You Can Build Your Own Add-On Greenhouse

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Be sure that the ground all the way across the area which will be covered by the floor is as level as you can make it, then dump or shovel in the gravel and smooth it out. The crushed stone is next topped with tar paper or a sheet of heavy builder's plastic (which will act as a vapor barrier), reinforcing bars and/ or mesh should be set, and the concrete poured, tamped, and screeded.

As the floor begins to cure, it must be troweled off smooth ... and after it has set up, the forms can be removed. If the weather is hot, the curing concrete should be covered with plastic or burlap and sprinkled with water from time to time to make it cure more slowly (which, it turn, will make the finished floor stronger).

You can purchase ready-mixed concrete for both the foundation and floor of the conservatory, or you can rent a powered mixer and mix your own (try one part cement, two parts sand, and two parts gravel with enough water to make the concrete easy to work but not overly "sloppy"). Work up each batch of mixer first, followed by the gravel and sand, and—finally—the cement. Mix the concrete thoroughly, then pour it and work it down quickly before it dries.

CONSTRUCTION: FRAMING

Redwood, cedar, cypress, and Douglas fir all resist rot and are all top choices for the wooden framing in your greenhouse. Use them if you can. And, if you can't, at least try for seasoned lumber that will grip nails tightly and which will not warp or twist. All the wood in the structure, of course, should be protected with any of the excellent heavy-duty preservatives or paints on the market today.

Although your local codes may have something to say about the matter, you'll generally find—for the kind of small add—on greenhouse we're talking about-that four or five 4 X 4-inch posts set up in the conservatory's "outside" wall are all the heavy vertical structural members you're going to need. Anchor them firmly to the lean—to's foundation by toenailing or bolting them to a 2 X 4 sill or bearing plate that is fastened to the concrete base with the "D" bolts mentioned earlier. It's a good idea to apply a layer of mastic between the footing and this sill to prevent capillary action from drawing water through the concrete and dampening (thereby rotting out) the bearing plate.

The horizontal beam running across the tops of the uprights in the accompanying drawing generally can be held to a 4 X 4 for four-foot spans, a 4 X 6 for spans of six feet, or—for eight—foot spans—a 4 X 8. Rafters, spaced either 18" or 24" on center, are usually heavy enough if made of 2 X 6's ... although—especially for flatter roofs—many folks like to use 2 X 8's for these members.

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