Six by-products are used as additives in adobe bricks to study the variability of mechanical properties by a three level design of experiments and to evaluate optima formulations. Corn plant, fescue, straw, and grounded olive stones are agricultural by-products; and rubber crumbs and polyurethane are wastes used as transport and appliances by-products. Results show that the three-level design of experiments defines properly the governing equations of flexural and compression strength, furthermore it shows the interaction between them. A maximum improvement of 89% and 26% of flexural strength in corn plant and fescue types, respectively, is achieved. The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under grant agreement No. PIRSES-GA-2013-610692 (INNOSTORAGE). Furthermore, the work is partially funded by the Spanish Government (ENE2011-28269-C03-02 and ENE2011-22722). The authors would like to thank the Catalan Government for the quality accreditation given to their research groups GREA (2014 SGR 123) and DIOPMA (2014 SGR 1543).