This document presents the results of a search for gluinos and squarks in events with exactly one lepton in the final state in addition to multiple jets and large missing transverse momentum. The ATLAS data analysed was recorded in 2015 and 2016 and corresponds to an integrated luminosity of 36.1 fb$^{-1}$ at a centre-of-mass energy of 13 TeV. In contrast to previous publications the latest results also contain a multijet channel, requiring at least 9 jets. To estimate the background in this regime a data-driven technique, based on the invariance of the transverse mass shape in events with different jet multiplicities, was developed. No significant excess was observed. The results are interpreted in a simplified model for two-step gluino decays and a subset of the phenomenological minimal supersymmetric standard model (pMSSM).