Lahar damage to buildings can include burial by sediment and/or failure of walls, infiltration into the building and subsequent damage to contents. The extent to which a building is damaged will be dictated by the dynamic characteristics of the lahar, i.e. the velocity, depth, sediment concentration and grain size, as well as the structural characteristics andsettingofthebuildinginquestion.Thefocusofthispaper is on quantifying how buildings may respond to impact by lahar. We consider the potential for lahar damage to buildings on Merapi volcano, Indonesia, as a result of the voluminous deposits produced during the large (VEI 4) eruption in 2010. A building-damage scale has been developed that categorises likely lahar damage levels and, through theoretical calculations of expected building resistance to impact, approximate rangesofimpactpressures.Wefoundthatmostweakmasonry buildings on Merapi would be destroyed bydilute lahars with relatively low velocities (ca. 3 m/s) and pressures (ca. 5 kPa); however, the majority of stronger rubble stone buildings may beexpectedtowithstandhighervelocities(to6m/s)andpressures(to20kPa).Weappliedthispreliminarydamagescaleto a large lahar in the Putih River on 9 January 2011, which inundatedandcausedextensivebuildingdamageinthevillage ofGempol,16kmsouthwestofMerapi.Thescalewasapplied remotely through the use of public satellite images and through field studies to categorise damage and estimate impact pressures and velocities within the village. Results were compared with those calculated independently from Manning’s calculations for flow velocity and depth within Gempol village using an estimate of flow velocity at one upstream site as input. The results of this calculation showed reasonable agreement with an average channel velocity derived from travel time observations. The calculated distribution of flow velocities across the area of damaged buildings was consistent with building damage as classified by the new damage scale. The complementary results, even given the basic nature of the tools and data, suggest that the damage scale provides a valid representation of the failure mode that is consistent with estimates of the flow conditions. The use of open-source simplified tools and data in producing these consistent findings is very promising.