MTC partners often ask questions about the necessity, benefits and differences between pilot & predrilled holes when installing structural self-tapping screws. As their very name indicates, self-tapping screws do not require any sort of predrilling to be installed in engineered wood products, however, in some cases, there will be some advantages to do so. This blog post will answer the most frequently asked question on that subject, including:
Approximately 6-minute read.
What are the differences between Pilot hole & Predrilling?
First, it is key to distinguish the two types of drilled holes that can be used to install self-tapping screws, Predrilled (or predrilling) holes and Pilot holes.
A fastener’s hole is considered Predrilled if the hole, in which the screw will be installed, is the full length of the screw’s penetration in the wood member. It is also required to drill the hole using the appropriate drill bit diameter based on the diameter of the fastener is installed.
In comparison, a fastener’s Pilot Hole does not have to respect the requirement for penetration length mentioned above as its goal is to help the fastener to settle in the proper alignment. Therefore, pilot holes will not bring the benefits of predrilled holes. It is still required to respect the appropriate maximum drill bit diameter.
What are the benefits for Predrilling?
The advantages brought by the predrilling depends on the type of predrilling holes considered as mentioned above.
Predrilling is the most beneficial as it allows the designer to reduce the spacing, end and edge distances geometry requirements while determining its building connection solutions. The geometry requirement factors are determinant and will participate in connecting system optimization. Smaller end and edge distances could permit to fit more fasteners into the same area, strengthening the connection, or, could permit to reduce the sections of the mass timber members used, reducing the overall costs of the project. The geometry requirements are essential to transfer forces safely and efficiently between materials and the ASSY self-tapping screws. We highly recommend you to read our blog post ASSY Self Tapping Screws Geometry Requirements – Part 1 & Part 2 to learn more on geometry requirements for fastener installation in wood material.
Predrilling also helps in reducing the torque needed to install fasteners. Installation torque can become critical when installing long fasteners, beyond 19” [480mm], in denser wood species, such as Douglas Fir or Yellow Southern Pine. Finally, Predrilling is a very good way of ensuring that the fasteners are settled properly in place as it will limit the risk of screws wandering in the wood material. Just as for the torque, screw wandering is a greater concern for long fasteners installation, in denser wood species, such as Douglas Fir or Yellow Southern Pine.
What are the benefits of Pilot Holes?
Pilot Holes are used to limit the risk of screws wandering and will help to settle the fasteners into place properly. Again, that concern mostly apply for long fasteners, beyond 19” [480mm], in denser wood species, such as Douglas Fir or Yellow Southern Pine. For more information, read page 38 of our Structural Screw Catalog.
Does predrilling affect the capacities of the ASSY self-tapping screws?
No, predrilling or using pilot holes does not affect the capacities of MTC Solutions’ ASSY self-tapping screws. At least three European studies, mentioned in Ringhofer edition 2017, are comparing the withdrawal resistance of self-tapping screws in wood, in predrilled and non-predrilled holes using self-tapping screws. These studies have shown that any difference can be considered negligible. This was also confirmed in a recent North American study. For more information, you should read our White Paper on the subject.
Is predrilling required for inclined screw applications?
When fully threaded screws are inclined to the wood surface, additional measures may be required for the screw tips to bite into the wood and initiate the self-tapping mechanism. Fully threaded self-tapping screws are designed to tap their own hole as they are driven into place. In cases where the screws are 90° to the wood surface, the screw tips bite into the wood, allowing the threads to engage and draw the screws in. When installed at an acute angle to the wood surface, fully threaded screws will tend to travel across the surface just like a drill when inclined to a flat surface. There are two options to initiate the installation of inclined fully threaded screws:
45° pre-drill jigs are recommended for steel-to-wood connection applications to facilitate the use of 45° washers. 45° pre-drill jigs not only facilitate workflow–they also create precise 45° angles for the pilot holes at the correct locations in the wood members so that the screws line up with the washers. Custom site-made jigs made from metal or wood may be used for other applications. Suitable pilot holes are only required to set the angle and initiate installation, and so are only a few inches deep, even for long screws.
The second method involves initiating short holes using the screws at 90°. Once the threads engage the wood enough to hold the screws in place, the angle of the screws is manually adjusted, usually by referencing a triangular framing square. With the angle established, the self-tapping mechanism is then employed as the screws are fully installed.
For more information on MTC Solutions self-tapping screws, download our Structural Screw Design Guide. This guide provides detailed instructions and considerations when designing connections with ASSY self-tapping screws. MTC Solutions technical support team is always available to answer your questions and assist you during your project design phase.
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