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There are some things you need to know before building a plaza or deck on your roof in Chicago. First you may want to contact an engineer to determine if your roof can support the weight. Most masonry structures will not have a problem. You will also likely need permit approval, and the engineer or an Architect can aid you in the design and permit process. Make sure to contact a carpenter who has extensive experience building roof decks, because not all Chicago deck carpenters have the experience it takes to build a roof deck. Roof decks are not the same as patio decks or decks for the ground.

Please note: The purpose of this article is to inform you of what you need to know about the ROOF installed in conjunction with the deck.  The point is, your new roof will leak if the roof is not designed for the deck and if your deck is not properly designed and built for your roof.

By far the most important one thing you will want to do before placing a deck on your roof is ensuring that you have a long lasting water tight roofing system protecting your building. As a Chicago area roofing contractor we repair several roofs every year where decks are involved. The typical causes of these roof leaks is unskilled carpenters damaging the roof, or installing a deck on an old roof which may not have much serviceable life remaining. Probably the most common cause for roof deck related leaks is the common practice of putting an expensive roof deck over a cheap roof. The roof is something you do not want to “cheap out” on.

When you are deciding what kind of roof to install before installing the deck you should go for the best roof your budget can afford. Ask your roofer the system ratings, or in other words the manufacturer life expectancy, of the roofing system he intends to install. The common “norm” for most Chicago area flat roofing systems is actually only a cheap 10-year rated roof. Do you want to tear off your perfectly good deck in 10 years so you can replace that cheap roof? Going cheap on the roof now will cost you thousands of dollars extra in the long run.

When I say to install the best roof your budget can afford I would highly recommend a 20 year rated roofing system. If you are installing a modified bitumen roof system, which is a common Chicago area flat roofing system, a 20 year roofing system would be no less than 4 ply’s of membrane. Single ply roofing systems, such as TPO, PVC and EPDM are becoming more and more popular (Learn about your Low Slope Roofing Options). If considering a single ply roofing system, install no less than a 60 mill thick membrane and preferably thicker 80 or 90 mill membrane. The thicker the membrane the longer it should last.

A commonly neglected step which will cause damage over time to your roofing system is the improper installation of roofing insulation. Roof insulation is a common and necessary component of a roofing system. However not all insulations are the same, and the commonly used insulation will be a Poly IsoCyanurate insulation which afford high R value. However Poly Isocyanurate insulation offers very little in compressive strength. Meaning that very heavy wood deck you put on your roof will crush the insulation over time and then the deck will likely tear the roof membrane. A simple solution would be to install a medium-to-high density fiber board over the Poly Isocyanurate insulation. Fiberboard offers very little in the means of R value but has a very good pounds per square inch (PSI) rating meaning it will prevent the crushing of your insulation beneath.

Now protect that roof before the deck is installed. A simple way of doing this would be to install some loose laid scrap roofing membrane, or preferably some neoprene (rubber) walk pads. The goal is that no object, especially the roof deck, actually come in contact with the water proofing roof membrane.

A quick tip about the roof deck and future leaks. Have the roof deck designed and built to be modular, meaning the floor panels can easily be removed with minimal tools. This modular roof deck design will save you on labor in the future if/when the roof does leak. When the deck is built modular the entire deck won’t need to be disassembled to find and repair a pin hole in the roof.

The key to a long lasting roofing system beneath a roof deck is all in the planning and proper construction of the roof and deck. Your roofer should work with your carpenter to ensure your roof lasts and that deck stays useable for a very long time. Ask yourself, does your contractor have the skills and experience to install your roof deck without damaging your roof. Did your contractor bring up any of the above information?

Post Author: Thomas Kral

One Reply to “What you need to know BEFORE putting a deck on your roof in Chicago”

  1. Two additional items that are absolutely critical:
    1. Check the occupant count for exitsing, an Architect is required.
    2. Check building code for deck materials allowed, non-combustible may be required.

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