Architectural sites suffers from weathering effects such as liquid sedimentation, lampblack or pollution deposit, moss growing or longterm erosion. Displaying their impact on present buildings could help deciders to plan relevant actions for preserving the architectural heritage. In this paper we present a user-friendly solution for on-line acquiring a semi-physical model from one or few images of a building, immediately followed by real-time simulations. The goal is certainly not to give an accurate prediction of what will happen, but rather to show a plausible state of the site under various possible degradations arising from natural effects. The solution relies on the coupling of two 3-D modelling programs: one dedicated to interactive reconstructions and a second to discreet manipulation for natural phenomena simulations based on a combination of different surface cellular automata. Several experiments were held to estimate the effectiveness of this approach and to identify future works in order to cope with present limitations. A first series applies a lampblack deposit simulation on a large-size 3-D model to demonstrate the solution feasibility and to provide indications about achievable performance. The second series focuses on the solution sensitiveness to both geometrical and semi-physical models granularity through the application of a stone erosion effect on a smaller but less regular-shaped building model. The third series demonstrates the discreet model ability to simulate multiple effects with the example of a vegetation growth simulation. Guidelines for future works are given in the conclusion of this paper.