Yes, you can add a solid roof to a pergola. A solid roof will protect any existing plants from the elements, and provide extra shade on hot days. To install a solid roof onto your pergola, firstly measure the size of the pergola frame. You then need to purchase materials that match those measurements – such as sheets of corrugated iron or polycarbonate panels for example. You'll also need screws, brackets and sealant to secure everything in place. The next step is to attach rafters to create an even framework for attaching the material chosen for your roofing structure - this could be wooden beams or metal struts with cross bracing depending on what type of material you have chosen. Once these are securely attached, it's time to fit your chosen paneling which should then be sealed against moisture ingress using silicone sealant at all corners and edges where necessary before finally securing them with brackets bolted into place along each side edge beam of the pergola frame itself. With everything secured in place you can now enjoy more comfort under your newly installed solid roofed-pergola!