It takes more paint to cover a rough wall than a smooth wall. Every bump or depression adds surface area--it's like a mountain range in miniature, and each peak and valley needs to be covered consistently. Whether your house came with roughly textured walls or you create the texture yourself, and whether they are just subtly nubbly or cratered like the moon, the right tools and preparation help achieve a professional-looking paint job.
Cover floors, carpet and furniture with dust sheets. Apply painter's tape to the edges of all floorboards, moulding, and trim.
Pour primer into a paint tray. Paint around the edges and corners of the walls with a paintbrush to create a stripe of primer that is 4 inches wide. Rinse out the paintbrush and leave it to dry.
Push a 3/4-inch-thick lambswool paint roller cover onto the paint roller frame until it snaps into place. Put on safety glasses. Dip the roller into the paint tray of primer, pushing it over the tray's ridged bottom to coat it thoroughly and evenly.
Roll the paint onto the wall slowly to avoid splattering. Paint a long, smooth, "X" pattern in the centre of the wall, then go back to fill in the empty gaps and push on the roller to spread the paint evenly from thickly covered areas to more thinly covered areas. Allow to dry for at least 3 hours.
Wash out the roller and paint tray. Remove the lambswool roller from the frame and leave it to dry. Snap on a new roller cover.
Repeat Steps 2 through 4 using paint instead of primer. Wash all tools. Allow to dry overnight.
Textured walls cause paint to splatter during rolling more than smooth walls; be thorough when covering floors and furniture with dust sheets. A natural wool roller with a thick nap will absorb a lot of paint--use caution when applying the first paint to the wall after dipping it in the paint to avoid creating drips on the wall that may dry before you see them. The primer coat diminishes the roughness of the wall somewhat by filling in cracks and depressions, so a smaller quantity of paint may be necessary to coat the same area.