Building Deck and Need Advise

[dfeck]

I'm in the process of building a deck for my parents. The deck is about 30" from the ground, attached to the house and I've using PT lumber for framing and Aztec decking (PVC Plastic). I want to use decking to close in the under side of the deck (skirting) rather than using lattice or nothing at all. I know deck ventilation is important so my question is; the deck boards will be spaced ~1/8" plus I'm going to be placing additional 4" round vents around the skirting. Will this be enough venting to create adequate airflow?

Thanks,
Doug

 

[tanglefoot]

I built my deck 15 years ago, and it is in need of new deck boards. The treated pine framing is still strong, even though some of it is in full ground contact all the time. Instead of skirting the sides, I used Windsor landscaping blocks to fill in the openings which range in size from 4" up to 16". Mine has never shown any sign of lack of ventilation. I believe the spacing between boards is for drainage. With my next set of deck boards, I plan to decrease the spacing to 1/8". The 2X6 cedar boards have shrunk to the point where the 3/16" gap I allowed has increased to 1/2" in places.
If you don't mind, what is the decking material going to cost?
Butch

 

[20 20]

Dfeck; I don't see a problem with your idea.

Tanglefoot; why not butt the boards up together, they'll still shrink, probably enough for what your wanting.

 

[dfeck]

tanglefoot - The deck boards are rather expensive. It's about $3.50 per board foot! Yes, $42 per 12 foot board. I'll need around 55-60 boards.

The PVC plastic boards do look great and they are suppose to be better in respects to stability, less weight and scratch resistant. The downfall is the cost. I'll be using a screwing system called Cortex which have hole plugs to hide the heads. I'm sure it will be labor intensive but should look great in the end.

As for the gaping. I built a 16' x 35' deck on my house a few years ago and I did NOT gap the boards. Of course this was PT deck boards. They do shrink in the summer and I get gaping of 1/8" when they shrink. They will swell up when it rains to no gap. If I had to do over I would have used composite decking but at the time it was an additional 3K in price and that was a big pill to swallow.

Thanks,
Doug

 

[tanglefoot]

2020,
If I go back with cedar, I won't leave a gap. But I'm looking at composite, and they recommend a 1/8" gap. I don't know whether that stuff shrinks or not.

dfeck,
That is expensive material. If I can buy in-stock material at Lowe's it will be reasonable, but I will need several 20-foot lengths, and none of the Lowe's in this area keeps that length. When they special-order composite, it sure gets expensive.
Butch

 

[EddieWalker]

I'm not a fan of any of the plastic deck boards, but I don't know all of them either. From what I've read in Fine Home Building and the Journal of Light Construction, it's not an expense that lives up to it's reputation.

As for venting, you need allot of it. I was under a deck that was just a year old that had 2x6's for decking and solid sides except for a few 6x12 vents in places that you couldn't see, but also not the best locations. The wood underneith the decking was growing mold and the joists were very damp with mold growing on them too!!!

I don't know how much ventilation you need for a deck, but I was surpised how wet it was under there and how much mold there was. I'm guessing that allot of water gets through the gaps in the decking and then the soil remains wet long after the rains are done. I can say that it had been several weeks since the last rain on that deck, and temps were in the low to mid 80's. Nice weather, but not hot enough to dry out the soil under tht deck. Of all the decks that I've built, I've never sealed in the sides.

I've never see moisture and mold like that under deck either, so it makes me wonder how much of that moisture is from the sealed in sides. The homeowner told me she had it done to keep out snakes. She's terrified of snakes, which I can understand, but I think she's caused herself some problems with her desire to keep the snakes out with a wood barrier.

I'd leave it open, but that's just me.

Eddie

 

[tanglefoot]

Eddie,
I live northwest of you about 80 miles. I enclosed our deck because I didn't want skunks to make their home there. I imagine there is a moisture buildup at times, though. What would be wrong with making the deck shed water and keeping the underside dry all the time?
Butch

 

[EddieWalker]

Keeping wood dry is the secret to it's longevity. Letting it get wet, they drying out, then wet again is the cylce that causes all of the problems. Pressure Treated wood will last longer then untrested, but it will still rot out on you. I'm not sure what it would take to make the deck shed water, but from what I've read, it seems pretty expensive.

Eddie

 

[dfeck]

I'm only closing in the deck for looks not keeping critters out. So it won't be tight. The plastic (PVC) material is suppose to resist mold growth and decay caused my water. My plan was to gap the boards along with keeping a couple inches from the ground. In addition to that my plan was to use 4" round vents placed every 3-4 boards around the deck skirting. I could go with larger square vents (6" x 16", etc) and strategically place them to allow airflow. The problem is I'm fining ZERO data on how much airflow is necessary under decks. The only data I find is how much gap to use depending on how high off the ground the deck is.

Doug

 

[EddieWalker]

Doug,

I think the problem if trying to figure out how much air flow to have is because the ground holds moisture and figuring it out is really going to be a case by case basis. Your soil may hold allot of moisture, or it might drain quickly. Sand versus clay type of thing. Your plan sounds well thought out, but the only way to know if it's working is by waiting a year or so and then to start doing periodic inspections. The deck that I was under doesn't show any signs of moisture under it, but while under there, it was overwhelming. They have a real problem that I told them about, and he told me he knows about, but that's as far as we got. I think they are just going to ignore it and hope it goes away by itself. I see this allot, and it keeps me in business, but it's still frustrating to deal with. Plastic on the ground should help, but in the few cases that I've seen plastic on the ground, it's been a mess and not actually doing anything. Mobile Homes do this, so I'm not sure how effective it is. In theory, it should work if done right, but for whatever reason, I always see it unrolled in areas, shoved into other areas and missing in others. Under the plastic can be very wet, or very dry, it's always a suprise to me when I come across these things.

