Existing deck to enclosed space


Old 03-26-19, 05:24 PM
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Existing deck to enclosed space

Good day all,

i have been been doing much research as I contemplate transforming a current deck to an enclosed space. I have watched and read a lot of material, and I while that type of info can be a great resource, I have found that forums such as this are very beneficial as well ( a couple of years ago I rebuilt and early Jeep CJ primarily with YouTube and forums).

Many forums I have visited, including this one, have questions about this type of project, and many answers talk about how it is a bad idea, building a room out of a deck will never work right, “tear the deck off and poor a foundation”.

While I agree that a foundation is a much better alternative to decking and posts, I was curious as to why this would be a bad idea? What are the pitfalls that have some refusing to do this? Frost heaves, sagging, etc...

A little about my potential project, it is a 10’ x 16’ existing deck that I would like to convert to a mud room with laundry. Deck is ledger board built and sits 16-20” of grade. Project is in Wisconsin
Please understand I am not asking to prove my intelligence, instead I am looking for things maybe I have not thought of? Or maybe I am just trying to talk myself out of this project 😀

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Old 03-26-19, 05:54 PM
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The reasons are quite numerous.

First off, you would need to get permits and submit plans to get the approval from your building dept. They would probably shoot it down. Why?

Your deck was built to be a deck. It's footings (assuming it was built correctly), posts, beams, joist size are all sufficient for a deck but they were not designed with a room in mind... a room foundation has to carry the load bearing weight of exterior walls, roof, and snow load. So your pier and beam are probably insufficient in size or overspanned or both. Also deck code has changed in the last 20 years since your deck was built. If the deck is not built to current code, (for example, if it's on 4x4s) or if the ledger isn't fastened correctly, or if the joists are undersized or overspanned or if the posts arent on a footing below frost... there isn't much sense building on it.

2ndly, your floor needs to be insulated, air sealed, then protected from vermin with sheathing on the bottom. Not sure how you plan to do that if it's too close to the ground to crawl under and work. Depending on your joist size, you may not be able to insulate your floor very well for the conditions. (Some areas will have insulation minimums that are required by code.) Plus, having the room open to the elements underneath means the room will be completely surrounded by the cold in the winter, except for one shared wall. So it will be COLD out there... very cold, unless you plan to throw heat at it constantly in the winter such as electric baseboard heat or a wall unit that has its own thermostat. Your water and sewer would almost certainly need to remain in the former exterior wall since if its in the floor, it will surely freeze.

Finally, decks don't usually last forever. A house addition on a foundation is going to last a whole lot longer than some posts in the ground. But I understand that is what some people want.

I have built several sunrooms over decks (at least 3 that I can think of offhand) but have always incorporated plenty of heat... 2x12 joists with plenty of insulation (2" foam + fiberglass)... pressure treated plywood underneath... lattice to keep out animals... and have never advertised it as being a 4 season room. 2 were SIP roofs (structural insulated panels) the 3rd was a framed gable roof, shingled and tied into the existing roof with 2 valleys.
Old 03-26-19, 06:02 PM
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Thank you for the reply, this is exactly what I was looking for, and I understand all that you are saying.

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