TopCap 90
RetrofitFor decks that are already built
Welcome to GAPCAP™ — your solution for enhancing the appearance of your deck by seamlessly covering corner gaps. Here's how to retrofit a TopCap 90 onto an existing deck.
Initial setup
Begin by inspecting the deck to identify the corner gap that needs to be covered. Look at the miter from straight on and from across the board run — the more honest the look, the better your final result.
Measure and mark
Use your speed square to align and identify the exact position where the TopCap will sit. The cap should line up with the outer corner of the deck.
Mark the width of the underside of the TopCap onto the deck miter. Your deck board will be cut exactly 3/16" away from the underside of the TopCap to allow for seasonal expansion and contraction.
Scribe and double-check your marks
Run the speed square along your marks to scribe them accurately. Make sure every line is dark and clearly visible — once you start cutting, you don't want to guess.
Cut the deck board
- Eye protection on. Always.
- Set your circular-saw depth so you do not cut into the framing underneath. The cap only needs the depth of its underside channel.
- Cut along the marked lines as far as the saw will let you reach.
- Finish each cut with a multi-tool to precisely remove the deck-board sections right into the corners.
Install the TopCap
- Clean out the newly cut groove thoroughly — sawdust under the cap is the #1 reason a retrofit sits proud.
- Insert the TopCap into the groove, ensuring it aligns perfectly with the corners of the deck boards.
- Secure the TopCap with the correct color-matched composite deck fasteners.
- Use a drill to drive the screws below the TopCap surface so the plugs sit flush.
- Install the provided plugs and orient them to match the grain direction. See the plug guide →
Final checks
Inspect the installation to confirm there's enough room for the deck boards to expand and contract through the seasons. Confirm the TopCap is securely fastened and sits flush with the deck surface.
By following these steps, you've effectively retrofitted a GAPCAP™ TopCap onto your existing deck — covering unwanted gaps and giving the corner a finished, intentional look.
Pro tips for a clean retrofit
- Plan around the season. Cutting on a 90° August day? The boards are already expanded — leave the full 3/16" anyway. Cutting in January? That gap matters even more.
- Pair a contrasting cap color with the railing for an intentional accent line. Black caps + black railings is a customer favorite.
- Dry-fit before you fasten. The cap should drop in with hand pressure; if it's fighting you, your groove isn't clean.
- Pre-drill into hardwood or older treated decking to keep your screw heads from chewing the channel.