For pitched roof conservatories you need to position the ridge and hip rafters first. Lean-to models have a wallplate bolted to the house wall. Ridge beams and wallplates are supplied with holes pre-drilled to accept the connecting rafters. All the rafters are pre-cut to length, and all are labeled and cross-referenced with your individual roof plan supplied as part of the instructions.
Each rafter has a captive bolt at each end which drops into pre-drilled holes in the ridge and eaves beam (wallplate & eaves beam for lean-to styles). You just have to secure the bolt from inside. No cutting or drilling is required on site.