Since I hate to give bad advice, I decided to double check myself. So, not to beat a dead horse or to argue structural problems vs physiological problems. I thought this an interesting piece from Joe DeFranco
"here’s the abbreviated version of what you will see most athletes do when squatting:
Upper body inclines forward, low back begins to round
* This problem usually occurs due to tight hip flexors, tight calf musculature, weak spinal erectors
Heels come off the ground
* This problem usually occurs due to tight hip flexors, tight calf musculature"
And from Eric Cressey (MA in kinesiology)
"Unfortunately, as Mike Robertson and I pointed out in our Neanderthal No More series, a large percentage of the population has tight calves and tends to over-pronate at the subtalar joints (leading to flat feet). As a result, dorsiflexion ROM is compromised, and if the lifter tries to squat deep with the toes pointing straight ahead, he must compensate by a) rising up on the toes, b) increasing the amount of hip flexion, or c) combining the two in what makes for an extremely ugly squat. "