Here's a lot of reading for you lol! Many products and methods for you to choose from.
Here is info for your roof
You can tar over it. Cover it with a rubber roofing material.
Asphalt Comp Roll Roofing. 3’ wide roll, covers 100 SF for $98.
If your slope is less than 2%, then you’ll need to adhere seal the overlapping seams in addition to using roofing nails.
I have friends that live in massive teepee made from carpet, felt, dirt & tires they used marine grade sealer/paint & rubber roof sealer stands up to adverse all year round conditions
Asphalt roofing will or sure do a job .. They are very inexpensive
Try. Waterproofing seal you paint on or cover with a tarp
There is a pour on asphalt roofing that is affordable. It is commonly used to fix metal trailer roofs.
Use Roofix, You can brush it on. It's gloopy and you can use it where ever you want it watertight. Comes in Black or White! Best wishes....
We shingled our shed: https://www.charlestoncrafted.com/diy-shingle-wood-shed-roof/
I prefer shingles, but the brush on sealer does work. The trouble with that type of sealer is that you will continue to have to reapply it to maintain it over the years. We put the same color shingles on our shed, as our house and they have held uo for 25 years without leaking. Good luck and stay safe!
Here's a video that might help - https://www.youtube.com/watch?v=_UbY2EUDjYs
Hi Simon, i hope these help you out, https://www.youtube.com/watch?v=QS59h4qLqRM