A solid mass of Portland cement concrete makes a good foundation for a gas engine, and is easily constructed. Solid brickwork is also used, but the excavation required is more than with concrete on account of working room being required for the bricksetters. The best shape for the foundation is as nearly cubical as possible; if made long and narrow, and deeper than it is wide, there is a tendency to rock. To prevent vibration being conveyed to the walls of the building such foundations are sometimes isolated by forming; an open trench all round; but if the site of the engine is near a wall it is better to lay a concrete floor under the footings of the wall and make it form part of the same mass as the engine foundation, so that the weight of the building helps to steady the foundation. A stone bedplate should be provided between the concrete and the engine bed. For securing the engine to the foundation, holding-down bolts with anchor plates at the bottom ends may be buried in the concrete, being first placed in their exact positions with the aid of a template marked off the bed of the engine. The upper ends of these bolts are screwed to receive the nuts which hold down the engine.

Another method is to cast holes in the concrete through which the bolts may be passed downwards, in which case the heads of the bolts may be at the top and the nuts are tightened up through hand-holes constructed at the bottom ends, but this necessitates leaving a trench for access to the hand-holes. Cotters at the bottom ends of the bolts are easier to adjust than nuts.