Porch project

[seamlyne]

Well, it would appear I did things in the wrong order. I demolished my old porch (concrete) before purchasing the materials (wood) for the new porch. My account at the local DIY store is in much worse shape than I thought, so now the house is porch-less, and I've no way to purchase the treated lumber I was going to use.

I DO, however, have a source for non-treated lumber for a ridiculously low price, and it got me to thinking. If I use treated for anything in contact with the ground - posts, stringers, etc - why not use regular lumber for the rest of the porch? It's relatively sheltered by the porch overhang, has full sun for a good six hours a day, will dry out quickly after rains and snow.

Entire structures were made this way for centuries. Leaving off the obvious argument that treated lumber lasts longer, if it's kept painted and maintained, why not?

[pushkins]

Yes you can build it with non treated lumber as long as there is no contact with the ground.

Now having said that, it's not advisable if you want a long lasting carefree porch, even when you say you will keep it painted and it gets plenty of sun etc...etc...The problem is that you cannot keep the untreated lumber protected on all sides and ends forever, you can treat it initially on the underside and ends but after it's laid you cannot again. This leads the under side to absorb or dry out or the ends and no matter what you do to the top your just "sugar coating it".

The issue with old homes that used pine for decks or porches etc... (and i hear that same line mentioned all the time) is that back in the day they used "old growth" lumber, after all back then why pick the little skinny tree when you can pick that big ole hefty tree, they also used products not available today like creosote, lead paint or in some cases used motor oil.
Old growth lumber is by nature much more dense, most pine (SPF) nowadays is harvested as little as a weeks when you see it on the store racks, the tree was probably only 10 -15 when harvested.

Species of lumber will also make a huge difference, if you use SPF (southern pine fur) which has very open grain the lifespan is short if you use SC (swamp cyprus) it will last forever as long as it's not in ground contact and can dry out, although it will turn silver grey left unfinished.

[Nestor]

No, that's not a great idea.

The reason you don't want to do that is because any time you build a structure out of wood, like a fence, you create places where rain water or snow melt will collect or tight crevices where capillary pressure will cause any rain water or snow melt to be retained. For example, where fence rails are attached to fence posts, that's where the two pieces of wood are close together, and that's where you're going to have water collecting or being retained in crevices by capillary pressure.

And, the reason that's not good is that the end grain of wood absorbs water very rapidly, whereas water is absorbed across the grain of wood comparatively slowly. In fact, it's that slow rate of water absorbtion across the grain of wood that's an important factor in protecting most wood exposed to the elements outdoors from rotting. So, where you have water pooling on wood or capillary pressure holding water in an area where it's in contact with wood, much more water will be absorbed into the wood in those places entirely because of the longer time the wood will be exposed to water. And, of course, the greater the chance of wood rot starting in those areas.

If you use pressure treated wood, or the heartwood of trees like Redwood or Cedar, the wood you're using is much more rot resistant, and can stay wetter longer than construction grade lumber without rotting.

So, now, having told you not to use construction grade lumber, I'm going to now tell you what things you can do to prevent deterioration of any wood outdoors, so that regardless of what wood you use, you can slow down the rate of wood deterioration so your porch stays good looking longer. You might think some of these aren't necessary, but I know a little about wood, but nothing about building porches, so you can decide what to keep and what to discard.

If your porch is covered by a roof overhang, then using construction grade lumber is much less of a concern. After all, all the exposed lumber in an unfinished garage will remain in good condition for decades provided it remains covered by a roof, and therefore stays dry and in the shade.

Regardless of whether lumber is pressure treated or not, keep in mind that water is absorbed into and evaporates from the end grain of wood 15 times as fast as it is absorbed or evaporates across the grain of wood. (The only reason I can think of why pressure treated lumber wouldn't absorb as much water as construction grade lumber is that it'll still be wet from the pressure treating process for the first two years of service.) Not only does that rapid water absorbtion at the end grain make the end grain the most common place for wood rot to start, but it also causes wood to split at it's end grain, like this:



(Keep in mind that this is a fence post that was cut off at the top horizontally, so water would collect on top of the post. If the end grain surface is vertical, water will drip off it and so there won't be as much opportunity for water absorbtion into the end grain.)

