Exterior-Trim Details That LastFlexible caulk joints combined with a drainage path for moisture

keep vulnerable trim elements rot-free

BY JOHN MICHAEL DAVIS

Caulk needs help to stay flexible

Caulk that is allowed tobond to all sides of ajoint cannot expandand contract withoutbecoming unstuck.

A foam backer rodenables caulk to behavelike an accordion inresponse to seasonalexpansion andcontraction.

Drainage mat gives moisture a way out. Because even the best caulk joint can fail, a -in.thick drainage mat is applied between this porch post and its base trim. Any moisture thatgets past the caulk is able to run out beneath the front baseboard, which is left uncaulked.

T he old saying "It's better to belucky than smart" certainly appliesto New Orleans, Louisiana. Con-

sidering the rot-acceleration chamber thatpasses for a climate here, it's remarkable thatNew Orleans has more 19th-century housesthan any other city in the country. Althoughit's tempting to credit the skill of old-timecarpenters who built things to last, the onlyreason all these houses are still standing isluck. When the first Europeans arrived, al-most every acre of land in New Orleans wasconcealed beneath a massive canopy of old-growth cypress trees. Clearing the land fordevelopment released a seemingly unlimited

supply of one of the heartiest building mate-rials on the planet.

The old-timers didn't worry about back-priming, drainage planes or caulking. Theyjust nailed together two pieces of wood andwalked away; no fungus was going to take abite out of a 2,000-year-old chunk of cy-press. Do that with the fast-growth sapwoodwe have to work with today, and you can besure the forces of decay will start moving inbefore the extension cords are rolled up.

Caulk is the first line of defenseAmong all the places where rot can secure afoothold in exterior woodwork, caulked

joints are probably the most vulnerable. Sea-sonal changes in temperature and humiditycause wooden trim elements to expand andcontract. Unless caulk has the flexibility toaccommodate this movement, sooner or lat-er, the bead will crack, and water will seep in.From that point on, the caulk actually doesmore harm than good because it allows wa-ter to soak into the wood while it restricts aircirculation that would promote drying.

My approach to protecting vulnerable exte-rior trim is twofold: First, I try to create acaulk joint that can weather many seasons ofexpansion and contraction. I've been aroundlong enough to know that even the best

Active drainage planes protect vulnerable trimThese drawings depict just three examples of how well-plannedcaulk details used in combination with an active drainage channeloffer two layers of protection to the most-exposed parts of a house.

caulk joint will eventually fail, however, so asoften as possible, I also build a path formoisture to escape when that happens.

Backer rod gives caulk the freedomto moveThere are two ways to create a caulk jointthat can handle expansion and contraction:Either reduce the amount of movement, orincrease the ability of caulk to move. I doboth. Certain species of wood, such as pres-sure-treated southern yellow pine, are inher-ently unstable, so I try not to use them whengiven a choice. For exterior trim, I prefer touse the highest-quality kiln-dried lumber I

can find; in my area, that's either mahogany,Spanish cedar or all-heart redwood. I alsofully prime every piece of trim, and I assem-ble permanent (crackproof) miter joints us-ing biscuits and marine epoxy (GougeonBros. Inc.; 989-684-7286).

I increase the ability of the caulk to movewith the wood by applying high-qualitypolyurethane caulk (sidebar p. 65) on top ofa backer rod to create a two-sided caulkjoint. (Caulk doesn't adhere to the foambacker rod.) The biggest mistake most ama-teur caulkers make is to fill the joint com-pletely where two pieces of wood meet. Thisprocedure not only wastes caulk, but it also

creates a three-sided joint that's prone tofailure: The caulk adhering to the bottom aswell as to both sides of the joint is left littleroom for movement. A two-sided joint—one in which the caulk bonds to the sides butnot the bottom of the joint—allows thecaulk to expand and contract like an accor-dion. (For more on caulks and sealants, seeFHB #61, pp. 36-42.)

