Recommended clearance between cab over and truck cab

I have/had duallies so the clearance is between the TC and roof lights.
On previous Ford I had to remove roof lights covers + bulbs or they would get squashed by TC.
On new truck I have over 1" to the lights, what seems to be good for driving, but I have to be very careful with loading or leveling as small adjustment wrong way can eat the 1" very fast.
You can shim the camper higher, but that brings friction issue, bigger frontal area (fuel consumption) and stability.
Not ever talking that some TC on new Ford exceed 13' of height, so you better learn to read the signs posted on bridges.

Thanks to all that responded. I believe I will use a sheet of the 2" dense pink insulation from Owens Corning like I had used on my 2005 F350 with my Lance 855.

When I got my camper, I was concerned about the distance between the cab and the cabover mostly because I have clearance lights on the truck..Now since putting a 3/4 inch stall mat under the camper,I would like to lower the distance the mat provides...I was probably alright in the first place.Thinking just a pickup mat would suffice.

My latest F250 (2012) had that problem also. I had a rubber mat with a 2013 Camp Lite 10'. Only about 3/4 of clearence. The braces for the black water tank also were touching the rear fender, even with the bumpers and the happijack brace against the front of the bed. I have a piece of 5/4"on the front brace to push the camper back a little, and 3 pieces of 1x6 synthetic decking attached front to back, bolted to the frame of the camper. Now I have about 2" of clearance, and the tank brace is safely in back of the bumper.

tcguy wrote:
Quick question. Is there a minimum or recommended clearance between the bottom of the cab over portion of the TC and the truck cab roof? I have a 2018 F350 and a 2018 Northern Lite 811 Q SE. I currently have a heavy duty rubber bed mat in the bed and with the camper loaded have 3/8" clearance between the roof of the truck and the bottom of the cab over. Is this enough? Thanks.

I think NL recommends at least 1.5" sheet (I"m using 2" foam sheet).

I found that if the two "bumpers" on the front bottom of the camper were in contact with the front of the bed, the tail lights might rub against the camper. I also put two cut sections of particle board between the front of the bed and the camper to move the camper back ~1" so there is clearance.

IIRC, Ford has a taller cab than the others which is why your clearance is smaller. As Bob said above, 3/8" is not enough and the 2-3" is more the norm.
You will need to raise the floor of the TC in the truck bed.

No! IMO you need 2 to 3" for frame flex. Look down between the cab and the bed, those frame members do flex.