What is the proper temperature for “mining room”?
Higher temperatures what kind of damages can they cause?
Mine is at 26-28C in the room.
Outside house temp moving around 35-40C at the moment.
Well …higher temp in the room => higher temp on the video cards, mb, cpu, PSU and all the rest of components.
you may need bigger/faster fans moving more air to cool things down.
Damage is relative “wording” 
you may get lower performance, restarts, freezing, etc initially. Good PSU have thermal protection usually, same for good video cards.
GPUs like GTX 1070 will lower its frequency to avoid reaching upper temperature barier (83C for GTX1070) which means worse performance. Don’t worry about any damages, your GPU will work at lower frequency or with higher fan speed. The maximum temperature for GTX 1070 working in standard mode will be around 81-83C, room temperature has no inpact on maximum GPU temp, it only affects frequency or fan speed. CPU and PSU should adjust their fan speed as well in order to avoid overheating. The only device you can worry is HDD - this device works on maximum 7200 RPM all the time and doesn’t have any cooling system, so room temperature has direct inpact on its temp.