The use of linen canvas as a oil painting material was first adapted in Italy in the 17th century. Rolled canvas was a portable alternative to wooden panels, which warped easily and were not readily available in all sizes.

Changes in climate will led the canvas to expand or shrink, resulting in a sagging or rippled canvas. The oil paintings will then need to be re-stretched onto a new stretcher if the problem is severe or a less serious sagging can be amended  by either change the tension provided by the corner wedges in the stretcher or by applying a special spay, available in art shops, that tightens the canvas

In order to more conservation you painting you should pay attention to the next matter

The first step is to join the stretcher bars.Lay them out on a flat surface, and push the corners together by hand. If necessary, tap the corners gently with a rubber hammer.