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.

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

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

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

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

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.

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

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.
         &lt;-- slope direction --&gt;
      ___________________________________________ &lt;---- lumber on edges 
   __|___________________________________________|__    (stapled to sheet)
 _|  ---------------------------------------------  |_
| | |                       ^-- roof              | | |
|_| |&lt;-- fascia or wall                           | |_| &lt;-- wrapped lumber