- I have noticed that on several older buildings the parapets lean inward. Is this related to deflection of the roof system?
In most cases, the inward leaning of parapets in older masonry buildings is the result of differential movement between the clay brick masonry veneer and the structural system of the building. It is especially common in old concrete frame buildings. Both the exterior brick wythe and the interior concrete frame will undergo inelastic shortening over time because of creep under compressive loads. The concrete frame, however, will have greater creep due to higher compressive loads from the structure's weight and will shorten from drying shrinkage. The clay masonry, on the other hand, will undergo permanent growth because of moisture expansion. This growth can be sizeable and in many cases will exceed or negate the shortening associated with creep. The brick wythe in old buildings typically is not interrupted at the floor levels. As the concrete frame shortens and the clay brick masonry expands, the parapet walls will be pulled down and curve inward towards the building.
A possible cause of tilting of parapets that are supported on shelf angles is corrosion of shelf angles within the exterior wythe just below the roof line. As steel corrodes, it expands. The expansion resulting from the corrosion of steel will push the masonry upward at the outer wythe. The backup wythes resting on the concrete frame do not undergo similar lifting.
A third possible cause of tilting is unaccommodated horizontal expansion. In older buildings, the masonry parapets typically do not have vertical expansion joints. As the parapets expand horizontally, the expansion is resisted at the corners. The masonry parapets can buckle inward because of this horizontal expansion causing them to lean inward away from corners.
When parapets lean in excessively, problems can result. Portions of the masonry can crack or be dislodged, creating small sections of the masonry that are not adequately supported and that could then fall. Joints could also be opened, allowing further deterioration because of water entering and freezing or corroding steel shelf angles.
If the lean is caused in part from corrosion of embedded steel, there could be a corresponding loss of support or cracking of the surrounding masonry. An investigation should be performed to determine if the inward lean of the parapets is an indication of a larger problem.