Hello - I have a 5 month old who has just learnt to roll, and now loves sleeping on her tummy. We always place her on her back to sleep, but sure enough she rolls onto her tummy within a few minutes. We have an excellent, firm, breathable mattress (Ecosa) but I wonder about fitted sheets? I have been sleeping her on the bare mattress without a fitted sheet, because there were a few times initially where she would have one nostril pressed against the mattress and I wanted reassurance that she could breathe. Is it ok to sleep baby on a bare mattress. Or alternatively, which fitted sheets are best to use? 100% cotton? Bamboo? Thank you in advance!

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 or side.
This is not something that needs to be stopped.
It is important to make her environment as safe as possible.

This new environment involves being in a cot (not a bassinet) & in a sleeping bag with arms out.
She needs the space of a cot & arms free to be able to roll well & safely.

There is no problem with using fitted sheets, but ensure they fit tightly.
Also, nothing soft &/or padded in the cot at this stage.

**Always continue to place baby on back for sleep.
Increase tummy time when awake especially if you are concerned to continue to build upper body strength

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