Joints in traditional wooden stairs can be constructed in many different ways. Connections between tread and string are commonly made with tenons and screw joints. It is often hard to know which joint rigidity should be used in calculations. To study this type of stair, a total of 12 small straight stairs with three steps each were tested. The stairs were loaded and the deformation was measured. Modeling of the stairs with a finite element program was also performed, using linear elastic models in the analysis. The conclusion derived from the study is that the tested tread-to-string connections are not very rigid. Tolerances and accuracy of manufacture of the stair parts are important for the behavior of the joints. Comprehensive knowledge about stair joints can be used for development of new cost-effective and reliable design tools for stairs. In a computer program for stair design based on beam elements, the treads in this type of staircase should, according to this study, preferably be calculated as being hinged to the string, or else the rigidity of the joint should be a function of joint parameters.