Use the 150mm flush-head screw where the timber joint genuinely needs more reach than the 100mm Impact screw can provide. The correct choice should be based on the thickness of the pieces being joined rather than the product name alone.
The main reason to move from a 100mm fixing to a 150mm fixing is the extra reach required by the actual timber joint.
Our 5.0 x 100mm Impact Professional Wood Screw is intended for relatively substantial timber where the screw needs to pass through one section and obtain useful engagement in the timber behind it. It is hardened steel with a zinc and yellow passivated finish, Type 17 point, milling shank and ribbed double countersunk head.
The 150mm Flush Head Screw provides an additional 50mm of overall length. Our supplied guidance specifically identifies substantial landscaping timber and certain railway sleeper joints as applications where that extra reach can be useful. It is hardened steel with a green organic coating, self-drilling point, milling shank and flush head.
The longer screw should not be chosen simply because it sounds more substantial. Check the combined timber thicknesses so the fixing can pass through the first member, achieve useful engagement in the receiving timber and avoid unnecessarily emerging from the far face.
Use the 100mm Impact screw where its length suits the joint. Move to the 150mm flush-head fixing where the actual timber dimensions require the extra reach.