To begin let me say that your question is somewhat misleading. I don't believe "many people" called Howard Roark selfless. Most of the conflict in the book is brought about by people who believe that Roark is arrogant and selfish. There is a section in "The Fountainhead" were, if the reader doesn't understand Roark's character, he could appear to be selfless. In chapter 15 Peter Keating is reaping the rewards for a design he did not create. He had gotten the design from Roark. He knows that Roark is about to lose his business. Peter goes to Roark and tries to give him a check to smooth over the guilt he is feeling for "borrowing" Roark's design. Roark refuses the money. This may seem selfless to some, but the reader must understand why Roark refuses help and assistance from others. It is not because he is "selfless," it is because he is "selfish."
Howard Roark refuses to bend his value system for anyone or anything. If it means he can never build a single building, he will not change his designs and won't bow to pressure.
"In her preliminary notes for the novel, Rand comments that Roark contains an "utter selfishness"— an "iron conviction" to "be himself at any cost—the only thing he really wants of life." He insists, "All that which proceeds from man's independent ego is good. All that which proceeds from man's dependence upon men is evil."