Mass. Gen. Laws ch. 142 § 12
No inspector of plumbing or inspector of gas fitting shall inspect or approve any plumbing or gas fitting work done by himself, his employer, employee or one employed with him, but in a city or town subject to sections one to sixteen, inclusive, the said inspector of buildings, or the board of health, shall in the manner provided in the preceding section appoint an additional inspector of plumbing or inspector of gas fitting as therein provided, who shall inspect plumbing or gas fitting so done. Said additional inspector may act in the absence or disability of the local inspector and for his services shall receive like compensation.
This section shall not apply to any city or town establishing an annual salary for an inspector of plumbing or inspector of gas fitting, and in such city or town an inspector of plumbing or inspector of gas fitting shall not engage or work at the business of plumbing or gas fitting; provided, however such an inspector may perform the work of a journeyman plumber or gas fitter outside the area over which he exercises jurisdiction as an inspector.
Mass. Gen. Laws ch. 142, § 12