I have Roads shapefile which are denoted by lines.
I want to create another shapefile of roads as polygons, by adding 10-meter buffer on line.
How do I do this using QGIS?
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QGIS should be able to do that easily without any plugins.
First, you will need to make sure you are using the correct projections. You may already be doing this, and setting projections can be cumbersome depending on where your shapefile is located in the world, what projection it is in etc.
In QGIS, go to the "Vector" menu, choose "Geoprocessing Tools", and "Fixed distance buffer".
You can then also choose whether you want the result to be a temporary layer (you will lose it when you close QGIS), or if you want to save it elsewhere. Note that you can still save the resulting layer afterwards by right-clicking it in the layers panel and using "Save As".
The above is for QGIS 2.18, but the same basics should apply for all versions of QGIS.
If you want them to be of varying thickness, you could always specify the thickness in a field in your Roads line shapefile, then use the "Variable distance buffer".