In principle, creating a two-sided caulkjoint is simple enough: Just cover the bottomof the joint with a bond breaker, any materialthat will prevent the caulk from bonding. Ifa joint is shallow and cannot be enlarged,special Fine Line tapes are used as bond

Corner boards(plan view)

Porch skirting(elevation)

Water table and drip caps

breakers, but these tapes can be difficult towork with. An easier, better approach is toenlarge the joint to make space for a foambacker rod.

Wherever there's a place that two pieces ofexterior trim must fit together tightly, mystandard operating procedure is to cut a

-in. wide by -in. deep rabbet into the in-side edges of the trim pieces (drawing p. 60).After the trim is assembled, I compress a

-in. dia. poly foam backer rod into the bot-tom of the joint. This -in. backer rodleaves enough space on top for a -in. wideby -in. deep bead of caulk.

Erosion mat gives moisture an outUnfortunately, the best caulking job doesn'tcome with a lifetime guarantee. No matterhow conscientious I am, at some point intime, somewhere along the joint, I know thecaulk is going to fail. Failure could be causedby a minor installation flaw such as a drop ofsweat on the wood or an undetected bubblein the bead. If I'm lucky, the joint will lastthe life of the sealant (20 years tops). But atthat point, no one else is going to lavish thesame attention on the work as I did. So toensure long-term survival of exterior trimdependent on caulk for survival, I build in ameans for moisture to escape.

An escape route for moisture could besomething as simple as a couple of circularlouvers near the top and bottom of a hollowcolumn to promote air circulation. On theother hand, a fully exposed trim elementsuch as a corner board, drip cap or porchskirting (drawing p. 61) that has nothing butcaulk to protect its innards from wind-driven rain or splash-back requires an activedrainage plane.

Not long ago, I had to repair built-up poststhat supported a railing on an uncovered sec-ond-story porch. In this case, most of the rotdamage occurred when water seepage be-came trapped between the applied base-boards and the bottom of the post carcase.To prevent the same type of damage fromhappening in the future, I created a simpledrainage plane to give moisture a way out(drawing right).

Any type of rot-resistant shim stock can beused to create a drainage plane, but the bestmaterial I've found is Enkamat #7010, a

-in. thick mat of entangled nylon filamentsthat is manufactured for use as an erosionmatting (sidebar p. 65). After cutting themat to size with a utility knife, I wrapped ittightly around the post and then fastened itwith staples.

To allow for the mat's thickness, I addedin. to the length of each baseboard. After

TWO WAYS TO LET MOISTURE ESCAPE1. Leaving the front baseboard uncaulked at the bottompromotes drainage and air circulation.2. The angle brackets that secure the newel cap to the post

Creating the perfectcaulk jointBefore I apply any caulk, I make sureall the wood surfaces are fullyprimed; if some of the woodwork inthe photos looks unprimed, it'sbecause my favorite primer(Primkote #8006-1) is a two-partepoxy that goes on clear (Abatron;800-445-1754). Wherever I'veinstalled backer rod, I use my fingeras a gauge to make sure the rods aredeep enough to allow a minimumsealant depth of in.

A professional-grade caulking gun isa must for applying the thickpolyurethane sealants (photo 4,right); I've never given much thoughtto whether it's better to push or pullthe gun while applying the caulk,however, because to me the gun issimply a delivery device. I tool everybead of sealant I apply, and myfavorite tools are my thumb andforefinger. (They're just always there;what can I say?)

Polyurethane has to be cleaned upwith mineral spirits, and over theyears, I've developed chemical sensi-tivity to prolonged exposure, so Iwear 4-mil disposable nitrile gloves(W. W. Grainger; 800-323-0620)almost all the time. It's not alwayseasy to get crisp drags with gloveson, so when the appearance of thejoint is crucial, I will use a bare fingerif nothing else works, but I prefer touse a tool.

I think I've tried every tool specifi-cally designed for dragging caulk,and I've never had much luck withany of them. What works best forme is a 3-in. artist's palette knife(photo 1, top right). Looking like aminiature bricklayer's trowel, apalette knife (Charette; 800-367-3729) has a long, thin, tapered bladewith a rounded tip that's flexibleenough to offer precise control. Theblade is also polished and slickenough to get a really smooth drag,especially if it's regularly wiped offand lubricated with solvent.—J. M. D.

