Md. Code, Business Occupations and Professions § 12–601

(a) Except as otherwise provided in this title, a person may not provide, attempt to provide, or offer to provide plumbing services unless licensed by the Board to provide plumbing services.

(b) Except as otherwise provided in this title, a person may not assist, attempt to assist, or offer to assist in providing plumbing services in the State unless licensed by the Board to provide or assist in providing plumbing services.

(c) Except as otherwise provided in this title, a person may not provide, attempt to provide, or offer to provide propane gas services unless licensed by the Board to provide plumbing services or certified by the Board to provide propane gas services.

(d) Except as otherwise provided in this title, a person may not provide, attempt to provide, or offer to provide natural gas services unless licensed by the Board to provide plumbing services or natural gas services.