Foot fatigue is a massive issue for guys who stand on concrete for ten hours a day. To combat this, it is incredibly common for workers to buy a pair of thick, gel-filled, or heavy orthotic insoles at the pharmacy and stuff them into their expensive safety boots. It seems like a logical fix for sore feet, but doing this without understanding the geometry of a safety boot can actually cause permanent damage to the boot and create new safety hazards.
When a safety boot is designed and certified, its internal volume is calculated very precisely around a standard, flat factory insole. The most critical measurement is the clearance between the bottom of your foot and the top of the steel or composite protective toe cap. If you take out the flat factory insole and replace it with a thick, contoured orthotic that has a deep heel cup and a high arch, you are physically raising your foot up by a quarter to a half inch inside the boot.
Suddenly, the top of your toes are resting directly against the inside of the protective cap. Within an hour of walking, you will be bleeding through your socks because the cap is chewing through your toenails. Furthermore, raising the heel pushes the back of your foot higher into the heel counter of the boot, destroying the fit and allowing your heel to slip, which causes severe Achilles tendon irritation and blistering.
If you need custom orthotics or aftermarket support, you cannot simply drop them into a boot and go to work. You have to remove the original factory insole first. Most high-quality safety boots have a dual-layer insole: a flat bottom layer that is glued to the bottom of the boot, and a removable top layer for comfort. You must only replace the removable top layer. If your aftermarket insole is thicker than the removable layer you took out, you are altering the safety geometry of the boot. In extreme cases, lifting the foot too high can even pull the tongue of the boot down, breaking the waterproof gusset and voiding the boot's waterproof warranty. If the boot doesn't fit perfectly flat without the insole, you need a different boot, not a thicker pad to cram inside it.