And, since the pressure treatment process only forces chemicals into the wood to prevent wood rot, pressure treated wood will behave the same way as normal construction grade lumber when it comes to splitting at the ends. Ditto for deteriorating due to UV light from the Sun.

Here's where I fly off on a tangent and explain what causes wood outdoors to split at it's ends...

What happens is that when dry wood gets wet, it swells, and when exposed to rain, the end grain of the wood is gonna get wettest because of that rapid water absorption rate there.

(Actually, the above statement isn't 100% correct. It should read: What happens is that when dry wood gets wet, the wood cell WALLS absorb water and swell to become thicker, and that's what causes the dimensional change in wood. But since wood cells are shaped like really long and really narrow drinking straws with walls at their ends too, the dimensional change ACROSS the grain of wet wood is vastly greater (about 80 times as much) than the dimensional change that takes place along the grain of the wood. And, that's entirely due to the the vastly greater number of cell walls you'd encounter going across the grain of wood than along the grain.)

Anyhow, back to the chase...

The water absorbed at the end grain is carried along the grain of the wood by capillary action so that wood up to 2 feet or so from the end of the board or post also receives moisture and swells. But now the rain stops and the Sun comes out and water starts evaporating from that wet board or post, and it evaporates 15 times faster from the end grain than it does across the wood grain further away from the end of the board or post. The result is that the wood at the end of the board or post now wants to shrink as it dries out, but is prevented from doing that by the still swollen wood further from the end. So, the end grain of the wood splits to allow the end grain to shrink while still allowing the wood further from the end to remain swollen. That releases the tension that has developed at the end of the post or board. Every time that wood gets wet, the cycle repeats itself and the splits in the wood at the end grain get worst (deeper and more of 'em).

You can see the end grain splitting and wood rot setting in on this exposed wood joist:




And here's where I come back to the subject at hand...

So, regardless of whether you use construction grade lumber or pressure treated lumber, I'd paint the end grain with an end cut preservative to prevent wood rot starting there, give that time to dry, and then paint those same ends with an exterior oil based paint to prevent water from being absorbed into the wood there. Most people use the end cut preservative to protect against rot, but that does nothing to protect against splitting.

And finally, you need to protect the wood from the Sun. It's UV light from the Sun that causes the surface of the wood to turn fuzzy and grey, like this:






Any deck stain will have UV blockers in it to protect the surface of the deck that's exposed to UV light.

So, even if you use pressure treated lumber, I'd still use a deck stain on it (or anything with UV blockers in it) to protect the wood from deteriorating due to exposure to UV light.

So, if you end up using construction grade lumber, the highest priority is that you keep the roof over your porch in good condition. The better your roof protects your porch, the less you need to seal the end grain of the wood or protect it from the Sun.


PS:
If you want to learn more about wood, and wood construction materials, an excellent resource is the website of the Building and Construction Technology program at the University of Massachusetts at Amhurst:

http://bct.eco.umass.edu/

which is basically a science/engineering course for people wanting to work in the construction industry as engineers, architects, construction managers and that sort of thing.

If you click on the "Publications" link at the top of that web page, and then click on "Publications by Title" you'll see a list of the publications available on that site. Unfortunately, Dr. Stephen Smulski stopped teaching at the U of Mass about 10 years ago to start his own wood failures consulting company, and so that site took all of Smulski's papers offline. However, you can still find one of the better ones dealing with wood shrinkage here:

http://www.paintsource.net/pages/solutions/new%20construction/wood_shrink.htm

[seamlyne]

Um...wow. Much more detail than I was expecting, and much appreciated. Thank you both!