Window and door openings are where most siding jobs are won or lost. Get the trim detail right, and the elevation reads as finished; get it wrong, and you have a water problem that shows up years later inside the wall.
HardieTrim® boards are designed to work with Hardie siding, and they carry a 15-year limited warranty. Here is how the detail goes together, and where it goes wrong.
The material cost is modest relative to the rest of a re-side, and the difference it makes to how the finished elevation reads is out of proportion to that cost. The work itself is straightforward in principle and unforgiving in the details around windows and doors.
Do You Need Hardie Trim Around Your Windows?
You actually don’t, if that’s the look that you’re going for. Hardie siding can run directly to the window frame where the frame projects sufficiently from the wall. Check James Hardie’s installation instructions for the specific product, since the required projection and the sealant detail are both specified there and vary by profile.
Running siding to the frame still requires the head flashing above the opening. The sealant joint is a finish detail, not a water barrier.
Why would you run the siding to the window and not use trim? The main reason is cost savings, and is usually done on windows on the side or the rear of the house that are not seen from the curb. We usually recommend trim, partly for appearance and partly because the trim gives you a cleaner, more durable edge at the opening than a sealant joint against a window frame. On elevations not visible from the street, running siding to the frame is a legitimate way to control cost.
Installing Hardie Trim Around Windows
Install the trim before the siding, not after. Work in this order: lay the water-resistive barrier, set and flash the window, fit the trim, then run the siding up to the trim and seal the joint.
Reverse that order, and you create the most common failure on this detail. Trim fixed over unflashed sheathing looks identical on day one, then lets water into the wall from the first storm.
HardieTrim® Tabs, an extruded aluminum backplate, hold the trim board in place. Fit the tabs first, and you need only a few finishing nails to secure each board.
Hardie Trim has many valuable features. Among them are that the trim comes in 12-foot lengths, which reduces waste and speeds up installation, and they have a proprietary back-groove design, which makes the product stronger, yet light.
Tools Needed For Installing James Hardie Siding Trim
You will need:
- HardieTrim® Tabs, concealed fastening tabs
- Head flashing for each opening, sized to the trim
- Pneumatic staple gun with ½” and ¼” staples
- Pneumatic finish nail gun with 6d siding nails
- Color-matched exterior sealant
- Respirator rated for silica dust, plus gloves and eye protection
Installing the Hardie Trim
Confirm the water-resistive barrier is continuous and the window is flashed before any trim goes up. Head flashing laps over the window flange and under the WRB above it.
Fix a tab to the back of each pre-cut trim board, overhanging the edge by one inch, and staple it. Add further tabs at 16″ or 24″ centers along the board.
Hold the board in position and fasten it through the exposed tab section into the wall. A long board needs a second pair of hands.
Work around the opening with the remaining boards, shimming where the window flange requires it so the trim sits flush. Leave a 1/4″ gap between the siding and the trim, and seal that joint with color-matched sealant rather than butting them tight.
Looking for James Hardie Siding for Your Next Project?
Window and door detailing is where we see most DIY siding jobs go wrong, usually because the flashing sequence was worked out after the trim went up rather than before. If you want it done properly, talk to us about your project. Five-time James Hardie President’s Club recipient, a recognition given to fewer than 1% of Alliance members.
