Answered by: Maulana Moinul Abu Hamza
Why is it okay for men to be shirtless?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
The awrah of a man is between the navel and the knee. Therefore, what is above the navel is not part of what is necessary to cover. However, hayaa (modesty) entails being considerate of the circumstances. For example, it is mustahab to cover the shoulders during salah.
Only Allah knows best
Written by Maulana Moinul Abu Hamza
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 Raddul Muhtaar, Volume 1, p.404