Under Decking Roofing Concerns

[MGH PA]

So we have an ~40' x 16' deck that has about 20' covered by a roof, with the remaining 20' exposed. Underneath, we have a full length/depth patio with furniture and a hot tub. In fact, here is a photo that shows what I'm talking about:

The hot tub and some furniture are under the non covered side, and we hate that it gets wet down there and is slow to dry out (not to mention we hate the rain on us when we're in the hot tub). I had looked at a lot of under deck systems, and decided (I thought) to build my own out of metal panels or the poly carbonate panels. I was going to pitch the panels from the house to the end of the deck at 1/4"/1ft, and then attach a gutter along the end of the deck. The more I read about this type of DIY system, I'm worried about a few things. Number one being cleanout/debris getting caught over time from the deck above (we used the conceal loc system from Timbertech). The other is lack of venting and air circulation trapping moisture in the deck joists which would then inhibit mold growth and/or premature failure of the deck framing.

Now, I know many of the manufactured systems are basically a variation of this (unless you do a pre-installed system like Trex Rainscape), so am I over thinking this? If it's a bad idea, what are my options/alternatives that I may be overlooking? We really wish we had the foresight to address this when we were building the house two years ago, but as they say, hindsight is 20/20.

I appreciate any input you all might have.

Thanks!

 

[deezler]

Looks like you have a lot of height to work with. Why not just drop your metal or polycarb panels down about a few inches from the decking? Then you wont be trapping any moisture in the deck assembly. Plus maybe you could fish a power washer in there to blast out debris and clean it up periodically. But if you use opaque metal panels, you don't have to care if they look grimy on top, either. Can't imagine it will really load up with much material (no trees overhead, etc).

Kind of a tricky situation, I've always wondered the best way to accomplish this.

 

[EddieWalker]

You have a very valid concern with cutting down the air flow and the build up of material on top of whatever you install. Mostly it will be dust that will become sludge that will eventually lead to mold. The joists and bottoms of the decking will remain wet longer and eventually turn black and then begin to mold.

I'm not aware of a system that actually stops the water from getting through decking, but allows proper air flow.

If it was mine, I think that I would extend the existing roof line from the house, to cover the deck. Probably change the pitch to get as much height as possible and might consider going with metal to compliment the color of the shingles.

 

[deezler]

Another idea could be a retractable awning over that open side of the deck, that at least covers the hot tub area. Would only really work in light rain conditions though, because you generally dont want those left out when a heavy storm comes through.

 

[ponytug]

Sorry to hear about your challenges. That looks like a gorgeous deck.

I looked into this when we replaced our deck, and came away with the impression that there wasn't a good way to do this. Or at least an established way to do this. In my opinion, you needed to be able to put a waterproof membrane over the joists, but under the decking, to funnel 100% of the water into the drainage system, otherwise your joists were guaranteed to mold and rot. Even that method left you with the challenge of getting it cleaned out. (Really long pressure washer wand?) For us, it also left us with a lot of rather flammable material under the deck, so I decided any upside wasn't worth the downside risk.

One thing I did do was use a joist sealing membrane along the top of all the joists to prevent water from sitting on the joists, or getting in along the fasteners in the hopes that it would slow any rotting of the joists. Time will tell.

I would go with @deezler's suggestion of an awning as the best solution that will preserve your deck joists, keep the rain off when needed, and be easily cleaned/replaced.

All the best,

Peter

 

[MoKelly]

Maybe a stupid question, but is there a way to include a small fan that, along with vents, could help keep the space between the wood and the metal dry?

Sort of like what is used in attics and ridge vents?

Sorry if this is foolish - but I understand why you would want a roof over the hot tub. It’s kind of neat being in a hot tub while it’s raining.

MoKelly

 

[PILOON]

Going on 20 years or so I made a nice room under my deck.
I used metal roofing material and 'slung' it under the deck joists.
At the rear (house side) I attached a 1" furring strip screwed to each joist.
Mid way I screwed a 2 x strip attached the same way.
Bottom (outer) I attached a 3" strip, again same way.
Barely, but enough, for water to drain away.
The metal 'ceiling' was attached via 1" at top of every dimple using self tapping screws.
Never a problem with any leakage what so ever.
One major bonus was the fact that sun never hit the metal 'ceiling' so the under deck was always cool. (and the under deck room faces afternoon sun).
I'm in snow belt country and again no problems as there is never any snow on my suspended 'tin ceiling' as the upper deck takes the loads.
It has been so successful that we even carpeted the under deck room and never even cover the sofas.
I did however need to get creative where the 'tin ceiling' met the foundation and solved that with metal flashing to prevent any water from sneaking in.
While I've not done so, I figured I could have created a DIY air conditioning of sorts by trickling water onto the metal ceiling probably by using a soaker hose.
We even installed a baseboard heater for those cool evenings.
Now during Covid my wife uses that under deck room every day to chat over the internet and play bridge with her friends an a daily basis.
OH, I helped a bit as the wall cavities are insulated but then all 3 walls are windowed with the cheapest single pane glass I could buy at the time.

To add, I have never needed to clean any debris from the area between the deck an my 'tin ceiling' either.
About the only problem has been the occasional wasp nest that gets installed, but we take care of that easily as they always set up on an outer perimeter.

 

[deezler]

^ boom, piloon sounds like he nailed it. Sounds awesome

 

[AnotherBueller]

~50 years ago my Dad had the same problem. 16 X 40 foot deck with 2X8X8 joists supported in the middle, we attached 1X3X8' stringers starting 2" down at the house side and ended at the far side ~3 from the end of the deck, which is too steep. We should have made it the same pitch as waste water pipe (1" in 10' so that the water will carry solids with it). We then bought fiberglass panels (18"X8'), bowed them, and installed with a 3" overlap, from the bottom ending ~3" from the band boards.
We then removed the deck board at the house, installed a piece of
2X4 on both sides of the joist whereat the end of each deck board and did not fasten that board.
Once or twice a year he'd lift up the first board and hose any debris out.
In the last ~50 years there's been no mold or mildew although we have had to replace the deck boards every 10 years or so.
Only in heavy rain did he have any drops at the joint/over lap. I think that if we had used 10' panels with the shallower pitch and a 2' overlap everything would have been fine.
Bob
Anothbueller

 

[MGH PA]

Thanks for all of the feedback. It's greatly appreciated.

~50 years ago my Dad had the same problem. 16 X 40 foot deck with 2X8X8 joists supported in the middle, we attached 1X3X8' stringers starting 2" down at the house side and ended at the far side ~3 from the end of the deck, which is too steep. We should have made it the same pitch as waste water pipe (1" in 10' so that the water will carry solids with it). We then bought fiberglass panels (18"X8'), bowed them, and installed with a 3" overlap, from the bottom ending ~3" from the band boards.
We then removed the deck board at the house, installed a piece of
2X4 on both sides of the joist whereat the end of each deck board and did not fasten that board.
Once or twice a year he'd lift up the first board and hose any debris out.
In the last ~50 years there's been no mold or mildew although we have had to replace the deck boards every 10 years or so.
Only in heavy rain did he have any drops at the joint/over lap. I think that if we had used 10' panels with the shallower pitch and a 2' overlap everything would have been fine.
Bob
Anothbueller

So, you are saying you would recommend pitching the roof to match sewer and drain pitching vs. the 1" per foot?