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Structural variables make additions tricky


The first question most people have when they're considering building an addition to an existing structure is: How will it work; how will it attach to the existing building?

The answer, like the old joke, is "very carefully."

The care becomes even more important when the new addition is being built on top of a previous addition. In that case, the problem becomes how to securely attach the new structure to the old one at the sides and at the floor/ceiling in a way that distributes the load, or weight, securely. Throw in a few variables -- no interior walls on the first floor, a wide span to bridge with rafters -- and problems can be complex.

Randy recently finished a "shell" second floor addition over an existing first-floor addition. The new addition is 21 feet by 26 feet, with the floor joists spanning the 21-foot dimension.

The homeowners wanted to leave the first-floor space open, with kitchen, dining room and living room flowing into each other.

Another complicating factor was that the first-floor's floor had been framed with 2-by-8 joists with a central beam. Over the years -- the first-floor addition was probably built in the '50s -- the central beam had sagged, and so had the floors. That meant the first-floor joists could not bear any more weight than they had (and that the house's owners will still have to jack up the center and solve the structural problem).

It also meant the new structure had to be designed to load the weight of the new addition on the front and back walls of the existing structure, so the weight was transferred down into the basement walls, which are supported by concrete footings. The resulting addition was somewhat more expensive than if the weight could have been loaded on interior walls, in turn loaded on the existing floor. But the homeowners are getting exactly what they wanted.

With the plan resolved, it was time to build. The longest regular joist lumber available in this area is 2-by-12-by-20-foot -- obviously not strong enough or long enough to span 21 feet. The solution was to buy manufactured plywood joists called microlams. They can be ordered in all different lengths, including 22 feet and longer (Randy has used a 34-foot microlam). Microlams are stronger than lumber, because they are laminated plywood. For this project, Randy bought 22-foot microlams and cut them to fit. They were expensive -- about $100 each -- but they can be cut and drilled and worked like ordinary wood. They are also manufactured straight -- without the "crown," or upward curve, that 2-by-12s have. The microlams were installed with a center row of 2-by-12 blocking bolted to the beams, and created a solid floor with no bounce. A subfloor of 3/4 -inch tongue and groove plywood nailed and glued to the microlams finished the floor.

As often happens when an addition is built over an existing structure, the original builders had not planned for the headers, or support studs, over windows to bear the weight of walls on top. In this case, the original addition was just one story with a roof, and the roof did not bring a lot of weight to bear on the headers. In one case, the window span was 10 feet, and the header was not designed for weight.

The solution was to tear out the drywall above and at the sides of the window and use two doubled-up pieces of an extra microlam to form a header. The jack studs -- which support the weight of the header on the floor -- were also doubled, again to transfer the weight to the basement walls and the concrete footings. Fortunately, the other windows had headers strong enough for their spans.

The fact is, with a little careful design work to distribute the load, even second-floor additions can go solidly into place. And it means homeowners who like their home and location can remake an existing structure to fit their dreams.

Mr. Johnson is a Baltimore construction manager. Ms. Menzie is a feature writer for The Sun.

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