Deck Drawings

Small snag on the deck plans. You see, we are planning to put a water tank under the deck, and the water tank needs 50" of height to fit properly (and 70" of width, which we have with no problems). We have 30" available with the current deck plans.

I've spent some time doing materials research (we're 99% likely to use Trex for the decking, with joists 12" on center to support it) and then making drawings like this:


The idea was to get the lateral bracing worked out (for earthquakes; those of you who choose to ignore the tectonic plates can skim this section). A structure can be just fine with a complete membrane in three directions, so on the right side there you'll see a small diagram of the lateral bracing running along two sides (for horizontal directions). I've still got to figure out what will work best over the horizontal (the floor plane), but I'm guessing that some sort of strapping will work fine. Trex is too floppy to make a real membrane out of the floor surface as would happen in a wood deck.

So, yeah, that 20" problem under the deck. I need to do some measuring (it may be less than 20", but not likely much less), but it looks like we're going to be in for some major digging there if we want to get a water tank (or two) under the deck. Darnit; everything else works out just fine.

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posted by ayse on 06/25/07


Can you use/find smaller water tanks? What are the tanks for? Gray water?

Standard tanks are all too tall for the space (they're mostly designed to be raised on towers, so they err on the side of narrow bases in geometry). I'm doing some more research but it's not pretty; the shorter tanks tend to be intended for buried installation, and cannot be installed above ground. More digging.

I need about 500 gallons of capacity for the irrigation; we have the sump under the house that emits 1000-2000 gallons of clean freshwater per day (currently dumped into the street and thus into the bay), so 500 gallons gives me the float to irrigate at solid intervals while using most of the capacity of the sump. Excess water will... well, I don't know yet, but I don't want to dump freshwater from the ground into the saltwater bay.

We do eventually plan to put some greywater in the garden, but it's a very bad idea to store it in tanks because it gets smelly really fast. Also, we kind of have enough water right now, if not exactly in the right place.

Oh, I see. That's a lot of sump water. Could you put the tanks in the basement and then install a pump to pump the water out for irrigating? (Though you've probably already considered that.)

While having to have a sump pump seems like pain, it's probably also nice to have a steady supply of "free" water.

First question is - is that Chewbacca standing on the deck?

Second is - how much weight is going to be sitting on the deck? The reason I am asking is that what I have read about earth movement is that the top heaviness of a house and no anchoring is what makes them slide off their bases. If the deck is not that heavy, wouldn't it just flex a bit, even in a big quake?

It is a teenaged wookie. Noel suggested it looked like a fish.

As far as lateral loads, it's a bit more complicated than just having weight at the top. A lightweight structure (for example, a deck without walls) may have less weight than a house (but not much less), but all that weight is on the top (at the decking). It needs to have structural stiffness or it will collapse. The problem is that the joints in standard construction are not designed to handle sideways (lateral) weight at all, because that construction was designed before we really thought about that sort of load. That's why decks are generally stiffened with bracing of one sort or another. Trex happens to weigh a lot more than wood, and is not as strong in tension, so it adds load and doesn't add stiffness. Hence the need for a small amount of membrane strapping on the floor plane.

Didn't know that about Trex. I have also heard that Trex is hotter to the feet as it takes in more sun. I heard that and laughed derisively. At least you won't have to paint the sucker.

It does have a higher thermal capacty than wood, but in another thread I was assured that it didn't get too hot to walk on.

Given that our nighttime temps are in the 50s even in the summer, a warm deck might not be so hard to handle.

My upstairs deck is Trex, not sure how old, but definitely not new. I have not checked how hot it feels in the middle of the day, mainly because it's just too hot overall to be out on it. One thing, though, when it heats up, you definitely get a faint "warm plastic" smell from it. It's not super-obvious, or offensive to me, but it's there.

Not having to paint or stain it is worth that trade-off.

you can span up to 24" with TREX

What joist spans should I use for my deck when using Trex decking?
For Trex 5/4x6 deck boards, the joist spacing should be no more than 16" on center for residential applications, which is the same span generally recommended for wood. Trex 2x material should be installed at no more than 20" on center for residential applications and no more than 16" on center for commercial projects. A high strength 2x6 product is available for 24" spans.

Hi Ayse,
Just stopped by from Chez Neumansky to see what y'all have been up to. We actually move in this Friday!!! You seem to be set on trex but I wanted you to think about ipe. The stuff is nearly indistrucable, super strong- the 5/4by6 deck boards can span 3' easy. With hidden fasteners you can get a nice seamless furniture grade look. If you don't want to finish it you don't have to. It will turn silver-grey and stay that way for years-heck the stuff is rated 20yrs ground contact!!! With ipe you could set your joice spacing 24"oc and still do a diagonal pattern. Your railgins, benches will be darn near indistructable. Anywho we built a deck out of it at our first place and really love it.

How high is the water table in our neighborhood? Is that tank going to float on the high ground water? If not I say hire someone (you know the industrious lads that hang outside of economy lumber)to dig that hole and sink that tank below the deck

Trex can span 24", but a) the high-strength product costs more and isn't stocked standard at our lumber yard, and b) the sag is still notable.

Dan, yay on the moving in! We should tell you our stupid toilet installation story some time. Ipe sounds interesting; I was being biased about recycled materials but it may be worth a look. As for the water tank, the soil next to the house there is engineered, so we really don't want to dig it out. (The water table is irrelevant because we have a sump that goes down to 12' below the soil surface.) On the other hand, we may have found a different water tank that wil fit in the space and do the job just fine.

Note: We're getting pummeled with spam comments, so I've turned off the ability to use any HTML or include any links for the time being. Email with any issues.

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