I priced underdeck ceilings at $12/sq ft installed. That is just way more than I am willing to pay, and I am just not that fussy, so I am going to do my own under deck ceiling using barn steel roofing panels.

I found several people doing this on youtube. Here is a sample of one: https://www.youtube.com/watch?v=-9WUiPOl9co

The existing deck is 12'x36' and I will be sloping the short side.

Right now, my intention is to:

  1. buy 3'x12' 26 gauge roofing panels from a big box store,
  2. Screw progressively thicker strips of wood to the joists (to provide slope),
  3. Screw the roofing panels to the strips of wood,
  4. Use a gutter to catch the water at the lower end.

My questions are:

  1. What is the ideal minimum slope? I am asking because I don't just want the minimum slope. I have plenty of headroom, so I can afford a couple of extra inches, but I don't want to slope more then what has a real advantage.
  2. How far apart should the strips of wood be such that the roofing panels will not sag over time?
  3. Do you have any other tips?
  • 1
    My concern is whether this traps moisture and accelerates decay of the deck.
    – keshlam
    Commented Oct 31, 2015 at 21:52
  • 2
    Trapping moisture is a legitimate concern; it is also a concern for the $12/sqft solution as well. Commented Oct 31, 2015 at 23:23
  • Is this retrofitting an existing deck or is the deck yet-to-be-built?
    – DA01
    Commented Dec 1, 2015 at 4:31

1 Answer 1


The slope or pitch of the under panels can be at any degree that will shed the water. If you consider it as similar to a roof than according to Code: "minimum slope is 1/4 inch vertical in 12 units (2% slope)" and ideally :"1/2 vertical in 12 units (4% slope)". This is for metal roofs that have a sealant between their lapped seams. My only suggestion would be to remember to use the same metal fasteners as the roof and get the type with the neoprene washer below the screw head. Also depending on the roofs profile pick-up similar support backing (i.e. wavy board).

  • Thanks for the ideal minimum slope. What is wavy board? Google did not turn up anything obvious. Commented Nov 7, 2015 at 22:15
  • Wavy board is a descriptive name for the wood support strips that are installed under fiberglass or metal roof panels. The most popular are shaped for the serpentine or Quonset type panels (wavy).
    – ojait
    Commented Nov 8, 2015 at 3:09
  • wavy board: lowes.com/…
    – ojait
    Commented Nov 8, 2015 at 3:18
  • 2
    I think he won't want to use wavy board, as he's fastening the roofing to the bottom of the joists, and that could prevent runoff. Also, you'll want to find a way to waterproof around the top of the screw holes where you fasten it to the deck. Otherwise water could get trapped between the washers and the roofing.
    – DrewJordan
    Commented Mar 31, 2016 at 12:41
  • 1
    @Michael potter, since ojait provided an helpful answer consider clicking on the up arrow this tells the system you have been helped and can guide others to this answer.
    – Ed Beal
    Commented Mar 29, 2018 at 10:10

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