b'ed on as-built drawings, can be recal-culated from review of the existing structure during a retrofit project.The roof decks can be structur-ally designed to have a slope prior to construction. This can be done with sloped concrete or installing a con-crete topping that provides a positive gradient towards drains to avoid any ponding water. Alternatively, a roof assembly that includes sloped or ta-peredinsulation,orevenlocalized taperedinsulation,providesslopes todrainsandassistswithclearing water from the roof. Perimeter back-sloped insulation, drain sumps and crickets are utilized for local tapered locations and can result in a 20- to 30-per-cent savings in tapered insu-lation over full tapered systems.To understand how roof drains work,youneednotonlyconsider pipe size and plumbing slopes, but youalsoneedtounderstandthe drainage anatomy. Flow control de-vices are installed in various designs to slow runoff into the storm system during rain, leaving more water or load on the roof for longer periods of time. Ballast guards are used to keep stone/debris from entering and blockingdrains.Inaretrofitproj-ect, depending on local bylaws and codes,EPDMsealsorU-flowde-vices are installed in retrofit drains, but these can decrease the size of the drain and reduce the effective drain-age capacity.Drain DetailRedundantdrainsareanother option to protect a roof from pond-ingwater.Shouldthemaindrains get blocked or heavy rainfall causeRoof defects and standing water dont mix.water back-up on the roof, overflowPhoto courtesy of W. Allen Partners.scuppers can be installed to relieve +Build SolutionsTotal Design F architek.com | 1 888 317 9226or Your Green Roof Projects.ORN THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ONTARIO ROOFING NEWSISSUE 4 2023/2413'