Putting a Deck on a Flat Roof: What to Check First

A flat roof with a deck on top is one of the most requested upgrades on Chicago two-flats, three-flats and garages, and one of the easiest to get wrong. The roof was built to keep water out, not to carry people, furniture and a party. Before anyone orders lumber, there are five things to settle: whether the building and the City allow it, whether the structure can carry it, what the deck does to the roof underneath, how it drains, and what it takes to keep people from falling off.

We are not quoting prices or timelines. This article is the checklist we would want an owner to have before the first call to a contractor.

Can you put a deck on a flat roof in Chicago?

Possibly, and the answer depends on your building and your zoning. We have not found a City page that lays out one simple "yes, here is the rule" for roof decks, so we will not make one up. What the City does say tells you how seriously it takes them.

The City's Express reroofing page lists a building with an unpermitted rooftop feature, and gives an unpermitted roof deck as the example, as not eligible for Express reroofing (City of Chicago). In plain terms: a roof deck that was built without a permit can block a clean roof replacement later.

Plan-based permits in Chicago require plans prepared by an Illinois-licensed architect or engineer, and for most projects an architect or structural engineer (City of Chicago). The City's Standard Plan Review page says all projects need zoning approval (City of Chicago). Whether a new roof deck is handled that way for your building is a question for the Department of Buildings (DOB) and a design professional. Ask before you design.

Can the structure carry it?

This is the question that matters most, and it is not one a deck contractor should answer by eye.

DOB says porches and decks must support 100 pounds per square foot of live load (City of Chicago). A City training deck on the 2019 code, though, lists a different figure for porches, decks, balconies and occupiable rooftops that serve a single unit (60 pounds per square foot) than for exterior stairs and porches that serve more than one dwelling (100 pounds per square foot) (City of Chicago). Do not use either number as a rule for your roof. Which load applies depends on your building and who uses the deck, and an Illinois architect or structural engineer sets it.

Water adds load too. The Asphalt Roofing Manufacturers Association (ARMA) says ponding water can overload the roof deck (ARMA). A roof that ponds under a heavy deck is carrying more than anyone planned.

If the roof is over a garage, ask whether the garage structure was designed to carry people, not only roof loads. That is a structural question about the walls, the framing or the slab, and it belongs with an engineer. For the permit side of garages, see do you need a permit to build a garage in Chicago.

What does a deck do to the roof underneath?

Three problems, all of them avoidable if you plan for them.

1. Penetrations. Every post, sleeper or anchor that touches the roof is a place water can get in. A deck that sits on the roof, not through it, still needs protection for the membrane below.
2. Hidden problems. A deck hides the roof. Ponding, a clogged drain or a failing seam under it can run for a long time before anyone sees it. ARMA lists clogged drains and membrane deterioration among the problems standing water causes (ARMA).
3. Rotting wood. Serious decay in wood needs moisture above about 30 percent. Rain and wet conditions, not humid air alone, are what get it there (USDA Forest Products Laboratory). A wood deck laid over a roof that holds water stays wet.

How does the deck affect drainage?

A deck should never trap water or block a drain. Drains, scuppers and gutters have to stay reachable for cleaning, and the water still has to reach them. NRCA guidance recommends positive slope, tapered insulation and proper placement of drains, scuppers and gutters (ARMA). A deck that is built flat on top of that does not change where the water goes below it, but it can hide the problem.

Ask your contractor, in writing, where the drains are, how you will reach them with the deck in place, and what happens if the main drain clogs. Our guide to flat roof drainage explains the system.

What about railings and guards?

A roof deck with an open edge is a fall hazard, and guard rules are exactly the sort of detail you do not want to guess. The American Wood Council's deck guide, citing the model residential code, says a deck more than 30 inches above grade needs a guard and that guard openings must not allow a 4 inch sphere to pass (American Wood Council). That is model-code guidance. It is not the Chicago code, and a roof edge adds questions a ground-level deck does not, such as what the parapet counts for. Get the guard height and design from the DOB and your architect or engineer.

Do you need to fix the roof first?

Often yes, and the order matters. The City says a permit is required for work on low-slope roofs, even on residential buildings up to four stories (City of Chicago). Roofing and waterproofing work may be done only by a licensed roofing contractor or qualifying party in Illinois (Illinois law). If you plan to re-roof, do it before the deck, and have the deck designed with the roof system in mind. For how long a flat roof lasts and why we do not quote a number, see how long a flat roof lasts in Chicago weather.

Also remember that a roof deck is a structure you have to maintain. The City's maintenance code lists exterior stairs, decks, porches and balconies, with guards and handrails, among the things that must be kept structurally sound and in good repair (City of Chicago). A deck you build is a deck you own.

What to settle before you build

1. Is a roof deck allowed on my building? (Ask the DOB and check zoning.)
2. Who is the architect or structural engineer, and what load did they design for?
3. What is the condition of the roof, and should it be replaced first?
4. How are posts or sleepers set, and how is the membrane protected?
5. Where are the drains, and how will I reach them?
6. What guard design does the City require at the edge?
7. Who pulls which permit, and what are the numbers?

For the roofing permit side, see our Chicago roof permit guide. The Chicago roofing guide covers the whole system. If you want a straight read on whether your roof can take a deck, our roofing team will look at it and tell you what we would fix first.

Common questions

This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.