If it doesn't fit completely inside the sedan, don't even try. Some people try strapping them to the roof. This is an incredibly dangerous idea, as cars have been destroyed by flying mattresses on the highway.Rent or borrow an appropriate vehicle.*Pad the roof of the car with a soft blanket to prevent damage to the paint. Place the mattress on top of the blanket, and then place the foundation (box spring)** on top of it. Fold or tuck the blanket so that it doesn't protrude - if the wind catches it, it will flap and pull and take on a life of its own. Also when your car moves at high speed, air flows across the hood and up the windshield. Try to position the mattress so that it is as far back as possible so that it 'catches' as little of this air as possible.***Open all four doors, and pass ropes or straps over the foundation and through the doors. From inside the car, tighten the rear rope or strap. Depending on the geometry of your doors, it may be necessary to do this with the doors closed, but you should be able to do it and then close the doors.
Open and close the doors a couple of times and inspect the rope or strap where it passes through, to make sure that it isn't cut or damaged. If it is, it may be necessary to pass the ropes or straps through the windows instead. This is a last resort, and should be avoided if at all possible.**** If, in your testing with the rear rope or strap you found that you were able to successfully tighten it with the doors open, then go ahead and tighten the front as well.These two straps will keep the mattress firmly attached to the roof. If the mattress hangs over past the rear of the roof, wrap a third rope or strap around it and the foundation to keep them together. Otherwise, the mattress will sag as you drive and obscure the rear window or worse.A rope or strap running down to the front of the car will help to prevent 'uplift' from the air that the mattress catches as you drive. If you have a convenient tiedown point at the front center, use it; if not, you'll need to create one: open the hood (bonnet) and locate the point where the hood latch attaches to the body (I'm assuming a regular rear-hinged hood).
Pass a short length of rope around that point, and tie the ends together to form a loop. Position the knot so that it will remain under the hood when you close it. Pull the loop forward over then edge of the opening, making sure that it doesn't interfere with the latching mechanism, and close the hood. This loop will be the forward attachment point. Pass another rope or strap from this point, back over the foundation, to the rear of the car. [*] please think twice about doing this.If you don't have a foundation, a piece of plywood the same size or a rigid frame of some sort should be used to keep the mattress from 'flying' as you drive. If you don't have something like this, please think thrice.If your roof isn't long enough, please think a fourth time. It's better to err toward having the load too far back than too far forward, but better not to have it too far anywhere.Please try no to do this. You're tying the doors closed with yourself in the car. Think a fifth time. Seriously.Now - AND THIS IS IMPORTANT - your car isn't going to handle like you're used to.
A lot of very smart people have put a lot of time into making sure that your car moves through the air the right way. You've now gone and strapped a sail to the roof. Keep your speed down. Be aware of the differences in how your car handles. Don't let stubborn make you dead. Speaking as a guy who wrestles mattresses and humps appliances for a living. (No, I'm not trying to be funny, that's my actual day job...)At the Distro Center where I work; we're not even allowed to help you tie a mattress to your vehicle roof. If you came completely unprepared, then we can give you some nylon packing twine...and some advice besides (please please consider coming back with a bigger vehicle!); but apart from that, once you sign off on the outgoing log...you're on your own.Here's your options in order of expense:Borrow your buddy's pick up truck or van. Most mini vans or small bed pickups will accommodate at least a double mattress. For a King or Queen sized matt you will want a full size cargo van (IE Ford F-350 van).
Don't know anyone with a pickup or a large van? On to option two:Rent a cargo van or trailer from someone like U-haul. Here in Canada, Home Depot (The Home Depot Canada) will actually rent you a truck or van (some of them are U-haul vehicles) for 90 minutes for about 30$. If 90 minutes are enough time to drive to where you're picking up the mattress, deliver it home, and return the rental to the Home Depot center you got it from; Bonus if you can also grab something from Home Depot for your home while you're there. Combine 2 trips in one! Of course, if you go with the trailer option, you will need a vehicle with a tow hitch already set up that is actually able to tow a large enough trailer.Have it delivered by the retailer's delivery contractor. They'll not only drop it off at your house, they'll also carry it inside (and wear booties if you insist), and then dispose of the packaging afterward. Our delivery contractor will also - for a small additional fee - pick up and take away your old mattress for disposal at a recycler.
Hire a delivery firm to pick it up and deliver it to your house. This applies to anyone outside the retailer's delivery area. Because we're a distro center, we can also ship to one of our retail stores nearest your home. Fair warning: if you choose to have it forwarded to a store, it will likely take a few days longer than sending a contract delivery driver to pick it up and take it directly to your home. However, this is by far the most expensive option.Please please please read Adrian Hsiah and Courtney Ballard's answers. They go into the dangers and ramifications of what you're thinking of attempting...If you have a small mattress it might fit in the trunk with the seats folded down.If the mattress doesn't have a really firm inner structure (which includes many foam mattresses), there's also manufacturers who sell mattresses that are vacuum packed to fit inside a box (look for things like Bed In A Box). These are small enough to fit in the trunk or backseat of an average mid size sedan.
However, if it doesn't fit in your car (like anything bigger than a standard twin mattress) avoid trying to transport it with the common method of tying it to your roof, especially if you're going more than a very short distance.Mattresses have a tendency to catch the wind and being soft and flexy, they will deform when doing so. Just because it feels heavy doesn't mean anything - the wind at normal driving speeds can easily lift the mattress if air gets under it, essentially acting like a giant sail.Aside from creating a ton of drag that affects your driving, worst case scenario, the mattress escapes whatever you used to tie them down and flies into the car or a pedestrian behind you. You now have a huge 50 or 60 lb weight flying down the road - a huge hazard. People have been in serious crashes and even died in accidents involving improperly secured mattresses. You'd face a tremendous amount of liability for doing this!In order to make it even remotely doable, you'd probably want a roof rack and you'd have to go buy a full set of properly ratcheting straps (never, ever use ropes or strings!) and probably attach length-wise to the car's bumpers (to prevent the wind lifting the front of the mattress) as well as through the rack and the car.