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The use of woodchips of local origin for heating purposes is a diffused practice in some areas, like northern Italian alpine and sub-alpine zones, where large woods and forests extensions occur. In recent years, many thermal plants producing energy using woodchips as fuel have been constructed, supplying single edifices and delivering heated water to small communities through district heating. Unfortunately, due to the Chernobyl fall-out, particularly relevant in many mountain areas of northern Italy, woodchips of local origin are often contaminated with relevant 137Cs traces: therefore, the woodchips burning aimed at water heating produces ashes in which the 137Cs activity concentration is highly enriched with respect to that of the raw material. Typical activity concentrations of 137Cs in such ashes span a range from a few hundreds to several thousands Bq/kg. These combustion ashes are subject to different fates, according to reuse opportunities and law restrictions (not referred to the radiological aspects). Landfill disposal is the most common general option, together with the use in compost production plants and concrete factories. In this paper, we focused in particular to the use of concrete containing highly contaminated ashes, being considered as the most relevant from the radioprotection point of view. Therefore, some evaluations of the public exposure to radiations coming from concrete utilized as building material and containing woodchip ashes with high radioactivity levels (specifically 137Cs and 40K) have been done. The dose estimates for a person living in a house built with “contaminated” concrete were made using both standardized gamma radiation exposure indices and simulation models. The results are presented and discussed. Finally, a new formulation of the activity concentration index I is proposed for a safe and radioprotection sound use of building material containing woodchip ashes.