Northern Tool sells electric fans with moisture sensors that I've installed. I'm not sure how effective they are, but don't see any real negatives to using them if moisture is an isssue. The times I've installed them, they seem to help, but don't address the real problem of stopping the moisture from getting under the house. Some clients like to go through the steps of trying everything but addressing the real problem. I give them my advice and then do what they are paying me to do.

Eddie

 

[The Dougster]

While I have never used the plastic decking, I have several friends that have, and to the man, have complained about it... I'm sorry I don't have specifics but in a couple of cases, they have had the plastic removed and gone to a wooden deck.
Your's seems especially pricey... have you priced other wooded material? (E-pay)... sorry, I don't know how to spell it.. lol

 

[dfeck]

Yea, I've been spending way too much time thinking about this. At this point I might be over thinking it. The deck will be 30" off the ground and mostly in the sun. The house is new construction so it's not under shade. I've viewed the Trex website and see some PICs in their gallery that shows decks that are skirted with their product. Similar to what I was going to do. I see no vents in the PICs but don't really know if there was something else that was done. At this point, I think I'm going to do the skirting and gap it 1/4"-1/2", keep it a couple inches off the ground and add the vents. I'll keep an eye on it and see how things are working out over time.


As for plastic, In my area most people are going with Composite or PVC (plastic). Real wood isn't being used much. Yes the price is very high but the benefits outweigh the price. Mainly due to longevity and maintenance. My parents are 67 and I don't want to be staining the deck for them every few years. I've also tried to convince them to use composite material so they could lower their cost. The difference is ~500 between the two.

Doug

 

[dfeck]

I found this on the Trex website.

Gapping

Trex® decking must be gapped, both end-to-end and
width-to-width. Gapping is necessary to facilitate
proper drainage and for the slight thermal expansion and
contraction of the Trex decking boards. Another reason
for gapping is to account for shrinkage of the wood joist
system. Following the proper gapping requirements will
ensure that your deck will look great year after year.

- Always follow Trex recommended gapping guidelines.

- Maximum allowable overhang for Trex is 4"
perpendicular.

- All decks require air circulation to keep them dry and
looking good. Leave openings under the decking or
increase gapping to 3/8" to improve air flow.

Fascia

- Trex used as vertical siding or fascia around the base of
a deck must be gapped the same as decking to allow for
air flow.

- Fascia should be attached every 12" with three Trex approved
screws. The top screw should be placed 1" from
the top of the rim joist, the second screw in the center of
the rim joist and the third screw 1” from the bottom of
the rim joist.


So, it seems that if I leave a gap of 3/8" I'd be ok. Agree?

Doug

 

[tanglefoot]

Doug,
It sounds like they are saying the ventilation should be adequate by increasing the gap to 3/8". I've read that Choicedek settled a lawsuit a few years ago that involved mold growth in their material. Now, they have a chemical in Choicedek (and Trex, I assume) that will combat mold formation.
I'm considering removing the old deck surface boards(2X6 cedar) and filling the joist spaces with gravel up to within an inch of the top of the joists. I realize this will hold moisture against the joists, but they are treated pine, so they should last 30 more years. This would leave no spaces for wasps to build nests or skunks to dig into. Then I'll resurface the deck with a composite material, leaving the necessary gap. See any obvious problem, anyone?
Butch

 

[CurlyDave]

So, it seems that if I leave a gap of 3/8" I'd be ok. Agree?

I disagree completely.

The problem you are going to have is that water gets under the deck in liquid form but has to get out as vapor, with thousands of times more volume. Throw in the fact that air is a most a few percent water vapor and you have to move maybe a million times more air out from under the deck as water went in. This is not going to happen with 3/8" gaps between fascia boards.

Unless you are in the desert, it is going to be dark & damp under the deck.

Even worse, if the ground is at all damp from ground water, you will get water vapor from the ground, which will also make it damp under the deck. Think about how many damp basements you have heard of. This is the same thing.

I would use a light-colored lattice backed by about 1/4" mesh hardware cloth to close off the area under the deck and keep animals out.

Under the deck will always be dark, and the light colored lattice will draw the eye and prevent seeing under the deck. If you ever do get mold under there you will have to remove it all before increasing the ventilation. Spraying some sort of nasty chemical in a space 30" high (minus the joist height) is going to be a hard job. Prevent the problem rather than abate the problem after it happens.

 

[MossRoad]

CurleyDave,
I think they were saying if you don't vent the sides you should gap the decking at 3/8".

 

[CurlyDave]

CurleyDave,
I think they were saying if you don't vent the sides you should gap the decking at 3/8".

I still don't think that is enough, and my experience has been that once mold becomes established in any area, it is very difficult to get it to just go away. Even if you change conditions by increasing vent area, the mold will not usually die off on its own.

The space under the deck is cramped and confined, remediating mold is not going to be a fun job. Do it right the first time & provide more than enough venting.

If the issue is not being able to see under the deck, fixed louvers will block any visual path while providing lots of air flow.

 

[dfeck]

I thought long and hard about this. I even called the manufacture of the product I'm using and they said it shouldn't be an issue without using vents. I also got feedback from a variety of sources while most were ok with the idea there were a few that had reservations. I'm going to use the product as vertical skirting. In addition to gaping I'll still place vents along the skirting and leave a 2" gap from the ground.

Doug

BTW - I checked some of the manufactures website photo galleries and you can see in some of the photos others had done this.