Basic Differences Between Structural and Spax Lag Screws

Most of our readers have asked us couple of times to publish a write-up explaining the general differences between two most commonly used fastener in the industry i.e. spax lag screws and structural screws. This edition will give them a clear idea about how both these screws are installed and their specifications are also clearly explained in this edition. Read on!

Structural Screws

These fasteners are equipped with high strength and so have gained huge popularity in the recent years over the age-old lag screws and are always preferred first for basic applications. This piece will highlight on the pattern, installation procedure and also the pricing, which are important to be consdiered, if you’re thinking between these two rivets.


There are usually three major types widely available in the market, which include the Spax (left), FastenMaster (right) and GRK (center). while the former two are easily found at ant physical store and home centers, the last one is a specialized fastener available only at lumberyards and online.


These types of screws are stronger than the geenral lag bolts and are thus ideal for more sturdy connections. Even though they are stronger than lag screws, they are easier to employed since they do not require any pre-drilled or pilot hole. In most cases, you can install all of those screws at real time even before you are done with drilling pilot holes for the lags.


The structural rivets are made up of high-strength steel, which are even heat-treated so that they will not snap off while fastening. While the shafts of these metal pieces are thinner than lags, they are equal substitutes for a lag.

The threads however vary from brand to brand. While one brand features a drilled tip embedded into the screw point, so as to remove dust while entering into the wood, other brands have different screw threads near the tip, so as to pave ways for the remaining screw threads, which grips the screw into the wood.

Besides above characteristics, these fasteners are either equipped with Torx, having 6 contact points or Spider, with 8 contact points drive heads apart from their traditional hex-head styles. These specially designed heads allow further connection between the screw head and the driver tip, so as to prevent disarming as seen with Phillips or slotted screws.


Some contractors may not be aware about the structural screws, so if you are planning tor enovate your basement or deck, you would then definitely have to speak with your the building inspector first, so as to ensure that they approve use of these fasteners. You can download the specifications from the Internet and attach it with your plans, so as to make them clear about your requirements.

In case of spax lag screws, you will have to drill two holes for every lag screw to prevent the wood from breaking away. This is indeed a time consuming task, in case you’re using lag screws in huge numbers. You may even need them in more numbers due to their reduced strength.

Even the structural fasteners have distinct disadvantages too, which lie greatly in their pricing and availability. While the GRK type screws are the most expensive and you can only find them at lumberyards, the Spax and FastenMaster types are sold by home centers, but again these are available in selective sizes, which are commonly used.

A 6″ long by 5/16″ diameter structural rivet may cost you about 85 cents, while you can easily get a spax lag screw at just 30 cents. Therefore, the lag fasteners are mostly preferred over these expensive fastening objects.


Lag Screws

    • These traditional fasteners are hundred years old and are quite inexpensive, as compared to other fasteners. Although these are difficult to be installed, they are still preferred by many.
    • You’ll have to pre-drill two holes to use them i.e. one small one for the threads and a larger clearance hole for the shaft.
    • The structural screws are only available at lumberyards and online stores, plus, their quality greatly varies with the brands stocked in most home center
    • While lag screws on the other hand are easily found at all home centers, webstores and hardware stores, they even cost a third of the price of the former ones
    • However, there are some hardened lag screws, which feature certain markings such as multiple lines, or a triangle on therir heads. These are bit costlier as compared to the normal lag screws
  • When it comes to connecting a wood joist or other framing member to a concrete wall, these metal pieces are always a better choice. In such cases, the special lead sleeves are employed into a pre-drilled hole and then the lag screw is fastened into the sleeve

Structural Screws

    • These types of advanced fasteners make faster connections, but are expensive as compared to general hardware supplies
    • They are are thin and sharp and do not require pre-drilling unlike lag screws
    • These are engineered to meet high standards as they are made up of hard and heat-treated steel
    • These are found only in select stores in certain size and length
  • These cost several times higher than a similar lag screw