Seismic risk mitigation in historic masonry buildings often relies on the use of perimeter ties and floor or roof diaphragms to prevent local collapse mechanisms, particularly out-of-plane wall failures. Among these, roof diaphragms are especially effective where perimeter ties and floor diaphragms are unfeasible. These systems transfer seismic actions from the roof mass and out-of-plane loaded walls to seismic-resistant elements, promoting a box-like structural response in the lateral force-resisting system (LFRS). Several conceptual design models have been developed to support the design of roof box structures, generally proving effective, except in long-span buildings, where their performance is limited and often leads to over-dimensioned structural components. This paper presents an enhanced approach that explicitly accounts for the in-plane contribution of longitudinal walls under transverse seismic loads— an aspect typically disregarded in traditional models due to conservative assumptions. Following a review of existing models, nonlinear parametric analyses are carried out using a detailed numerical model aimed at assessing the accuracy and robustness of the approach. Results highlight the critical role of in-plane walls and provide a comprehensive evaluation of prevailing design practices, outlining both their strengths and limitations.