Is it alright to place the access point in the roof, the front facing the floor.
How will the radio reception be affected? I can´t find any online documentation regarding placement.
I am assuming this is a MR33. In a single AP environment it would be ok. In a multi AP environment best practice is to mount them with line of sight in a triangle or square placement.
If you are worried about signal you can place it temporarily and do a site survey
Info about range and mounting
MR33 Install Guide
Other MR install guides
In addition to the above links, I would also recommend the below links
To mount your MR33 on a drop ceiling T-rail, use the included drop ceiling mounting accessory kit.
It depends on the material for how the radio may be effected, but I do think all MR's are plenum rated, basically meaning they can be installed in the ceiling without causing a fire hazard.
For safety, I would be sure to mount it securely so it can't fall on someone's head if they open a ceiling tile, for example.
I once had to install ~200 APs of another manufacturer above the ceiling grid for "architectural reasons" (they were ugly). That reminds me of another suggestion- label/mark the ceiling where the AP's are placed so you can find them again 😉
Mounting the AP on the top side of the drop ceiling is definitely possible and can run in production. It just might not get the results you want or be the easiest to access.
I have done this before with MR16s, MR18s and MR20s. It works fine. Access points in general are somewhat directional so with them pointing at the floor you probably will not get the best coverage, however if it is your only option and you have enough points to cover the area needed then it will work.