I was wondering if I need to buy cross rails In order to strap my bike to the roof. I was planning on taking off the pedals and front tire I order for it to lay flat on a towel and be held on with ratchet straps. Would this method work or do I need to purchase an accessory part?

 

what year/model is your car?

personally I would use a proper bike rack and not strap anything directly to the roof if you value your paint (blanket or not). You should be able to find different options - including cross bars (ie Thule systems), trunk mounted (good for sedans and some hatches), or if you have or can install a trailer hitch, a hitch mount.

Check out sites like REI, etrailer, Dicks Sporting Goods, Thule etc for ideas and prices. You may also want to look into whether Uhaul rents something that fits your car.