Combined with "Ying Zao Fa Shi", the size of the four type of song dynasty wooden components and related contents in the design of the roof are introduced. On the basis of the facts that the weight of roof became heavier because of rain and snow, a method of calculating the weight of the roof was proposed under different cases considering construction and using period. The simplified method of working condition combined with the calculation results was put forward. The relationship between yellow pine and larchen had been clear. There was a corresponding relationship in the load effect between the various modular. The results would provide reference for calculating similar wooden roof weight, and further exploring the similarity relation in CaiFen system and model tests.