Under-deck Ventilation Necessary?

Tinstaafl

Occasionally I'll see comments on here about leaving room for air flow underneath a deck when the sides are being closed in. For most of the decks I've dealt with, that's a non-issue, as generally the HO wants at most a little [gawdawful] lattice to pretty them up.

But now I'm looking at one where the HO wants the underside buttoned up tighter'n... uh, a really tight thing. He doesn't want so much as a mouse being able to get in there.

The material is plain old boring PT lumber. Given that (1) it is after all PT and supposed to be at least somewhat decay-resistant, and (2) the inevitable shrinkage leaving gaps between all the boards, is extra ventilation really an issue?

Anyone know of a couple good articles on the subject?

TaitINC

I'll see if I can dig up some old articles that I read in the past. As far as ventilation, with any wood species you will need adequate ventilation. Without it, the boards will cup from the top drying and shrinking and the bottom staying moist. I will see if I can find those articles I read.

MALCO.New.York

If Water can find it's way in, it MUST have a way out. Evaporation must be allowed to it's job.

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knucklehead

And if it stops flowing it will take on the shape of what is holding it.

MALCO.New.York

....It becomes the Bawttle......

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Tinstaafl

Yep, not arguing that. The question is though, what constitutes adequate ventilation. In this particular case, the deck boards alone will be covering a 7x25' area, running the short way (dictated by existing framing). After shrinkage, that means it'll have 50+ ~1/8" gaps, each 7' long. I sure wouldn't want that ventilating my living room in the middle of winter!

Any article links would be much appreciated.

Malco, back in your bottle! Earth is permeable. Water will drain.

deck king

The issue isn't so much about the wood as it is about the growth of harmfull molds growing under the deck and a breeding ground for misqutos. There has been a recent trend by a lot of villages in my area to requireing side vents in the skirting under decks. It's probably not a bad Idea! As far a water effecting the deckboards goes, the best way to lay the boards is to have the radius of the crown in the grain facing the sky! Think of it this way, if you put a soup bowl out in the rain, set up right, it will hold water!
if you put it up side down, the water runs off!
The down side to treated pine is the expansion of the wood. Pine has larger pulp baffles to hold sap ( this is why pine trees last longer in draughts than other trees) that the tree store for the dry season. the pulp is softer than the ring structure of the wood. when the wood becomes wet the pulp expande first at a quicker rate than the ring then the ring becomes satured. if the wood is cupped the wrong way, it's like water sitting in a bowl causing the wood to expand & crack repeatedly
which distresses the wood. Sorry, I got on a rant !

Tinstaafl

Rants are quite common here. We like to saddle 'em up and ride them until they die thrashing in agony.

I think you have to be a pretty young pup to be unaware of which way the rings should go, but it does bear repeating. As for mold, that can certainly be a factor in some cases.

In this particular instance, we're actually dealing with a roofed porch which gets plenty of afternoon sun, and the underlying soil has good drainage. I'll probably wind up closing it in just as the HO wants, but it would be nice to have some authoritative references to either back the idea or shoot it down completely.

MALCO.New.York

That was my point.

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neolitic

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(Sorry....it seems there really are dumb questions)

John Hyatt

Your looking for Cross Ventalation,,,air in air out. There are a lot of ways to do this. With Pt decking lumber it hardley matters. J.

Tinstaafl

Exactly, that's been my admittedly limited experience. I'm by no means a full-time deck guy, though I do 'em fairly regularly.

I'm just looking for a way to quantify the situation, similar to HVAC formulas. Near as I can tell at this point, the research hasn't been done--other than the occasional old-timer who just "knows what works" from long years of experience. I don't have that much time left.

John Hyatt

Its a thing I dident learn until I started with ipe/ S American lumber back then it was pau lupe brand name the rep had a video of a very cool ipe deck with raised pannel skirting before and after. The after was not pretty that 5/4 x 6'' ipe fase fastened looked like a roler coster total warp city. The Guys had closed it up tight with perfect joints.

I just started doing what I thought it would take your right there is no actuall in writing how to do it. I put screened vents in the skirting in a way that lets air move in and out, say a 16' x16' deck 4' high butted up to the house, two 12'' x 6'' vents on the side and two in the frount I make them look nice with a router detail all around.

If the project is going into an area like a U or L shape that is going against the house on two or three sides, I put floor vents in the decking with a couple in the skirting where I can.

I did a low to the ground T&G ipe project in an L shape with several floor vents close to the house and as maney as I could in the skirting. The first week those floor vents dripped water thats how much they work. If there had been no place for that water heavey air to go It would have warped that T&G material like crazey.

The gaps between the decking will not help at all if there is no air movement Under the deck with solid skirting. J.

Tinstaafl

Thanks, John; that gives me a little better picture. The plan is to use decking material for the skirting, so shrinkage will give a tiny bit of air flow--but you're right, it can't hurt to add more.

The frustrating thing is that no matter how wonderful a job you do building 'em, 90% of the owners slack off on maintenance sooner or later. <sigh>

Island1

A big reason why we stopped using those "built by" plaques.