Abstract The integrity of flat end to cylindrical shell connections (flat endcaps) operating in the creep regime has been questioned since some failures have occurred in the recent past, mainly as Type IV cracking in the weldment. Failures in endcaps are potentially dangerous due to the fact that a crack will evolve from the inside and final failure will take place as ‘break before leak’, which in turn makes detection difficult. The codes treat the component by calculating the cylinder and the flat endcap separately, whereas failures occur in the transition between the two in a cross-section not covered by the codes. This paper presents reference stress solutions for endcaps derived from collapse loads using the Finite Element method. The computed reference stress can be used for design and remaining life predictions. The reference stresses are compared to the predictions by some codes, and it is demonstrated which combinations of geometries have the potential for premature failure.