Be very careful with heat, don't try and use anything that builds more heat than a propane torch.   It is best to use a tip that gives a "broad" flame as opposed to a "pointed" flame.  When attaching a top rib, use one of the "heat stop" paste compounds on the bottom rib to help stop the heat from effecting it too.  Keep the heat on the barrels and let the heated area melt the solder at the juncture.  

Important, be very careful with the heat. Too much heat will cause the barrels to swell more than necessary and when the barrels cool, they will "shrink" where they were heated.  That will cause the barrel tube to be smaller (ID and OD) where they were heated.  Just as soon as the solder flows take the heat away. The temperature of the barrels will continue to rise for a few moments.  Don't force cool them. 

Of course there is a lot more than mentioned here, depending on if it is just a spot repair or laying the entire rib, methods of holding the rib ... ect.

The selection of the correct solder is critical to provide a good secure connection where needed without damage to the barrels, barrels connection or the other rib connection.

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