How do I temporarily protect a flat roof until a permanent fix can be made?

You'll need some plastic sheeting (reinforced, ideally, as in a medium-weight tarp or mesh-embedded poly), and you'll need some lumber. 1x4s and 2x4s in eight- to twelve-foot lengths would be good. Even better if they roughly match the width of your sheeting roll or tarp.

1. Roll out a length of sheeting that's as long as the slope of your roof plus 6 feet.

2. At one end, roll a board into the sheeting for three wraps. Keep it neat and tight.

3. Lay another board over the wraps and screw it to the wrapped board, sandwiching the sheeting.

4. Do the same at the other end. The result should be leave you with the roof dimension plus about 16" between the wrapped boards.

5. Drape this assembly over the roof, with the lumber hanging off the high and low edges of the roof. It should be completely flat so as to not catch water anywhere. The dropped ends shouldn't be so long as to sway in the wind and cause damage or lift. Shorten the assembly if needed.

6. If needed, repeat this process and install additional runs with a 2-3 foot lap on the side.

7. Lay lumber down the edges of the runs, stapling from below to keep it from sliding off the roof. It's critical to keep the wind from lifting the edges. If that happens the whole thing becomes a parachute.

            <-- slope direction -->
         ___________________________________________ <---- lumber on edges 
      __|___________________________________________|__    (stapled to sheet)
    _|  ---------------------------------------------  |_
   | | |                       ^-- roof              | | |
   |_| |<-- fascia or wall                           | |_| <-- wrapped lumber
   

Now that we've seen the picture...

Hot glue, rubber cement or contact cement may hold.

• Hot glue will definitely come off cleanly with a minimum of effort, but I'm not sure how well it would hold to either the siding or the plastic sheeting.

• Rubber cement will also come of cleanly, but again, I'm not sure how well it will hold.

• Contact cement should hold pretty well, but I don't know how well it will come off.

Since both hot glue & rubber cement should clean up very nicely, it would be a simple matter to test them out and see how well they hold.

If neither proves suitable, contact cement would be worth a test. If you can find a scrap of (vinyl?) siding in the garage, it would be worth testing it on that while you're trying out the others, to see if it will clean off reasonably enough. If you can't find a scrap, look for a construction site around town somewhere and ask if you can have a scrap of siding. They sure won't care if you're carting off some of their garbage for them, but it's nice to ask first. Of course, if nobodies on site in the evening, and you're just taking a little scrap left on the ground...

A clear silicone caulk would certainly hold, and if it's only up over the winter, it should come off mostly easily in the spring. Any amounts that may get stuck into the embossed wood grain in the siding should mostly disappear since the caulk is clear. You may notice some yellowing/dirtying in 5 or 10 years and decide to get more aggressive about removing it then.