<b>ABSTRACT</b> </br>

The need to reduce heat-related thermal stress calls for a reassessment of shading in outdoor urban spaces with the aim of achieving more habitable and sustainable cities. In this research, a methodology based on mathematical formulas is presented and applied to determine the most appropriate geometric properties and position of the trees to be planted in an urban canyon, based on its E-W solar orientation and the height of the buildings. The methodology relies on shading the largest possible surface area of the pedestrian zones of a canyon through the appropriate autochthonous tree species, but it also requires a thermal-comfort evaluation of tree transmissivity. The case study is validated in Bilbao, where a combination of measurement and modelling techniques (Solweig model) are used to verify suitable guidelines for urban design. The results will be of interest to architects and city planners for decision-making when integrating trees, so that urban thoroughfare development can improve thermal comfort in actual or new canyons. The methodology could be applied to other climatic regions, although some of the parameters used both for the evaluation of the heat-related critical moments and in the formulas should be adapted.</br>

<b>ACKNOWLEDGEMENTS</b></br>

The authors wish to express their gratitude for funding from UPV/EHU (contract PPGA20/26), SHELTER EU project (GA 821282), and the research groups: IT1314-19 (Basque Government) and GIU19/029 (UPV/EHU).