MTC Solutions’ partners often ask questions about the necessity, benefits, and differences between pilot and predrilled holes when installing structural self-tapping screws (STSs). Due to the design of their proprietary tips, STSs generally do not require any degree of predrilling to be installed in engineered wood products. However, there are advantages offered by predrilling and, in some cases, it is a recommended practice.
This blog post will answer the most frequently asked questions on this subject, including:
Approximately 6-minute read.
What are the key differences between pilot holes and predrilling?
First, it is key to distinguish the two types of drilled holes that can be used to install STSs: predrilled holes and pilot holes.
A fastener’s hole is generally 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 being installed.
In comparison, a 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 offer the benefits of predrilled holes (as described below). It is nevertheless required to respect the appropriate maximum drill bit diameter; however, smaller diameter drill bits are also acceptable in this case.
What are the benefits of pilot holes?
Pilot holes are used to improve positioning, centering, and alignment, as well as to promote wood bite and initial thread engagement, resulting in a more rapid onset of screw draw and a more efficient installation process. Furthermore, pilot holes can limit the risk of screws wandering during insertion, a concern mostly applicable to long fasteners (over 19” [480mm] in length) and in denser wood species, such as Douglas-fir and SYP (Southern Yellow Pine).
What are the benefits for predrilling?
Predrilling, on the other hand, offers the benefit of reduced geometry requirements (spacing, end distance, and edge distance) to the designer for determining and detailing their building connection solutions. These geometry requirements are key design constraints and will largely dictate the degree of optimization possible in each connection assembly. Smaller end and edge distances could permit the installation of more fasteners into the same area, strengthening the connection, or could allow for the reduction of section size 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 STSs. They vary depending on the angle at which force is applied relative to the direction of wood grain and are tabulated as linear functions of the fastener diameter (D). These requirements work in tandem to ensure the safe transfer of forces and to mitigate the risk of wood splitting.
Additionally, predrilling helps to reduce the torque needed to install fasteners. Due to the dynamic nature of a typical screw installation, allowable torque values during installation are reduced from the torsional capacity of the screw to protect it from excessive distress and potential damage. Without predrilling, the torque experienced by the screw during installation can be a credible concern in cases such as when installing longer fasteners (over 19” [480mm] in length) or shorter fasteners (less than 8” [200mm] in length) into denser wood species, such as SYP. Predrilling, even if only to a partial depth to suit available drill bit lengths, is recommended to reduce this installation torque in cases such as these or other similar conditions where insertion is expected or observed to involve substantially more effort.
Finally, predrilling offers the benefits of a pilot hole by improving positioning and alignment, promoting wood bite and initial thread engagement, and limiting the risk of screws wandering in the wood material during insertion.
For specific information on pilot hole and predrilling diameters, as well as bit sizes, see the Appendix of our Structural Screw Catalog (page 80, US Version; page 89, Canadian Version).
Does predrilling affect the capacities of the ASSY STSs?
No, predrilling or using pilot holes does not affect the capacities of MTC Solutions’ STSs. At least three European studies, mentioned in Ringhofer edition 2017, have compared STS withdrawal resistance in wood in predrilled and non-predrilled holes. These studies have shown that any difference can be considered negligible. This finding has also been confirmed in a more recent North American research paper.
Is predrilling required for inclined screw applications?
When fully threaded screws are inclined to the wood surface, additional measures may be required to prevent the screw tip from skating along the surface by further promoting wood bite and initial thread engagement. The tips of our fully threaded STSs are designed with the intent of drilling their own hole during insertion. In cases where screws are installed at a 90° angle to the wood surface, their tips bite into the wood, allowing the threads to engage and draw themselves in. When screws are installed at an acute angle to the wood surface, their tips and threads are less directly engaged by the wood surface, and they will tend to travel across the surface, just like a drill when inclined to a flat surface. The following options are recommended to initiate the installation of inclined fully threaded screws.
The use of a Pre-Drilling Jig is recommended for steel-to-wood connection applications with machined holes or where wedge washers are used. These jigs not only facilitate workflow, but they also self-align and create precise angles for the pilot holes automatically centered in the hole to promote a successful and high-quality screw installation. Unless required for other purposes, suitable pilot holes are only required to set the angle and initiate installation and therefore only need to be a few inches deep, even for longer screws.
The second method involves initiating short holes using 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 initiated as the screws are fully installed.
Regardless of the method followed, we recommend the use of installation jigs to ensure that the angle of installation is as intended and specified.
Predrilling and pilot holes can introduce additional labor and time demands to a project schedule, underscoring the importance of clear guidelines regarding the instances and reasons for employing these two installation steps. Over the last couple of years, we have created a few resources in the form of technical blog posts and design guides that offer detailed instructions and guidance on:
Feel free to browse these resources and reach out to our Technical Service Team with your connection questions. 😉
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