The Nelson family of Lakeville, Minnesota, already had a fine deck overlooking the lake behind their home. But when they decided to add this sunroom, they had to tear down the deck and rebuild from the ground up. Had the deck been constructed as discussed here, the homeowners could have saved time, effort and money and simply built atop the existing platform.
The Nelsons are not alone. Many of us fall into the same old trap. We begin dreaming of a beautiful deck so we can spend more time relaxing and entertaining outdoors. We design. We build. We relax. We entertain. We soak in the compliments. And we want more: an enclosed space with a roof to shade the midday sun, large crank-out windows with screens and double patio doors. Then we discover that the platform won't support the extra load, and the elevations are all wrong. So we must rebuild the deck before we can add the sunroom.
We asked Club members Jim Hollingsworth of Mount Vernon, Illinois, and John Durocher of Maple Grove, Minnesota, to use the Nelsons' project to demonstrate what a deck would need to support a sunroom - without skipping a beat. Jim is an architect, and John is a carpenter who specializes in room additions and decks. With a little guidance from Jim, John built this deck and sunroom almost single-handedly.
“Decks and sunrooms, whether enclosed or not, are built differently from the ground up,” says Jim. “If there is the slightest chance that you are going to want windows where there are screens or a roof where there is an open deck, you have to engineer the structure for the ultimate use.”
“With an open deck, the load is sparse, and it usually is distributed fairly evenly. The engineering is straightforward,” he continues. “But as soon as you have a roof and walls, you have to think of engineering it as you would any other wall in your house. And you can expect your inspector to be a lot more particular.”
An elevated deck that supports a porch or sunroom must be able to carry more dead load (permanent weight) and live load (people and movable objects). John used post-and-beam construction, including a structural ridge and eaves, for this project.
“I like the post-and-beam approach because the whole roof is supported by the two ridge posts and the four corner posts,” he explains. “That lets you do whatever you want with the walls.”
In this case, we show 4x4 posts for the two outside corner posts and stick-frame the walls with 2x6s set 24 in. OC for superior insulation around the windows.
Don’t hang out
In deck building, it is common to support a beam with two or more posts and let the joists overhang the beam by a foot or more. Sometimes the beams are just bolted to the sides of the posts. Do not cantilever joists or sandwich beams like this if your long-term plans call for a porch or a sunroom. The beam must bear on the tops of the posts, and you need to position the posts directly under the corners and the ridge.
It’s OK to hang the middle joists off a ledger that is bolted to the rim joist of the house as you would when building a typical deck, but the end joists that support the side walls cannot be hung. They must rest directly on posts at both ends.
John accomplished this by extending end joists into the back wall of the house and supporting them with structural posts that rest on the bottom plate of the walkout basement wall. He also doubled the end joists to carry the extra weight of the walls.
Porch and sunroom considerations do not end here. The height of the deck is important if you want to integrate a finished room with the rest of the house someday and avoid water damage.
In this case, John made the portion of the deck that was to be enclosed a step higher than the rest of the deck so rainwater and melting snow and ice would not seep in from the exposed deck platform. And because the homeowners wanted the sunroom floor to be level with the adjacent room, the joists were set to anticipate the 3/4-in. plywood subfloor that would be placed over the 2x6 decking that was installed at the all-deck phase.
If it is not possible to drop the joists low enough to accommodate both 2x6 decking and plywood, the decking could be removed at the porch phase.
This is a big sunroom — 16 ft. square. That’s quite a distance to span with dimensional lumber, even for an open deck. John was limited to using 2x10s because of the double doors lead¬ing to the basement directly below the sunroom. So he specified Douglas fir and laid the boards out 12 in. OC rather than the typical 16 in. OC. John also installed solid bridging between joists to stiffen the spans and prevent the floor from bouncing.
John made the most of the windows by using structural ridge and conventional rafter design. The cathedral ceiling let the tops of trapezoidal (trap) gable windows follow the roofline.
Jim approves. “It might have been easier to frame the roof with scissor trusses, but then the ceiling would have been at a different angle, and it would have made the gable look like the wings on a plane,” he observed.
Just as the trap windows follow the line of the ceiling, the lower windows follow the width of the traps. You might be able to run the casement windows against the corner posts, but you won’t be able to frame the trap glass that close. If you want your windows to line up, you have to size your casements and traps accordingly.
Truth is, John’s original plan called for two custom-made trap windows and four pairs of stock double-casement windows on the gable wall. But after he calculated the actual size and position of the trap glass, he switched to two pairs of triple casements.
If all this has you thinking you should consult an architect or an engineer before you build your deck, you have the right idea. Design centers at home centers and window stores often have computers with design software that can help you get the windows right, but you can’t expect them to do structural engineering. Making windows work together, lining up floors and creating a sturdy structure is no small matter. The point is, you have to think about all the details of all three phases before you drive the first nail or pour the first footing. But when you do, dreams can become reality.