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A little history - We bought this condo unit last year and just noticed that the paint the previous owner used on the deck boards were bubbling and peeling.

We inquired with the condo management to confirm that it is our responsibility to replace the decking boards.

Upon further inspection, some of the boards were noticeably moist and rotted. Some areas were worse than others. We initially thought it was due to the thick coating the previous owners used which may have trapped moisture and decided to replace all of the boards.

There is one area right above the floor drain that is collecting moisture on the underside of the boards. The new boards in the other areas of the terrace that were just laid down (not screwed down yet) and seemed to be ok.

This area in the winter is constantly warm and snow does not build up. Based on what we see, there seems to be a good amount of moisture in this area.

The structural base of the deck (and the deck boards) is constructed with pressure treated 2x6 boards which is sitting on top of a membrane.

Would larger gaps in between the boards (currently at 3/8”) allow enough airflow to prevent future rotting issues? I’m thinking that something more needs to be done to allow for more venting and prevent it from building up in the first place.

This is the drain area: enter image description here
NOTE: The deck boards are flipped up on their sides to help them dry out.

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    A few pictures would really be helpful. Why is there a floor drain under the outside terrace deck?
    – JACK
    Jul 15, 2020 at 15:59
  • The terrace is on the roof of the building and (from what I’ve been told by management) is that each unit that has a roof top terrace has a drain underneath the deck. Not the greatest picture but here is the drain. The deck boards are flipped up on its side to help air out. The moisture can be seen on the boards closest to the drain. postimg.cc/jDFBYQ6K
    – Zed
    Jul 15, 2020 at 16:22
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    That pic is very helpful. I'd be willing to bet that the left-right sleeper closer to the camera is sealing well enough to slow the water flow from where you're standing to where the drain is, thus allowing the moisture build up. I would drill some holes or carve out some small arches along the bottom edge of that beam to allow the water to flow faster to the drain.
    – FreeMan
    Jul 15, 2020 at 16:40

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You should replace the boards with composite decking.

I don't know what the old boards were or how old they were. A roof top deck that covers a membrane is always going to have moisture.

The deck structure looks to be 12" oc so you should be good to use composite.

If you flood the membrane with water chances are there are low spots that pool the water for portions of the year. It will never be perfectly dry in there. You don't want larger gaps in between your boards unless you want to loose coins, keys, etc in between the boards. Larger gaps would probably help somewhat with ventilation but I doubt it would be a magic fix.

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    I don't think there's 12" OC spacing there. The edges we're looking at are the edges of the decking itself, rotated to allow them to dry. It's not a layer of joists at close spacing over sleepers/skids directly on the roof surface.
    – FreeMan
    Jul 15, 2020 at 16:42
  • Correct. Specifically for this patch above the drain, the joist measurements are 12” on the outer channels and 16” towards the center. The other sections are 12-12.5 OC. I was hoping to switch to composite initially but the boss decided to match the existing materials and wood has been measured and cut already unfortunately. The moisture concerns me in terms of having to redo this again in a short amount of time. Definitely will switch to composites in the future but hoping to find some sort of temporary solution to hold a bit.
    – Zed
    Jul 15, 2020 at 17:32

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