Abstract: The invention relates to a method for leak detection in roofs and interior and exterior floors covered or clad with sealing layer products and an apparatus for carrying out the method. Smoke gas is fed into and made to spread in a space between the sealing layer and the support on which the sealing layer rests. Any of the smoke gas penetrating in an outward direction through holes in the sealing layer is detected when it escapes from the holes on the outer side of the roof cover, whereby their locations can be determined. The apparatus for carrying out the method according to the invention comprises a device for generation of smoke gas, transport elements to move the smoke gas to the space where the smoke gas is made to spread, and a generator that creates an air flow for transport of the smoke gas.