2. Mineral spirits remove excess caulk.

4. Polyurethane is the first line of defense.

cutting all the baseboards to length, I pre-assembled three sides of the box before slip-ping it around the post. While applyingslight pressure on the corners to compressthe mat, I checked the reveals by sight beforeI tacked the baseboards using an air nailer(photo p. 60).

After the final side of the baseboard wasglued and tacked in place, I filled the gaparound the top with -in. backer rod. Thebest tool I've found for forcing backer rodinto a tight gap is a window-screen splineroller (photo 3, left). Because this tool hasdifferent-size wheels on each end, I choosethe end that fits the situation best and sim-ply roll the rope in with one hand whilestretching it slightly with my other hand.Using a finger as a gauge, I roll back andforth over the backer rod until the top is atleast in. below the top of the baseboard.Then I cover the backer rod with a -in.bead of polyurethane caulk (photo 4, left).

Hardware creates breathing roomOther weak points on the built-up post arethe post-cap assembly and the joint wherethe railings meet the post. If the railing I'minstalling is thick enough, I'll make space for

-in. backer rod by rabbeting the edge thesame way I described earlier. If I'm installinga narrow railing, like the one on this job,however, I cut the railing in. short andmount it on top of a galvanized angle brack-et (drawing facing page).

Those handy angle brackets also make iteasy to create a breathing space while mount-ing the post cap. Using a scrap of cardboardas a temporary shim between the top of thepost and the cap, I installed two 1-in. anglebrackets on each side to anchor the post tothe cap; then I removed the shim and coveredthe brackets with a bed molding. I caulkedthe joint where the molding met the cap butleft the bottom of the molding uncaulked topromote air circulation.

Older trim needs help, tooAlthough modern lumber is more suscepti-ble to rot, even old-growth cypress can dete-riorate after years of neglect. When I have torepair localized rot damage on significantstructures such as windowsills or porchcolumns, I try to rebuild the missing or dam-aged sections in place using epoxy (see FHB#107, pp. 60-65). Wherever a failed caulkjoint was the source of moisture, I retrofitthe structure with working caulk joints. I al-so try to install air vents and weep holes thatcould help to promote drying (drawing p. 64).

Recently, I used these strategies to createworking caulk joints on the base of a 10-in.

1. Palette knife tools visible joints.

3. Backer rod allows caulk to flex.

Retrofitting for backer rod. A corner-grooving tool carves a -in. dado into the jointwhere the column shaft meets the upper torus of the base (photo left). Custom-mountedon a Plexiglas base, a flush-cutting saw cuts a rabbet where the edge of the plinth meetsthe brick pier (photo right).

Innovative bond breakers (backer materials) fill small gaps. When standard backer rodsare too thick, other materials are pressed into service. Some -in. silicone weatherstrip-ping fits the -in. groove at the top of the base (photo left). Fine Line tape backs up thecaulk joint between the upper and lower torus sections (photo right).

round column. As with most cases, the worstrot damage occurred at the top and the bot-tom of the base. At these places, I used a cou-ple of specialty tools (not made for the pur-pose) to carve out the space for backer rods.

At the top of the base, where the columnshaft rested on the upper torus ring of thebase, I was able to use a corner-groovingtool, which is intended for installing weath-erstripping in old door frames (ResourceConservation Technology Inc.; 410-366-1146), to carve a -in. wide by -in. deepdado (photo top left). I've also been able tomake this cut, although not as easily, using acordless drill equipped with a -in. ball-shaped die-grinder bit. The only problemwith carving a narrow groove such as thisone is that standard backer rods are too wide;fortunately, I have found an alternative,which I'll describe in a moment.

Before completing the caulking detail onthe top of the column base, I used the flush-cutting saw I'd normally use for trimmingdoor jambs (Crain Power Tools; 408-946-6100) to make a space for standard backerrod underneath the base (photo top right).The flush-cutting saw allowed me to place a

-in. deep saw kerf, in. up from the bot-tom. I needed a few swipes with a sharpchisel to remove the waste, and the base wasready for backer rod.

