Do some searching on the site for roof tents, there have been lots of discussion about the same topic. It's not just the crossbars, you have to consider the overall weight on the vehicle, both static (while parked with people sleeping inside) and dynamic (while driving).

 

Crossbars are easy to order online and I’m sure they’ll handle the weight with occupants.
Oh roof tents… they are fast, easy,………..
Keep in mind that they are very exposed to wind being up that high. When people roll around or move, the whole vehicle will roll with the suspension. Most important is that you need to be extremely diligent to dry your tent out. Weather and respiration condensation will dampen the tent. If you pack it away wet/ damp for a few days it will mold and you’ll never get that smell out.
it’s totally doable, you just need to be very diligent. Roof tents are fast and easy but take a lot of care and have drawbacks.
another option would be to invest in a nice large roof box. I have the Thule Sweden Touring box and it is insane how much you can fit up there. I personally got the Luno air mattress with lifetime warranty. It is also fast and easy without lots of diligent care. When I plan to camp / road trip there is plenty of storage up top to empty the rear of the car. Just a few thoughts.

 

also remember the smallest roof top tent weights like 95lbs.. and that is the size of a very small to man tent. unless you want to drive around with it on your car all the time it takes two people to put it on and take it off the car. i had a tipui ayer 2 tent on my tacoma. it was cool but its kind of a hassle. once you put up the tent you cant leave.. you have to climb up and down a ladder.. the small tents have no where to put your shoes. your car has to be level or the tent will not be level, seems like an easy thing to do, but some campgrounds in the mountains will have level spot for a normal tent but the parking spot is on an incline.., also they are not that good of quality for how expensive they are.

 

Agree

 

From Subaru Drive:

"The Ascent, Crosstrek, and Forester all have a roof rail capacity of 700 pounds when the vehicle is parked on a level surface. That means that the tent and the occupants inside it can’t weigh more than 700 pounds.

You should also be aware of the weight capacity of the roof rails when the vehicle is moving. At speed, all of the aforementioned Subaru models have a roof rail capacity of 176 pounds, meaning that the tent – when stowed and at speed – has to weigh less."

 

your car has to be level or the tent will not be level, seems like an easy thing to do, but some campgrounds in the mountains will have level spot for a normal tent but the parking spot is on an incline.

Solution is a couple of items I have in my old 1976 Toyota pocket RV to level it out on sloped campsites/parkings.

Install a couple of bubble levelers inside the vehicle, one front, one side. Might be challenging to find good spots to mount inside the Crosstrek cabin. These particular ones work well in that they indicate how many inches the vehicle is off level on each corner. $10 for a pack of two.

Get a set, or two, of Lynx Levelers, or comparable. $30 to $45 a 10-pack depending on where you buy them. Each Lynx block gives 1-inch in height. Yeah, it is more stuff to cram into an already limited storage area in the back of the Crosstrek. The calculation is a bit crude for doing the inches and setting out the correct number of levelers, but it's generally close enough. I swear by these. Set them out and drive up on them.

Here are the Lynx Blocks in use.

 

I have known people with rooftop tents, and as was mentioned earlier, they are kind of a pain. Once you're set up, you can't move the vehicle, you have to climb, etc. My wife and I have a Leesure Lite motorcycle camper. It's a Full-size mattress on wheels. When it's being towed, it has room inside to store things, a box on the tongue to carry stuff, and lastly a cargo rack. When it is open, we have room to stand (getting dressed is much easier), room for a small potty (no going outside to pee at night), and it literaly goes from a trailer to a camper in 30 seconds (get out of rain quickly). It weighs 260 pounds, so you can't even tell it's back there. Just giving you another option.