In the article as an alternative оf strengthening of existing rafters or replacing them by new timber elements there is an offer to consider the replacing timber rafters by elements of steel thinwalled structures (STWS). The authors have proposed the methodology for calculation the statically indeterminate frame structures with the use of force method. Selected sections of rafters made of STWS and timber for various constructive schemes of pitched roofs were analyzed. The results shown that using of STWS reduces the major repair cost by 13.5 % due to less material consumption compared with a timber structure.