Is It Safe To Install a Roof Box With A Gap Between The Box And Crossbars?

Updated 02/19/2026 | Published 02/17/2026

Question:

My Prescott Design cross bars do not allow my Thule box to sit on its bottom where it has a recess. When I place the box on top, there is a gap. I bought your kit with t-bolts and knobs to tighten inside the box. Can I mount the box safely if it is not seated flush on the bottom area that is recessed? Thank you. Rex Bergsma

asked by: Rex

Expert Reply:

Hey Rex, thanks for reaching out.

With roof boxes like your Thule, the most important thing is that the mounting hardware is clamping securely to the crossbars and that the box can't shift once tightened down. A small gap from the recessed bottom area is not automatically a problem, as long as the box is sitting evenly on the bars and the T-bolts and knobs are fully tightened.

After installing it, I would grab the box and give it a firm shake side to side and front to back. If there is no movement between the box and the bars and everything stays snug, you are good to go. Just make sure the box is not flexing excessively at the mounting points, and recheck the hardware after your first drive to confirm it remains tight.

expert reply by: Jesse M