Twenty pounds per square foot. That is the ground snow load most of eastern Missouri designs to, and it is the number that decides whether the shade kit you bolted to your deck last spring was a bargain or a problem you have not priced yet. A cover over a 16 by 20 foot deck is 320 square feet of roof. Loaded with wet snow, that is 6,400 pounds pushing down through posts and into a frame somebody sized back in 1994 for a grill, a picnic table and four people standing around it. So the blunt answer, the one the deck builders Wentzville MO homeowners call tend to give on the first site visit, is that a cover is a structural change to the deck and never an accessory bolted onto it. The box it shipped in says otherwise.
The Myth That Any Deck Can Hold A Roof
The myth goes like this: the deck is already standing, so whatever you set on top of it is decoration. Reality is duller and cheaper to respect. Every pound of that cover has to travel somewhere, and it travels down the posts, through the beam, into the footings and sideways into the ledger board fastened to your house. Job after job, the frame turns out to be perfectly fine for people and furniture and nowhere close to ready for a roof. The ledger is where it usually shows first.
Do not add a roof, a solid cover or a heavy pergola to an existing deck until a licensed pro has evaluated the footings, posts, ledger and fasteners under a permit and an inspection. The North American Deck and Railing Association states the reason plainly in its guidance on ledger boards and deck safety: a deck attached to the house with nails alone is unsafe, because the ledger connection is the failure point, and the association recommends a licensed inspection of the whole structure every year. If you already see split posts, a ledger pulling away from the siding, or joists that bounce when you walk across them, stay off the deck and call a professional rather than loading it further.
What Snow And Wind Actually Load Onto A Cover
Snow does not read the assembly instructions. PermitDeck, a permit outfit that tracks joist span tables jurisdiction by jurisdiction, said in a July 2026 rundown that once ground snow load passes roughly 50 pounds per square foot the prescriptive IRC R507 span tables stop applying, and the local office then wants either reduced joist spans or a structural engineer to verify the design. Eastern Missouri sits well under that threshold, which is the good news. The bad news is that the tables you do get to use were written for a deck holding people, not for a deck holding a roof that holds snow.
Wind is the half that gets forgotten. A solid roof on a second story deck catches air from underneath as well as over the top, so the connections have to resist uplift and not just weight. That is why a real deck roof over either carries its own posts down to new footings or gets the existing posts, piers and ledger verified before anything ties into them. A bolt together kit arrives with lag screws and brackets sized for the kit, not for your frame, and nobody at the warehouse ever saw your house.
Here is the part I cannot pin down. Nobody counts how many of these kits get strapped onto existing decks in a year. The manufacturers do not publish it, and the permit office only ever sees the projects where somebody pulled a permit, so I have no honest figure for how many are quietly overloaded out there right now. What I can tell you is when the phone rings about them, which is February, after the first heavy wet snow, and by then the repair is no longer a repair.
Questions Worth Asking Before Anyone Sets A Post
Run the money before you run the tape. Say a 16 by 20 foot roof over on that second story deck: two new footings with posts at roughly $2,600, framing and a shingled roof structure at $8,900, demolition and haul away of the old kit at $650, and permit plus engineering review at $900. That comes to $13,050, which leaves real room under an $18,000 budget for gutters, a fan circuit and the stain the deck boards have wanted since 2019. Treat those figures as an example scenario rather than a quote, because span, roof pitch and whatever the inspector wants under your posts all move them, but the shape of it holds. The structural work is about a quarter of the job, and skipping that quarter is what the kit was really selling.
Ask these before anyone sets a post.
● Who pulls the permit and meets the inspector? A good answer names the municipality and says the builder handles both.
● How will you check the existing footings, posts and ledger before you tie into them? A good answer involves digging and opening something up, not a look from the yard.
● What ground snow load and wind speed is this cover designed for? A good answer is a specific number that matches what the local code office publishes.
● Will the cover posts land on new footings or on the existing deck frame? A good answer explains which one, why, and puts it in the contract.
If your deck went in before 2000 on 4 by 4 posts and shallow footings, plan on the cover carrying its own posts straight down to new piers. If the frame is newer, with 6 by 6 posts on proper footings and a bolted ledger, tying the cover into it is often reasonable once an engineer or the inspector signs off on the numbers. Either way the answer comes from what is under the deck, not from what the cover weighs on paper.
Build For The Load You Will Actually Get
A three season room over a deck is a real thing, and plenty of them get built on 1990s subdivision houses every year. They just do not start in a carton. They start with somebody under the deck with a shovel and a flashlight, working out whether the footings reach below the frost line and whether that ledger is bolted or nailed. Everything after that is carpentry and money.
Judge the work by the questions you get asked, not by the photo gallery you get shown. Any of the deck builders Wentzville MO homeowners trust should be talking about footings, ledger bolts, snow load and permits well before they talk about ceiling fans and recessed lighting. That conversation is the product. Get the frame right and the space is usable in October rain, January glare and the wet snow that shows up on a Tuesday, and get it wrong and you own a very expensive shade structure nobody wants to stand under.
