- Question: I have seen cases where roof shrinkage has broken off the top courses of brick masonry. How can parapets be detailed to prevent this from happening?
Answer: Roof membrane shrinkage, along with warping or detachment of wood blocking, can often damage the tops of masonry walls. This can be a problem both in masonry walls with gravel-stop edge details and in masonry walls with low parapets. Many PVC roof membranes, particularly unreinforced ones, shrink over time. Modern reinforced PVC membranes will shrink less; however, shrinkage can still occur. Shrinkage is also a problem in EPDM membranes. Shrinkage of roof membranes can cause the gravel stop to rotate. Depending on the method of connection to the masonry, shrinkage and subsequent rotation can cause the upper courses of the masonry veneer to break off.The best way to prevent masonry failures caused by roofing-membrane shrinkage is to anticipate that shrinkage in the design of the roof's flashing details. With low parapets, this can be done by detailing the perimeter of the roof with a movement joint to accommodate some shrinkage along the parapet and by attaching the roof membrane to the face of the masonry walls. The top edge of the membrane should be covered with a counter flashing.
Extending the membrane over the top of low parapets causes problems if the roof shrinks enough to "tent" along the perimeter. When this occurs, the masonry at the top of the low parapet must resist tension in the membrane.Accommodations for movement can be provided at roof-to-wall interfaces that have gravel-stop edge details. First, the wood blocking and gravel stop should be attached to the roof deck, not the masonry. Depending upon the roofing material, the flashing detail can be looped over flexible foam rod that can widen to accommodate shrinkage of the roof membrane. An alternate approach involves the use of an expansion bellows that caps a curb, similar to those used at typical roof expansion joints.Warping of blocking can also cause problems. If the blocking is attached to both wythes, it is possible that cupping of the wood blocking could break the bond of the first bed joint down from the top. Attach separate pieces of blocking along the exterior and interior wythes to prevent this.