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Exterior Porch Details for Traditional Materials

Drive through a new subdivision today and you’re greeted by homes covered with large swathes of vinyl and plastic. Step onto a porch and meet more plastic—floors, railings, columns…the list goes on and on.

In many homes, plastic has replaced wood and permeated almost every exterior building product. Not surprisingly, builders have become fluent at installing plastic, while wood skills have begun to disappear. Just when we need them most.

Recently, our company was privileged to build a home for a client with traditional tastes; the desire for an authentic look—especially on his back porch, where he wanted wood, was specified for ceiling, walls, floor, and columns. The challenge: using sound techniques for installing traditional materials in an exterior environment. In this article, I’ll focus on the floor and columns, which bear the brunt of sunlight and water.

Wood is hygroscopic

Wood absorbs water and when it does, it moves. Sometimes that’s a good thing. Each December, our families gather to slaughter hogs. A half-round wooden trough is filled with water for scalding—the first step in making lard. This trough, when brought out of storage, always has visible gaps between the slats. Around Thanksgiving someone places burlap bags in the trough and keeps them wet, swelling the slats.

Moisture and wood movement can also be a bad thing. We recently had a callback where water leaked through the joints of a factory window, causing water damage to a new hardwood floor. Just as with the trough, the boards swelled and they cupped, with the whole floor becoming wavy.

These experiences weighed heavy in my mind as we began designing the porch floor and framing.

Detailing for wood movement


(Note: Click any image to see a larger version. Hit “back” button to return to article.)

During the design stage, we identified several objectives for the supporting structure of this Douglas fir T&G deck.
Building it sloped to the exterior, with ventilation, were priorities. Early on, we ruled out dimensional lumber because of the proximity to grade. We decided on a detailed concrete slab. Our subcontractor poured a 4″ thick slab sloped 3/16 per foot to the outside and we allowed it to cure. Next, we installed a layer of Ice & Water membrane, so that rainwater dripping through the T&G decking would shed and drain off the slab quickly. The membrane also prevents ground moisture from dampening the underside of the decking.


For decking supports, we ripped a PT 1X6 in half and used spacer blocks to create ventilation channels. A 3/8″ gap at the perimeter allows air to circulate freely. Minor shimming of the sleepers prepared the way for decking installation.


Anyone with experience installing T&G decking enjoys the simplicity: nail through the tongue, slide the next one in place, nail through the tongue, etc. Right? Well, not so fast when you’re installing the material outside, or even inside, these days.

Understanding moisture content

Thinking about that cupped and wavy hardwood floor, I pulled out my trusty MMC 220 Moisture Meter by Wagner and took a few readings. Setting the specific gravity for Douglas fir (.50) and checking various boards, I found the average moisture content (MC) to be 11%. Next I consulted two charts. The first one can be found in a Forest Products Laboratory publication titled: Moisture Content of Wood in Use. PICT0445-croppedThe second chart is found in Chapter 12 of the Wood Handbook, also published by the Forest Products Laboratory.

Consulting these charts, I found 12% to be the equilibrium MC for wood in an exterior environment. However, the second chart stated the MC would rise in the winter to almost 15% in my regional area. Faced with these facts, I decided to gap the decking as I installed it. Using a piece of plastic banding and spaced the boards apart, nailing as I went. My weapon of choice is an SN60 loaded with 2 ½-15 gauge stainless steel nails. The SN60 seemed to work better than a flooring stapler, as it didn’t drive the boards tightly together. PICT0446My helper precut the boards and routed the drip while I nailed. Many donuts later (yes we eat them in Illinois, too) we finally had a finished floor. I enjoyed watching the gaps in the decking widen a little first, but then came a rainstorm. The gaps closed completely. Was I glad for that spacing!

The posts

With the flooring in place, we turned our attention to the four solid cedar posts that support the second floor and the roof above. The weight of the posts also got our attention during installation—trust me, they’re heavy—but that’s another story. The client wanted some sort of trim on the 10×10 posts so we got our creative juices flowing and made a mockup of a 9 1/2″ base and 4 1/4″ capital. With the customer’s approval, we turned to the next challenge—making miter joints that can withstand the expansion and contraction of outdoor wood movement.

