Introduction: Drywall and Plywood Cartop Carrier / Roof Rack

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Out of the choices of Cheep, Quick and Good, you usually only get to pick two. This project satisfies all three goals of most of my projects. I needed to lug home some 1/2" sheets of fragile yet flexible 4' by 8' drywall with only my Honda Civic. I built a roof rack that will allow me to haul 4 by 8 foot sheets of plywood and drywall. It is easily torn down to re-use the lumber or adapt it to a different car. The lumber for this project cost $10.

Step 1: Materials

For this project you will need:
Supplies
(2) 2x4 4 feet in length (I cut an 8 footer in half)
(4) 2x4 6 inches in length
(4) 2x2 8 feet in length (if you are carrying more rigid materials you can use 3)
2 1/2" Deck screws
Foam Padding, or carpeting - closed cell foam sleeping bag pads work great
Shims - but any thin scrap wood will do
Rope, strapping

Tools
Saw
Screwdriver
Staple Gun (tacks and hammer will do)

Step 2: Cut and Wrap the Feet

Cut four 2x4 pieces to about 6 inches in length
These will be the feet that rest on the roof of your car. To prevent them from scratching the paint, and to provide a little more friction than bare wood. We'll wrap them in some padding. I used some closed cell foam sleepingbag pad.
Cut four pieces of padding to about 6 1/2 by 6 1/2 inches
Using a staple gun or tacks (roofing nails have nice big heads too) fasten the padding one the EDGE of the 2x4's so that your staples/tacks/nails to not come into contact with the roof of the car.

Step 3: Attach the Cross Members

Place the feet on the corners of the car where you think they could support the most weight. You can test the location by pushing on the feet to see if your roof is sturdy. What you do NOT want to see if a lot of flexing of your roof, if you do, pick anoher location.
Lay one of the 2x4's across both feet and try to center it as best you can. You will notice that due the curve of the roof, the 2x4 crossmember does not lay flat on the feet. This is ok.
Place a shim (a wedge shaped piece of wood) or any thin scrap wood in the gap between the foot and the cross member and drive one screw through the cross member nearest the point where the foot and cross member touch (but at least 1/2 inch from the edge of the wood). Drive a second screw in the opposite corner of the same joint, through the shim/scrapwood.
Don't worry when the shim/scrapwood breaks and falls out, we only needed it to provide a spacer. Once the screw is on place it has performed it's function.
Repeat for the remaining three feet.

Step 4: Add the Rails

Lay the 8 foot 2x2's on the cross members. Center them fron to back so that your load will put equal pressure on all four feet. You will notice, again, that due to the curve of the roof the rails do not lay flat on the cross members.
Try to allign your cross members and rails so that they are square. You can use your car for reference.
Use a shim or scrap wood to fill the gap while you drive two screws from each rail into each cross member.

Step 5: Get a Rope

At this point you're pretty much finished. But before you drive off, you will have to fasten it to your car with lots of rope and/or strapping. It is important to have the rack anchored to your car so that is does not slip forward when you stop, backward when you accelerate, or sideways when you turn (or from wind forces).
While driving, PLEASE keep an eye on your rack and the materials you are transporting. The wind forces that can get under your drywall are proportional to your driving speed (and wind conditions in general). Take back roads, do not drive on the highway, be aware and be careful.