Susan...
Do NOT use plastic & Tape... It will melt perhaps even with double walled pipe.
DO use the 'black tar' roof sealant that has been recommended.
If your roof is old, there are many commercial sealants that will cover the entire roof. You should apply these periodically to make sure that the roof remains leakproof.
Hope this helps..
Never mind the above advice:
- Since you're short of money, the plastic and tape might just get you by, which it seems is what you are looking for.
- the black tar is another temporary but again might get you by
- Of course there are commercial sealants, but it doesn't sound like you are financially ready for these and any other permanent application.