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Self-Drilling Screws Selection and Application Guide

Self-drilling screws use in ventilation systems

How to choose the correct self-drilling choose? When and how can we use self-drilling screws in the ventilation ductwork assembly?  Keep reading to find out!

Screw types

Screws are mostly divided by head type. Alnor product offer consists of screw types: hex socket head, Phillips head and PAM-HEAD. 
The question is Which screws to choose? – it depends on the personal preference of the duct fitter and tools available. However, for ventilation ducts, we recommend screws with a flat head bottom.

Screws have different lengths which allow matching the thickness of the joined surfaces. Alnor offers screws between 9 and 25 mm in length. Screws can be easily used with a special wrench and screwdriver heads.
Check most popular fixings and fasteners for mounting circular and rectangular ducting

Self-tapping screws

Illustration no. 1
Self-drilling screws & bit tip holder

What are self-drilling screws used for? 

For joining ducts

Self-drilling screws can be used in for connecting both circular and rectangular ductwork components. However, when joining spiral ducts and fittings- correct installation method is crucial to reach needed airtightness class. What to know more out Ductwork Airitghness - see our article on that subject. 

How to make a joint tight?

In order to make an airtight joint between two spiral duct sections or spiral duct and another duct component, a duct fitter needs to make sure that the following requirements are met:

1.  The correct selection of the screw size and quantity. When installing spiral ducts, it is required to match the number of screws to the selected duct diameter in order to make sure that the joint will be airtight (see the table below for reference).

Tabel no. 1 - The number of screws to duct diameter

Min. screw diameter
The number of screws 
80 – 250 3.2 3
280 – 500 3.2 4
560 – 710 3.2 6
710 - 1250 4.0 12
1400 1600 4.0 16


2. The correct sequence of screws tightening. To join spiral ducting, it's recommended to install them crosswise, at a distance of 10mm from the duct edge/female coupling edge (see fig 2 and 3). Improper order of screw spacing and the tightening sequence creates folds which cause duct leakages. 

Incorrect order of installation

Fig. 2 Incorrect installation - wrong screws tightening order

Incorrect order of installation

Fig.3 Correct installation - correct screws tightening order

3. Type of self-drilling screw. For joining spiral ducting any self-drilling screw should be enough to reach ductwork air-tightness class A and B. However, to maintain higher air-tightness of ventilation system, i.e. class C and especially D, it's recommended to use special self-drilling screws where dp < d (see fig. 4). These self-drilling screws are not flattened on the bottom of the threaded shank - they are pointed, which results in smaller hole diameter, limiting the risk of leakages.

Correct self tapping screw

Fig. 4 Correct self-drilling screw

All the abovementioned steps are important when using duct fitting with rubber seals, which offer class "D" airtightness (the highest class airtightness) according to EN 12237:2005. Why? Because the highest airtightness class of the ventilation system can be reached only when: a) we select the airtight ducting system (high-quality airtight spiral ducts and duct fittings with factory-installed EPDM seals) b) we correctly mount the chosen duct system. Our spiral ducts and duct fittings are manufactured according to Swedish Technical Approval no. 0103/07 issued by RISE Research Institutes of Sweden AB | Certification (formerly SITAC). Here You can check the Technical Approval 

For duct suspension

Screws can be utilized to a great effect with other installation accessories, such as duct suspension solution - duct suspension brackets type V, R, L and Z.  

Duct suspension bracket type V is used for quick and secure mounting of all types of installations. Trapezoidal suspension bracket type “V” and self-drilling screws can be used to suspend different types of rooftop components made of the trapezoidal metal sheet. Type R duct suspension bracket is used for suspending circular ducts. The suspension bracket is attached to the duct surface by screws. L and Z duct suspension brackets are used for quick installation of square ventilation ducts, especially with threaded rods and self-drilling screws.
More about Duct Suspension Bracket type V, R, L and Z. 

Duct suspension brackets type-l type-v

Installation of duct suspension brackets  EQRS/EQVS

For rectangular duct "production"

Self-drilling screws also allow for joining rectangular ducts - even at the construction site. Contractors who don't know the exact duct length may purchase ducts with the loose frame on one end, this allows to cut the duct down to the desired length and use the screws to attach the frame, which allows joining the duct with other components of the installation. Moreover, the self-drilling screw can also be used to install attenuating baffles in rectangular ducts which facilitate easy installation of the acoustic insulation layer on the duct surface. 

Rectangular ducting production

Framing assembly on rectangular duct

For connecting prefabricated components

Screws are also used for the production of components of the ventilation system with prefabricated parts - both circular and square ones. Self-drilling screw facilitates easy and fast installation of side parts and damper blades to create the final product - wall mounted air intake/exhaust louvres or multi-blade dampers. 

Multiblade air damper porduction

Installation of Multiblade Air Damper

Self-drilling screws are perfect for installing PVC damper mechanisms. They allow for quick and easy joining of the plastic mandrel with the revolving damper surface. 

To sum it up, a wide range of fixings and fasteners are used to mount ventilation systems. Without a doubt, self-drilling screws are the most popular and universal solution out of the entire selection. They enable fast and easy joining of components, installing duct under roof suspensions. They facilitate own assembly of ventilation dampers. Most importantly – the installation can be disassembled quickly and easily. Screws facilitate easy future maintenance and cleaning, enabling quick disassembly of any component.

More information about the range of Self-drilling Screws You can find in technical sheets.