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How to ruin a good siding job

Even the highest quality siding installation can be undone if you don't apply the caulking right. Caulking is primarily used to fill gaps between siding and where trim intersects, or where fixed items such as hose bibs, gas lines, or HVAC piping come through the wall. If done correctly, the caulking should be virtually unnoticeable.

The right caulk

There are a number of different caulk types and formulations. You might find four or five options for exterior caulk suitable for siding applications at your local hardware store. Keep these tips in mind when making your selection:

  • Pick a caulk that remains permanently flexible and meets the ASTM standard C920 class 25. Class 25 is the important part; it indicates that the caulk bead can expand up to 25 percent of its width. This means a 1/4-inch caulk bead can expand up to 5/16 of an inch. The bigger the caulk bead, the more the caulk expands.
  • If you are painting over the caulk make sure the tube says it's paintable. Some silicones and other caulks are not. Colorized caulks that match the finish on the siding are widely available. OSI Quad, for example, is color-matched to all of James Hardie's ColorPlus colors.
  • If you are using colorized caulk, pick the color that matches the body of the house -- not the trim color. The edge of the trim should look clean and straight. By matching the caulk color to the trim, you make the trim look out of shape with wavy edges.
  • You might think that you can save a buck when it comes to caulk. Don't. The best caulks on the market cost between $8 and $10. A modest-sized home requires about 24 tubes for a total cost of less than $250. If you pick a poor-quality, low-cost caulk, you are sure to regret it if you have to re-caulk in no time. A couple of extra dollars up front can pay off.

Follow the directions

When it comes to caulk, not all products are applied the same. Some caulks allow for -- or require -- tooling. Tooling means either using a tool or your finger to push the caulk into the gap you are filling, or using a tool to shape the caulk. Many people, even professionals, feel compelled to tool caulk even when it is completely unnecessary or not advised by the manufacturer. Not only can this ruin the performance attributes of the caulk, it can become a real mess. Some of the higher-quality caulks are not water soluble and if you get them on you, they can be tough to remove. Getting them off requires a solvent such as mineral spirits.

Bigger is better

Don't forget that a bigger bead of caulk will expand more than a smaller bead. The mistake most people make is thinking that caulk is meant to fill a gap, but this not the case. The bead should make contact with the two surfaces and lay over the gap. If you are covering a 1/8-inch gap, use a bead close to 3/8-inch to ¼-inch.

Practice makes perfect

Applying even the best caulks is as much an art as a skill. If this is your first time applying caulk, pick an out-of-the-way spot to practice. A steady fluid motion using long strokes is best. Avoid quick starts and stops, which can give the caulk a lumpy appearance. Smooth is the name of the game.

With good products and a little practice, caulking should be no problem for either the novice or the professional.

About the Author

Matt Spencer is the National Installation Manager for James Hardie Building Products, the largest manufacturer of siding in North America. Matt and his team educate installers, builders, and design professionals on the proper techniques to install fiber cement siding. Matt also works with James Hardie's R&D group to develop and improve installation practices for new and existing products. Matt has been with James Hardie for eleven years holding prior positions in sales and product development. He earned a master's from Northwestern University in product design and development.

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