A three-season room can be made from an existing roofed porch so you don’t have to start from scratch by building on an expensive new room. For comfortable dining and relaxing through the summer, spring and fall, there is no need to install heating, plumbing, or insulating that you would need for a permanent living area.
Using pre-fabricated doors and panels eliminates the need for expensive labor, carpentry, and milling. You must get a local building permit before you start which involves developing a plan and getting it approved. There will be fees involved with this process. Building without a permit may invalidate your home insurance and is also illegal.
Step 1 - Build a Floor
Build a solid base for your porch, if you do not already have one. Decks can be used, or you can pour a new slab. If you have a solid existing floor, you may need to add additional supports to hold the new weight of the walls and fittings. If you have an existing concrete slab, make sure water cannot flow into the room during storms. There should be a one to three-inch slope to allow water drainage.
Step 2 - Lay Perimeter Boards
Replace any rotted or unsuitable posts. New 4 x 4-inch posts are suitable with metal top plates. Lay the 2 by 4-inch boards around the perimeter, except for where the door will go. Use a square to make sure they are square. Attach them to the concrete with concrete anchors and hardware.
Step 3 - Construct the Walls
Construct the side walls. Anchor them to the outside wall. Using screening is the cheapest option. If you are not using pre-made screening panels, fasten the screens in place, then cover the edges with batten boards. Cut the wire screening to the correct size for the openings, making sure they overlap by one inch. Staple the top edge. Pull the screening down and fasten the bottom edge with staples, working from the center out.
Making sure there are no wrinkles, staple the sides, beginning in the middle and work alternatively from one side to the other. Nail the battens over the screen edges.
Step 4 - Hang the Door(s)
Make the frame for the door. Hang the doors. Install splashboards along the bottom of the boards to repel rain and mud from coming in. If you are hanging screen doors, use springs or pneumatic closures so the doors spring back and prevent the entry of insects.
Step 5 - Replace Roofing
Replace some of the roofing with clear roofing materials if more light is required.
Step 6 - Finish the Details
Paint all wood surfaces. Install fans and lighting. Make sure your fittings are approved for outdoor use.
If you are using a pre-made window for the walls, you will need to make extra framing to support them in the walls.