Our 5 month old is very adept at rolling from back to tummy, but can’t roll back. She is doing this in her sleep and we regularly find her asleep face down - ie she has not placed her head down on it’s side but rather face first into the mattress. She does not seem in distress in this position and she is breathing, but we’re concerned about air flow. Do we need to move her out of this position every time? We’re exhausted!

Most babies will be rolling over by 4- 5 months…...some earlier, some later.                              This is normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy & some seem to face plant on occasions.
This is not something that needs to be stopped. It is important to make her environment as safe as possible.

**Firstly, now that your baby is rolling, she needs to be in her cot for all sleeps & not wrapped or swaddled, if you are not already doing this.
Baby needs arms free & space of the cot to roll safely.
Increase tummy time when awake to help build upper body strength.

Ensure that the cot mattress is firm & flat & there is nothing soft or loose in her cot, especially as she is face planting at times.
You can try to turn her face if you see her with face down, but often, like rolling to tummy, babies will just go back to their position of comfort.
Have you tried to roll her back, when she is on her tummy?
Most parents find there is not much to be achieved by trying to roll her back from her tummy. Most babies will just roll straight back onto tummy.
It is important to make her sleep environment as safe as is possible.
**Always continue to place baby on back to sleep.

Red Nose has the following general information now that she is rolling. 

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over