Having seen older buildings with miter joints that have separated, I wanted to do something more than simply nailing molding around a large solid post. Remember, MC of exterior wood in central Illinois fluctuates between 11% and 15%! The installed posts measured 12.5%, which mean they’d swell about 1/8 in. in the wet summers. And they’d shrink, too, maybe even more in the winter. We were already seeing substantial cracks in the posts. The idea of wrapping trim around a moving post was…well…unsettling. To solve the problem, I cut a groove into the post to receive the trim and accommodate any potential wood movement.

Start your routers

I decided to rout out the wood to a depth of 3/8″ and then install the moldings 1/4″ into the post. 100_0795With an 1/8″ gap at each side of the trim, the post could swell up to 1/4″ before it would begin to stress the moldings. My preferred router for this work is the Bosch Colt because they’re easy to grip and maneuver, especially working at the top of the posts from a stepladder.

I set the angled base for the Colt at 10 degrees and cut a groove 4 3/8″ from the top of the post, using a scrap of wood as a guide. 100_0793A sharp reader will notice that 4 3/8″ is 1/8″ more than the height of the capital molding. I planned for this small gap, along with the slight angle, to allow water to escape and also encourage drying—an important detail for exterior trim.100_0797

After cutting the groove on an angle, I used another Colt with the standard base (multiple routers are good thing!) and a straight bit to excavate the wood above this groove. Some places were difficult to reach with the router, like right under the beam and against the house. I used a Fein Multimaster to complete those cuts.

All bare wood received two coats of sealer before the moldings were attached.

Pre-assembly is quicker

I cut the abacus and echinus to length and biscuit joined them together, and chamfered one edge of the echinus. Biscuits reinforced the miters while I dry-fit the entire assembly. All sides of the moldings were sealed except for the miters. We began installation by coating the miters and slots for the biscuits completely with Titebond III. Next we fit three sides together and slid the assembly around the prepared groove. We secured the last piece of molding using spring clamps to hold the joints tight. Centering the capital on the post, we secured them by nailing through the abacus into the beam. The finished capitals can battle moisture with their secret design weapons for drainage and expansion totally invisible to the client.

Protection for the plinth

The process we used for installing the plinth was similar to the capital, except we used only a single groove rather than mortising out the entire height of the base.  100_0751Using the Colt again and a straight edge, I routed a groove 3/8 deep by 3/4 wide into the post. With the other Colt and the angled base, I cut a 45-degree chamfer at the bottom of the groove so that water wouldn’t puddle in the groove and cause rot. A Multimaster and chisel finish the cuts against the house. As before, we sealed all raw wood twice.

I chamfered the baseboard on a table saw, and cut a rabbet so the top of the base would fit into the groove on the post. Then I mitered all the pieces. I didn’t want the plinth sitting down flat against the deck, so I added a special detail—a scoop cut out of the bottom, which was easy work using a Colt with a flush-cutting bit and a template.100_0752

We assembled all the pieces with Titebond III and used clamps to hold everything tight. After the glue set, we centered the base assembly on the post. Here we ran into a snag. Although the base was centered on the post, the base was not parallel to the deck boards because the posts were twisted! Aren’t they always? But our solution to moisture movement also provided a solution to the twisted posts.

Final details

We had enough wiggle room to rotate the plinth detail slightly and make the sightlines almost perfect. One nail through each plinth holds the assembly in place.100_0802
To dress up the posts a little more, we chamfered the edges using the largest bit from Freud 100_0800 available chucked into a DeWalt router with an offset base for better stability. But that raised a new issue: the routed chamfers were much smoother than the rough sawn cedar. To solve that problem, we first tried a Multimaster blade and then a hand saw to create the illusion of wood grain by dragging the teeth along the wood until the proper roughness was achieved. After cleaning up mountains of sawdust and painting the chamfers we stepped back to admire the finished product. 100_0805The rough posts had been transformed into eye pleasing columns. And best of all, we felt satisfaction in doing our part to make them last!



John Butler is the foreman of a small construction company. He works with two other carpenters doing mostly new construction and general contracting, along with the occasional remodel thrown in. He’s feeling the shortage of new homes to build, but still continues to work 40+ hours a week. John’s homes are generally much smaller than the 6000 sq. ft. beast discussed in this article; most of them are three bedroom, single story and 2300 sq. ft. They sell for 300K, including the lot. John enjoys chainsawing and having the family over for a wienie roast. He’d also like to install an outdoor wood burner, if he can ever find a home in the country for sale.


