Dimensional difference by welding deformation is usually occurred at steel bridge manufacturting or multi-pass welding used at joining of thick plates. Be this, root gap out of standard is frequently developed at butt weld joints. For example, although standard root gap below 6mm at butt weld joints of plates under 15mm thickness, maximum 30mm root opening is developed at the weld field. At this case, 24mm parts out of standard is generally built up. But, there are no accumulated data and restriction about this built up welding pars. In this study, therefore, the accumulation of data for built up parts and the verification of the mechanical properties of weld part as root gap is performed. It is included that tensile, bending, impact, hardness test and microstructural review for each welding specimen of 0mm, 6mm, 30mm root opening.