screw and possible damage to the truss or rafter.  The best shear strength is with the shank embedded in the wooden truss .75 inches for a ¼ inch diameter lag screw, 1.1 for a 5/16 screw, and 1.12 for a 3/8 screw. So for shingle roofs the best shank is 1.25 inches long.

Overall length

The length of your screw thread should be limited so it won’t break during the installation and yet can still fulfill the requirements for minimum withdrawal load.

Pilot holes

Pilot holes are necessary for lag screws.  A 5/32 inch diameter pilot hole is best for a ¼ inch lag screw, a 3/16 inch diameter hole for a 5/16 lag screw, and a ¼ inch diameter hole for a 3/8 inch lag screw.  To reduce the chances of splitting the wood, drill to .50-inch less than the ideal lag. The height of the lag screw should be .50 inch less than the sum thickness of the shingle, plywood, mount, and truss, to make sure that rafter’s wood does not split.

Screw shank clearance holes

Though shank clearance holes are usually required for lag screws, they aren’t optimal for rooftop installation.  And while clearance holes are effective in reducing the potential split of lumber under a shear load, most of this load is shouldered by the plywood sheathing anyway.


Article written by Jennifer Coleman of Sun Source Solar Energy Brokers, providing solar brokering, brokerage, and solar energy consulting services in Santa Rosa, Marin, Sonoma, Napa, Solano and San Francisco Counties.  For more information, please visit


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Contact Info

Sun Source Solar Brokers
525 East Cotati Avenue, #220
Cotati, California 94931