Will using a board under a mattress help with low back pain?

Possibly: Placing a plywood board under a mattress can help with low back pain, especially if the current mattress is old, too soft or sagging. If, however, the mattress is firm or relatively new, the board will probably do very little in terms of helping with back pain.

Perhaps: I generally recommend that mattresses be firm but not rock hard. A board under a mattress, particularly an older mattress that may be losing its support may be helpful. It is a matter of individual preference. I know when i go to a hotel and they have mattresses that are very soft, i wake up with a back ache the next day.