Need Advice about my aging parents. They are always asking me to do laborous jobs like roofing, painting, etc?

[P.S.]

I am NOT trained to climb up on roofs and lay down shingles. People have broken their back or neck and become paralized doing this. I also HATE climbing on ladders and painting houses. This is also dangerous, even if a bit less dangerous than roofing.
I love my parents, and it's heartbreaking that my dad can't do what he used to when he was younger. But I tend to value my own health as well, and generally prefer not to be paralyzed, or maimed for life. I am kinda clutzy when it comes to that kinda work, not afraid to admit. WHY CAN"T MY DAD STOP ASKING ME, and why can't he hire professionals to do this ??
IS IT OK TO SAY NO to him? Or am i an ingrateful evil son?