In accordance with standard PN-EN ISO 9692-1, when making welded joints using a backing strip, the maximum distance between elements being joined should amount to 15 mm. In certain cases, the use of a gap greater than the normative one could be technically justified. The above-presented issue inspired tests aimed to identify the influence of a gap between elements being joined on the operational service life of these joints. The tests involved the adoption of a fatigue criterion. The article describes results of fatigue tests along with related conclusions.