How To Build a Roof Over a Porch

House with porch

A roof structure over a porch, often referred to as a porch canopy, is a cantilevered roof system that ties into the existing building and roof, with supporting posts at the outer line of the canopy and a bearing system that supports the canopy roof rafters.

Begin by removing any existing roof overhang rafters or other that may impede the attachment of the canopy to the building. Remove these to flush with the exterior of the building and to the desired width of the canopy area.
Support posts at the outer edge of the canopy will be anchored into a concrete base, which may be either the porch surface or 12” diameter concrete footings.

Measure from the face of the building to the desired length of the canopy and make a perpendicular mark on the surface using a chalk line or wood stakes and a string line.

Deduct for the desired roof overhang on each side and mark for the (2) post locations with a marker or additional stakes. Check the configuration for square and consistent dimensions.
To anchor posts into an existing concrete surface, drill (2) 5/8” diameter holes to 3” deep into the concrete and install a 5/8” threaded rod with epoxy. The rod should extend above the surface of the concrete a minimum of 6”. Next, using 4” x 4” wood posts, drill a 5/8” diameter hole, 6” into the center of the post bottoms. Slide the posts over the threaded rods and brace it plumb.
To anchor posts on new concrete footings, dig a 12” diameter footing, 16” deep at the staked locations and install post concrete with a Simpson post base anchor. When the concrete has cured, stand a 4” x 4” wood post in the anchor, mark for the bolt holes and drill the post. Install the post in the Simpson anchor and bolt it in a plumb position. Brace the posts.

The header system will be constructed of 4” x 6” structural Doug fir, or larger depending on the span. (4” x 6” headers will span 8’ as roof supports)

Install 4” beam hangers on the face of the building so that the top of the beam will flush with the top of the building wall. Mark a level line from the bottom of the hangers to the posts and cut the posts at this height. Install the 4” Simpson post caps atop each post, drill the holes and bolt them to the posts. Install the 4” headers at each side, extending from the building and resting in the post caps. Bolt the headers in place and cut the protruding ends to allow for the desired overhang at the front face of the porch.
Next, install Simpson 4” beam hangers to carry the front header between the posts. Cut the front header, place it and secure it to the hangers. The roof ridge rafter can now be installed.

Mark the center of the front header and install a short 4” x 4”post on end. Using a rafter guidebook calculate the height of the ridge and mark the post, deducting for the width of the ridge board. From this mark, level back to the existing roof and mark that intersection. Measure from that point to the face of the porch and cut the ridge board, allowing additional for the desired overhang in the front. Install the ridge board and brace it in place.
For the rafters, begin my stacking 2” x 6” Doug fir rafters on edge, spanning (2) sawhorses. Measure from one end and mark a perpendicular line across the tops of the rafters to attain the desired overhang. From this mark and using a rafter guide book, determine the length from the mark, or seat cut, to the ridge board in accordance with the desired roof pitch. Mark the rafters and cut the plumb cuts at the long end of the rafters. Make layout marks per the rafter span on the ridge board. Attach the rafters using #16 box nails at the ridge and at the connection points on the headers.
Once the rafters are installed over the porch area, cut the fill rafters to complete the section over the building. The porch roof framing system is now in place. Install the ½” roof sheathing with #6 galvanized nails and trim the edges flush with the rafter tails. The roofing may now be applied.


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