How to Build a Porch Roof on Top of Vinyl Siding

What You'll Need
Half Inch Plywood
Metal Flashing
Roofing Sealant
Roofing Underlayment (Tar Paper)
Galvanized Roofing Nails
8 and 10 Penny Nails
Lag Bolts
4-inch by 8-Inch and 2-inch by 4-inch Boards
Support Beams

Building a porch roof is a complex and ambitious project that should not be taken lightly. Make sure to follow the steps below and consult a building inspector if there are any questions about permits or porch roof designs that are acceptable in the area.

Step 1 – Plan Ahead

Adding a new roof to a porch will require quite a bit of preplanning. Be aware of power lines, tree branches and satellite dishes that may need to be dealt with before beginning work on a porch roof. Most importantly though, remember to plan for the new roof to match the pitch and shape of the roof that covers the rest of the house. A building permit, as well as approval from a building inspector, may be needed before beginning the project.

Step 2 – Check to See If the Porch Is Level

Place a level on the floor in multiple places to determine if the porch is level. A level porch is ideal since it will be the most stable base for the roof that is being built and will allow the all of the supports to be cut to the same size. If the porch is not level, it may be necessary to go beneath the porch and build it up in places to level it out. This can be accomplished by raising part of the porch with a floor jack and filling in the gaps below it with concrete.  It is also possible, if the floor is just barely uneven, to cut the supports for the roof to different sizes so that the new roof will be level. If this is the case, one must be sure that the porch is not continuing to slant further as this may cause problems in the future.

Step 3 – Place the Support Beams

First, choose whether to use square or round support beams. The support beams for a porch roof, should be place directly above the support beams of the porch itself. In other words, the support beams for the new roof will be extensions of the already existing support beams. Toenail the posts into position and then brace them with 2-inch by 4- inch boards. Finish connecting the support beams by installing a post connector that is the proper size for the wood that is being used. Finally, use a level on two adjacent sides of each beam to ensure the support is plumb. Plumb supports take the most advantage of gravity to transfer loads to the ground.

Step 4 – Install the Top Plate

The top plate is basically a board that is placed along the top of the support beams to link them together and create a stable box shape onto which the rafters can be secured. Use sanded 4-inch by 8-inch boards as the top plate and join them in the middle of each support beam with lag bolts.

Step 5 – Install the Rafters

Along the top plate, mark a place for each rafter about 24-inches apart. Remember that the rafters used should overhang about 12 to 14-inches off the front of the porch. Once marked, the rafters can be temporarily installed by toenailing them into position with 10 penny nails. When all the rafters are in position and the structure looks both symmetrical and stable, permanently install the rafters by pounding 10-inch spikes through them and into the top plate.

Step 6 – Cover the Roof

Cover the entire area of the roof (including the overhang) with plywood that is half of an inch thick. Once done, secure the panels with 5-inch panel screws. With the entire roof covered by plywood, cap the overhanging areas with 2-inch by 4-inch boards that are trimmed so that they flush with the top, bottom and sides of the roof. The boards must be flush so that no snow or water collects on them throughout the year.

Step 7 – Connect the House and the Porch Roof with Flashing

Join the area between the porch roof and the vinyl siding with metal flashing. First use roofing nails to secure the flashing to both surfaces, bending it in the middle if needed. Then, coat the areas where the flashing meets the siding and where it meets the new roof with roof sealant. To complete the weather-proofing needed for the roof, roll out a roof underlayment, or tar paper, across the roof. Be careful to ensure the underlayment is lying completely flat so that no air bubbles exist between the weather-proofing and the plywood. Staple the underlayment down to secure it.

Step 8 – Shingle the New Roof

Beginning at the overhang, make sure to use a chalk-line to ensure the first row of shingles is perfectly straight. Once in position, use four galvanized roofing nails in each shingle to secure it. From there, overlap the next row of shingles 2-inches over the first row and stagger the seams. Repeat this process all the way back to where the house meets the new porch.