Build Strong and Stylish PorchesDesigning the structure to complement a home

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If the porch is to be screened, I do a couple of things. I stretch screening across the top of the joists before installing the decking, to keep insects from entering through the joints (Figure 4). Locating the screen on top of the joists rather than underneath performs better and is less visible. Also, when there's an adjoining deck, I bump up the floor of the screened porch 1 1/2 to 2 inches to keep rain water from running off the deck into the porch. As long as there's a door to alert occupants to the floor elevation change, it's not a trip hazard (Figure 5).

Figure 4. Screening under the decking helps prevent bug invasions from below.

Figure 5. Porch flooring raised slightly above an abutting deck helps keep out water. This detail should only be done where traffic enters through a door, so that occupants are clued in to the elevation change.

Support Walls
On walled porches, I always use pressure-treated material for the bottom plate. As with a deck, porches more than 30 inches above the ground (even those with screens) require a railing (Figure 6). Solid kneewalls can satisfy that requirement and at the same time provide privacy and a place to install electrical outlets (Figure 7). To keep the walls dry and avoid rot, use water-resistant materials such as fiber-cement siding or Azek, especially for the kneewall cap. Lap siding that matches the home's exterior often works well on both sides of the kneewall.

Figure 6. Elevated porches -- even when screened -- require guards to prevent falls.


Figure 7. Kneewalls provide guards, privacy, and a place for electrical outlets.

My preference for wall posts is cedar 4x4 or 6x6. Although more expensive than treated lumber, cedar has a finer grain that looks better painted. I lay out 4x4 posts about 36 inches on center, which works well with my favorite roof construction -- timber rafters stacked directly above the posts and capped with 2x6 tongue-and-groove roof decking (Figure 8). With 6x6 posts, screened openings can be larger, but the downside is that wind loading may in time cause the screen to sag.

Figure 8. Timber rafters that stack atop the posts provide a clear load path. Tongue-and-groove 2-bys above provide both a finished ceiling and a roof deck

To provide bearing for the rafters, I double 2x8s or 2x10s and build a continuous beam that runs along all three walls. Continuing the beam across the front of the gable helps brace the ridge support column, keeping the gable wall from shaking. If the walls will be 9 feet or higher, I offer the option of transom inserts (Figure 9). They add a design element, and the additional horizontal members stiffen the wall posts.

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Figure 9. Transoms placed low must be made of tempered glass. They provide a clear view and aren't prone to tears and dings, as screens in the same spot would be.

I take a different approach with open-air porches. Because there are usually fewer posts than on a screened porch, I use 6x6 posts for a more substantial structure. The posts should be located where the side wall beams meet the house, at the two outer corners, and mid-span as needed. I often incorporate column and beam wraps to achieve a more custom look (Figure 10). Using fewer posts may require the beam they support to be upsized, depending on roof loads. Tables in the IRC are useful for sizing beams, but when in doubt, have the design engineered.

Figure 10. Open porches have fewer posts than screened porches. To maintain the look of adequate support, the author wraps such posts with trim to increase their width.

Cathedral Ceiling
Gable roof framing must be designed to keep the ridge from sagging and the rafters from pushing out the side walls. Accomplishing either of those ends automatically takes care of the other, and there are several ways to go about it.

Traditionally, ceiling joists within the lower third of the rafter span tie the bottoms of the rafters together in a stable triangle. Though many people think that's the purpose of collar ties (the horizontal members found between rafters near the top of the roof), it's not. Collar ties help keep the top of the two sides of the roof together under wind loads and do little or nothing to prevent the bottom of the rafters from spreading. Trusses are a more modern version of this framing. Either method is fine for houses, where the busy framing is concealed in an attic.

On a porch, though, I prefer an open ceiling. Vaulted ceilings make even small porches seem bigger. So instead of focusing on the rafter bottoms, I use a beefy ridge beam to span between the gable ends and support the tops of the rafters (Figure 11). If the tops of the rafters don't sag, the bottoms won't push out the walls.

Figure 11. Cathedral ceilings rely on stout ridge beams, rather than a rafter-joist triangle, to prevent sagging.

The key is to adequately size the beam. I'm often able to use a 4x12 ridge beam, depending on the pitch of the roof and the distance out from the house. The span may call for an LVL or glulam, which can be wrapped to blend with the rest of the ceiling. Most beam suppliers will handle the engineering for you.

I prefer not to extend porches more than 16 feet from a house -- that's about the maximum distance a reasonably sized ridge beam can span without either a post in the center or intermediate ties between the bottoms of the rafters.

Each end of the ridge beam must be well supported. A post at the outer end is part of a continuous load path to a footing. At the house end, you'll need to cut a pocket in the siding to provide access for running vertical framing members down to the foundation. If there's a window or door below this post, verify that its header can take the additional load. And be careful when sliding the beam into the pocket or you'll find yourself repairing drywall inside the house. Depending on the other loads the wall carries, it may be possible to bring the ridge support post down onto a header that sits above the wall and transfers the loads down through several wall studs.

When the ridge beam intersects the house roof rather than a wall, I extend a post up from the wall plate to support the ridge beam. Usually, the ridge beam can then cantilever back to the existing roof. In any case, go inside the attic to verify what framing is there to tie the ridge to.