(Figuring that many readers would like to know more about those hog troughs, we asked John to write a thorough description of their use. After reading John’s vivid account, we decided that photographs weren’t necessary. Aren’t you glad?)

John: My uncle has always taken care of the scald trough prior to butchering. The trough is half-round, composed of approx. 3″ wide by 1 1/2 ” thick slats, cut on a bevel—you can see gaps between the slats. The trough won’t hold water at first. We have to lay burlap bags in the bottom to keep the trough damp. There are two or three metal rods that run through all of the slats with a nut on each end to draw the slats tight. The half-moon sides are made of similar slats set into a dado. Because we make lard from the hog’s fat, it is necessary to remove the hair first. This is where the scald operation comes into play. The trough is filled with water, heated in cast-iron cauldrons over an open fire. Optimum temp is around 160 degrees depending on outside temp, humidity, temp of the hog, sign of the moon, and any other old saying one can think of.

A length of thick rope is laid in the trough to roll the hog. One man holds the two ends of the rope, while the other holds the middle of the rope; they rotate the hog until the hair begins to loosen. Then the hog is rolled out of the trough onto a wooden table where 4 men promptly use a round metal scraper with a wooden handle to remove the hair. It’s a tricky operation: if the water is too cool, or if the hog isn’t rolled enough in the trough, the hair refuses to let go. And if the water is too hot or the hog is rolled too long, then the hog’s skin can tear and shred in the process.

After scalding, the hog is taken inside and hung with the head hanging down, where it is gutted and then cut in half. When the meat has cooled and the halves are being processed, the outer fat/skin layer is removed and cut into 1″ cubes. These cubes are cooked in the cauldron over an open fire. After about an hour, much of the fat has turned to liquid and the cubes of fat start to float in the liquid. The cauldron is removed and the fat cubes run through a press to squeeze the lard out. The result, when cooled, is exactly what old timers used instead of Crisco. If the lard is cooked properly (not “green”) it doesn’t need to be refrigerated and will not spoil. Old timers also kept cuts of meat in with the lard to protect the meat from spoiling (before freezers). Some of my family uses this lard for baking though the majority goes to a local bakery.


10 Responses to “Exterior Porch Details for Traditional Materials”

  1. Ben Davis

    This is an excellent article. It would be very nice to see close-up pictures of the final trim work in addition to the overall finished product. Very nicely done!

  2. Hartley Edmonds

    It’s gratifying to see the basic grain conflict addressed that occurs when rapping posts. I used a similar system on some exterior columns about 15 years ago. When I last swung by to check. they were still tight as a tick with no signs of degeneration. Great job!

  3. Hartley Edmonds

    Rapping posts…there’s an image. I meant wrapping posts.

  4. Brandt

    On our 1923 Sears house (“Columbine” model) front porch, the word “dentils” is used – seemingly referring to something about the double columns. Can you clue me in on that word? Thanks. B.

    • James

      Not sure about the relation to double columns. I have always associated the word dentil with a molding(dentil molding) Try googling dentil molding for a picture of what I am referring to and see if that fits with something on your house.

  5. Ryan Nowell

    Really like this post. Had a quick question on how you mounted the Sleepers to the membrane and concrete? Concrete screws perhaps? I have a cement slab currently for my front porch and this is exactly how I would like to mount the sleepers and the T&G on top.


  6. Aaron Curtis

    I love this article. Not only does it give me hope that there are still clients who want traditional forms of carpentry and construction but it also provides an exceptional example of how to do so. I personally find it inspirational as well!

    Well done.


  7. Norb Warnes

    It’s wonderful to see this thoughtful attention to detail. I have a similar problem but it involves concealing Simpson CCQ post cap connectors. This article offered useful and helpful ideas for my own project. Thanks!

  8. Joe H

    When attaching the plinth assembly to the column the article states one nail was used to hold it in place. Was a shim affixed to the column (or backside of the plinth) at this fastener location to keep the plinth assembly centered about the column? Then the other three sides are ‘floating’ so to speak? Just curious how this was done at the base whereas the top has the luxury of attaching to the beam and being completely independent of the column.


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