How to Decide When to Use Structural Screws or Lag Screws
High-strength structural screws have gained wider popularity in the past decade over the traditional lag screws as the fastener of choice for structural applications. This article will review the design, costs, and installation requirements you need to consider when choosing between structural screws or lag screws.
Types of structural screws
Shown here are the three major brands of structural screws. Spax (left) and FastenMaster (right) are available at home centers and hardware stores. GRK (center) is a premium screw available only at lumberyards and online.
Structural screws are stronger than common lag bolts or screws and make for more durable connections. While they are stronger than lag screws, they are also easier to install since they do not require pre-drilling a pilot hole. For most jobs you can completely install all of the structural screws before you have even finished drilling pilot holes for the lags.
Structural fasteners are made from high-strength steel that are also heat-treated so they will not snap off when tightened. While the shafts are thinner than lags, they are equal replacements for a lag. One manufacturer has a drill tip embedded into the screw point, so that it removes material as it enters the wood. Several brands have different screw threads near the tip that saw a path for the remaining screw threads that anchor the fastener into the wood.
Structural screws also feature Torx (six contact points) or Spider (eight contact points) drive heads in addition to traditional hex-head styles. These heads allow for more contact between the driver tip and the screw head to prevent stripping as seen with Phillips or slotted screws.
For your project you will need to drill two holes for every lag screw to prevent the wood from splitting. This is a very time consuming if you are using a large number of fasteners. You may also find that you need more lag screws due to their reduced strength.
The downside to these fasteners lies in their cost and availability. GRK brand screws are the most expensive and are only available only at lumberyards. Spax and FastenMaster brands are sold by home centers, but only the most common sizes are typically in stock. A 6" long by 5/16" diameter structural screw costs about 85 cents, while you’ll spend about 30 cents for a similar lag screw. This increase in cost will be offset with time and labor savings.
Some building inspectors may not be familiar with structural screws so if you are planning on upgrading your deck or renovating your basement, you may want to speak to the building official first to make sure that they approved. You can download the specifications and structural data from the website and attach it to your plans when you submit your drawing.
Structural screw design
Lag screw design
These traditional fasteners that have been around for over one hundred years are inexpensive, but are difficult to install.
• Lag screws require you to pre-drill two holes: one for the threads and a larger clearance hole for the shaft.
• Structurally rated lag screws are available from lumberyards and online, but the quality varies with the off the shelf brands stocked in most home centers.
• All home centers and hardware stores carry a large assortment of lag screw sizes.
• Lag screws cost a third of the price of structural screws.
• Hardened lag screws will have marking on the head of the screw such as multiple lines, or a triangle. These will costs more than standard strength lag screws.
Lag screws or bolts may be a better choice when you are connecting a wood joist or other framing member to a concrete wall. In this case special lead sleeves are inserted into a pre-drilled hole and then the lag screw is secured into the sleeve.
These new fasteners make connections quick, but they are expensive.
• Structural screws are thin and sharp and do not require pre-drilling.
• Structural screws are engineered to high standards from hardened, heat-treated steel.
• Home centers carry the most commonly used structural screw styles and sizes, but usually only one brand.
• Structural screws cost several times more than a similar lag screw.
• Structural screws are designed not shear off.
If you live in the west or southwest you may already be familiar with installation considerations for fasteners like lag or structural screws. In arid conditions the wood tends to shrink more and has a smaller nominal thickness, this is referred to as “scant lumber.” Some manufacturers offer screws that are different lengths so that you can install them without excessive protrusion.
The shaded area indicates where excessive wood shrinkage can occur
American Wood Council
Application Of Technical Report 12 For Lag Screw Connections
Simpson Strong Tie – SDW Structural Wood Screws