TopCap around
corner postsNotch the cap, keep the look
When a railing or corner post lives right where your TopCap wants to sit, you don't skip the cap — you notch it. Here's how to wrap a TopCap neatly around a post and still get a clean miter line.
Prepare the TopCap
If your deck design includes a corner post, you'll modify the TopCap to fit around it. Hold the cap up against the corner, mark the outline of the post on the cap, and double-check from both directions.
Using a jigsaw or a multi-tool, carefully cut the TopCap to accommodate the post. Cut just inside your line so you can dress it back with a file or sanding block for a tight fit.
Fit the TopCap
Align the modified TopCap with the corner of your deck. If there's no post, simply fit it over the corner as usual. If you've notched it around a post, ensure it wraps neatly around the post and still aligns flush with the corner miter.
Dry-fit twice. Pull it. Vacuum the groove. Set it again. Any rocking now will only get worse once it's screwed down.
Account for weather conditions
Leave a 3/16" gap between the deck board and the underside channel of the TopCap to allow for seasonal expansion and contraction.
Install the TopCap
- Slide the TopCap into place around the post and into the corner miter.
- Confirm it fits flush and enhances the corner aesthetics of your deck — no rocking, no gaps at the post.
- Place a composite deck screw in each exposed hole of the TopCap once it's seated. Drive each screw just below the surface so plugs sit flush.
- Finish with the included plugs, grain aligned. See plug guide →
Add visual interest
Consider using a contrasting color for the TopCap to create an intentional accent line. Black railings are popular — and pairing them with a black TopCap (even on brown or gray decking) makes the corner pop in a way that feels designed, not patched.
Tips for best results
- Ensure every cut is clean and measure twice. The TopCap forgives small inconsistencies in the deck board — but not in its own notch.
- Plan for seasonal changes when cutting the seam for the GAPCAP™ TopCap. Same rule as a standard install: 3/16" for composite, tight for treated.
- If the post is wrapped in a sleeve, dry-fit the cap around the sleeve, not the bare post. Sleeves are usually 1/4"–1/2" larger.
- Enjoy your beautifully enhanced deck with GAPCAP™.