The Problem with Roof Decks
By William Decker, CMI
It is a typical Chicago story. We just love our Cubbies, the Chicago Cubs baseball team, even if they are almost always losers. An afternoon spent in the “Friendly Confines” of Wrigley field is a good afternoon, even if the Cubbies lose, because of the beauty and history of the oldest Major League Baseball stadium especially when the experience is topped off with beer. And, again in typical Chicago style, many people choose to not pay for an actual ticket to the game, but still want so see the game. They accomplish this by watching the game from a perch located on top of the many buildings that surround Wrigley Field. Some of these facilities are humble, consisting of a few people with folding chairs, but some have morphed into elaborate, bleacher seats that the building owner charged for. This lead to a legal battle when Chicago Cubs organization sued these building owners for copyright infringement. Eventually, the suit was settled and the building owners agreed to pay the Cubs 17% of their gross revenue, but the problem of rooftop decks on flat roof buildings continued. By the way, I am not very well liked in the Wrigleyville neighborhood.
What are Roof Decks? In the Chicago area, there are many buildings that have flat roofs. The most common type of roof covering for these roofs are either the older felt paper and tar, multiple layer roofs or the newer widely used modified bitumen roofing membrane. Modified bitumen is a fancy name for a special type of rubber, think of it as sheet rolls of car tire. This membrane is rolled out on the roof and overlapped at the edges. These edges are then “torched” which is exactly like it sounds. The roofer takes a propane fired torch and applies it to where the edges of the membrane overlap, melting the edges and sealing the two sheets of bitumen. The roofs are, essentially, covered with one big piece of rubber. These roofs are required to be covered with a reflective, aluminized paint which reflects sunlight and helps to prolong the roof membrane life. Modified bitumen roofs are also called rubber roofs.
The easiest and most commonly built type of roof deck consists of simply laying 2 x 4 wooden sleepers (2 x 4’s laid on their edges) directly on the roof, sometimes with a second piece of roof membrane as a cushion, and nailing or screwing wooden planks to these support sleepers. This leaves a nice looking wooden deck for people to leave their patio furniture on, bring up a grill, buy a keg or two and have parties. I regularly see people who use these rooftop areas as play areas for their children or as a “sanitary run” for their dogs. When you live in an urban area, you find your open spaces where you can. As long as the parapet wall around the roof deck is at least 4′ high, providing a barrier to keep people from falling off the roof, and the structural load design of the roof joists is sufficient to carry the load, the City of Chicago allows roof decks to be installed. But I, as a State Licensed Home Inspector defect them and inform my clients about the problems with roof decks, and here is why:
So, what’s the problem? The problem with roof decks that are installed, directly, on the roof membrane are three fold:
1) They violate the roof’s warranty:
Placing the sleepers directly on the roof causes pressure on the membrane material. As people walk on the deck and as the deck members move because of changes in temperature and humidity, the deck sleepers rubs against the roof membrane and tears it. Almost all roofing membrane manufacturers will void the roof’s warranty if a deck is installed on it. Most reputable roof companies will not repair or replace roofs that have wooden decks installed on them. When the roof gets torn up it leaks and has to be repaired or replaced. The added pressure from the deck sleepers often cause the torched seams to open, causing leaks that have to be repaired. This greatly shortens the life of the roof and the roof manufacturers want nothing to do with this situation.
2) The roof’s drainage is ruined:
There is no such thing as an actually flat roof. Seemingly flat roofs have to allow for rain and snow melt to be drained, usually to rear gutters or to side wall scuppers. To facilitate this drainage a rigid foam material is installed between the roof plywood and the roofing membrane and this foam is shaped so as to provide a slope towards the gutters or scuppers. When these wooden roof decks are installed over this foam, the deck’s weight crushes it and causes low areas. These depressions allow water to pool under the deck and because the deck blocks sunlight, these pools of water are very slow to evaporate. Thus, the weight of the roof deck causes water to pool and work itself between the seams and tears of the roof which increases the frequency of roof leaks.
3) The cost of roof replacement is increased:
Eventually the roof membrane has to be replaced, whether its full service life is reached or not. To replace the roof, the roof deck has to be removed. This involves man-hours removing the screws that hold the planks to the sleepers and is very labor intensive (read, expensive). Because most roof decks are three or more stories above the ground, the wood is moved to one end of the roof while the other end is replaced, then moved back to the replaced portion while the other end is replaced. When completed, the wooden roof deck has to be re-built, and take it from me, it never looks as good as it originally did. In my experience, replacing a roof with a wooden roof deck costs about 3 times more than does replacing a roof with no deck.
Rather than having a wooden roof deck, with all these problems, here are a few alternatives that do not damage or increase the cost of replacing the roof membrane but still provide an open space for urban living.
Rather than a wooden roof deck built on sleepers, use wooden roof tiles. These are square wooden tiles with a special rubber cushion backing that spreads the weight of the tile more evenly and avoids damaging the roof membrane. These tiles interlock so they don’t move around and tear into the roof. They come in a wide variety of colors and wood types and are very attractive. Best of all, they are very easy to remove and stack up in one corner when the roof has to be replaced. They are manufactured to be water resistant and because of the raised sponge rubber backing they still allow rain and snow melt water to easily drain.
Many of you have seen the new sponge rubber mats that are being sold for installation in children’s play areas. These mats have been scaled up for use as roof deck material. These mats are made of recycled rubber and also serve to protect the roof membrane while providing an attractive and non-slip surface. These mats come in many colors and also in patterns resembling stone, wood and even grass.
There is one manufacturer, Buzon, that has long worked with roof membrane manufacturers and has a pedestal roof deck system that is endorsed by most roofing companies. A pedestal roof deck system uses adjustable plastic pedestals to support either the individual room deck sections (stone, wood, composite or plastic grid sections) or the roof deck sleepers. The base of the pedestals are specially cushioned and adjustable to compensate for the roof’s drainage slope. Because the system is modular, it can be easily removed for roof replacement and provides the ability to change the roof’s appearance when styles and tastes change.
So, when buying a house or into a condominium association building, thing ahead. You can have an open and attractive space for entertaining and still enjoy long life without roof leaks. Stay away from the cheap wooden plank deck alternative and take advantage of the new composite materials (most or which are made of recycled material) that will give longer life and require much less maintenance time and money. Be smart and think ahead.
Hope this helps.