After blowing out the sawdust, I coatedboth grooves with Primkote #8006-1 (Aba-tron Inc.; 800-445-1754), a fast-drying epoxyprimer that did not clog the groove as mostprimers would have. Five minutes later, whenthe primer was dry, I inserted a -in. foambacker rod in the groove beneath the base. Inthe narrow groove on top, I inserted mystand-in for backer rod, a length of -in.silicone tubing (photo bottom left) that, likethe cutting tool, was also intended for weath-erstripping (sidebar facing page).

Although the top and bottom of this col-umn suffered the most rot damage, I alsocaulked the two middle joints where the in-dividual torus sections came together. Bothjoints had enough cleavage between themfor caulk but not backer rod, so I used bond-breaking tape instead (photo bottom right).To prevent a three-sided bond that wouldcrack and fail, I lined the bottom of eachjoint with -in. wide (#218) Fine Line tape.Once all the bond breakers were in place, Icaulked all the joints with polyurethanesealant and tooled them smooth, confidentthat this column will be around a lot longerthan I will.

John Michael Davis is a restoration carpenter inNew Orleans, LA. Photos by Tom O'Brien.

Modern materials protect historic trimMoisture that seeped in through failed caulk joints causedextensive rot damage to this 100-year-old column base.After the damaged areas were rebuilt with epoxy, air ventswere added, and a variety of modern tools and materialswas used to retrofit working caulk joints.

THE RIGHT STUFF MAY BE HARD TO FINDAlthough some of the materials mentioned in this article can befound at local supply houses (depending on your location), manyare available only by mail order. So plan ahead.

Enkamat #7010 erosion matting creates adrainage channel between trim boards(Colbond Geosynthetics; 800-365-7391).

Wascoseal 10-mil elastomericmembrane serves as flashingbehind drainage (YorkManufacturing Inc.;207-324-1300).

One-in. Midget Louvers and-in. thimble vents allow

moisture to escape fromwithin hollow columns(Midget Louver Co.; 800-643-4381).

Foam ( in.) isthe standardbacker rod fortwo-sided caulkjoints(Macklanburg-Duncan; 800-654-8454).

Silicone tubing( in.) serves asalternate backerrod for narrow(retrofitted)caulk joints(R. C. T. Inc.; 410-366-1146).

THE BEST CAULKS ARE FLEXIBLE AND PAINTABLEPolyurethane sealant is mypreferred exterior caulk. Al-though more than twice as ex-pensive as premium acrylic la-tex, polyurethane's vastlysuperior adhesion and flexibil-ity make it a bargain. I useSikaflex 1a, a Europeanpolyurethane available onlythrough marine suppliers orby mail order (Resource Con-servation Technology Inc.; 410-366-1146). Recently, my localsuppliers have begun stockingpolyurethane sealants fromMacklanburg-Duncan (now GESealants; 866-275-4372) and PL(ChemRex; 800-433-9517). Oneday, I'll give them a try.

Although polyurethane is mystandard exterior caulk, forsuper-critical joints—such asthe front edge of a thresh-old—I use polysulfide. Polysul-fide costs three times as muchas Sikaflex and takes three toseven days to dry. Neverthe-less, polysulfide retains signif-icantly more flexibility farlonger than polyurethane andis sandable. Where appearanceis critical, sandability allowsme to create seamless fillets. Iuse 3M #101 polysulfide (3MCo.; 800-364-3577), but haveonly small (3 oz.) tubes onhand because they don't keepwell after opening.

Silicone is highly flexible andadhesive, but I don't use it of-ten because—despite somemarketing claims—it can't bepainted. Where wood hasbeen painted or where trim el-ements are metal, glass or ma-sonry, silicone is a more effec-tive caulk than polyurethane.All silicones are not alike,however; in my opinion, DowCorning #795 (Dow CorningCorp.; 517-496-6000) is thick-er, more adhesive and moretoolable than anything I canget at a hardware store, and itcosts only a dollar or twomore a tube.—J. M. D.