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metal joist hangers. One common cause is shiners, as they are called—nonbedded nails that lay along side a joist and rub as the floor structure deflects. These must be removed before any lightweight or other concrete fill is poured.
Another cause is unevenness in the top surface of the joists, either due to imperfections in the wood or in the case of joist hangers, to differences in the joist level, which allows motion of the floor diaphragm against the nails. Gluing the plywood diaphragm to the joists prevents much of this panel motion and increases damping. Joists can also be shimmed at the hanger to assure even floor support. In tongue and groove flooring the individual planks can move relative to one another. Applying paraffin to the plank edges helps prevent this cause of squeak.
In some cases, subflooring, made of wood strands bonded together with a resin material, has been found to contribute to floor squeak. When these materials deflect, they rub against the nails that powder the binder and open up a small hole around the nail. This in turn loosens the grip of the nail on the board. This effect can be offset somewhat by gluing under the flooring and using a gripping ring shank nail. Ring shank nails are rec- ommended for nailing all wood diaphragms since they provide some additional grip on the plywood. To repair existing wood floors screws can be added to cinch down the flooring to the joists and reduce panel movement. Glue should be applied from below along the top edges on both sides of the joists.
Summary
The achievement of adequate isolation between dwelling units is becoming more routine with the recognition of the fac- tors influencing noise transmission and with the introduction of products on the market that provide vibrational separation between floor-ceiling components. While the use of each of the techniques cited above does not guarantee a perfect result, the author has used them to improve the isolation between spaces in multifamily dwellings and achieve results in line with the standards recommended in this article.
Where dwelling units are separated by design, good results can be achieved without complicated construction techniques. For example, in multifamily dwellings a town- house plan is preferred over stacked units to avoid common floor-ceilings. When multi-story units are necessary, a plan that stacks similar rooms, one above another, avoids incom- patible uses such as a bathroom located above a bedroom. Closets and other non-sensitive spaces can be located on party walls to provide additional shielding.
Modern buildings are constructed from lightweight mate- rials, usually wood or light gauge steel studs, and the sound transmitted between spaces can be relatively high. In the older masonry and concrete structures, the mass law of building acoustics insured that sound isolation would be very good. The exigencies of cost and time have pushed building con- struction towards lighter and cheaper materials, and hence to greater sound transmission. Given these very real constraints, it is incumbent upon architects and engineers to find ways of providing adequate sound isolation in residential structures using commonly available and affordable materials.
Finally, we should not be lulled into a sense of compla- cency by compliance with minimum code standards. These
Fig. 5. Spring-isolated ceiling systems.
to support hat channels, have recently become available. These give somewhat better floor isolation than resilient channels (STC 61 vs STC 53) and can support a double layer of drywall. They are installed on 24 inch (0.6 m) centers in one direction and at the joist spacing (typically 16 inches or 0.4 m) in the other. They provide the advantages of a resilient point—mount support along with ease of installation.
The most effective structural decoupling in wood floor- ceiling systems is a resiliently supported ceiling hung from spring hangers shown in Fig. 5 (STC 73). Note that the hang- ers are located high on the joist to preserve as much ceiling height as possible. Spring hangers are more effective than a ceiling supported on separate joists since with the latter sys- tem there is still the possibility of structural transmission through the joist supports. When a spring-hung ceiling is installed, unless the springs are precompressed, it will drop by the amount of the hanger isolator deflection. Hence, the ceiling drywall must not extend beyond the top of the wall drywall or else the walls will support its weight and the ceil- ing will bow. Once the ceiling has come to its final elevation the gap between the ceiling and wall material may be caulked. Molding or other trim pieces can then be added since they are non-bearing.
Spring precompression can minimize the actual deflec- tion; in practice, however, this is somewhat tricky since the final load must be determined carefully. Springs are located at 4 ft (1.2 m) on center and if they support 16 sq ft (1.5 sq m) of ceiling, at 5.5 lbs/sq ft (27 kg/sq m), they will each carry about 90 lbs (41 kg). A spring located along an edge will carry a little more than half that load and one in a corner somewhat more than one quarter. In irregularly shaped ceilings or one with coffers and light fixtures the loading is more complex. It is prudent to have springs of several different sizes at a job site in case the odd hanger is needed. When stepped blocking and a resiliently hung ceiling are used in combination, the black iron can run parallel to the joists just below the block- ing. The hat channels run perpendicular to the joists just below them. When the drywall is installed, its weight will pull the hat channel away from the joists so it does not touch.
Floors should be structurally decoupled laterally as well as vertically. Joists should not be run continuously across a party wall separation but should be supported on the nearest side of the party wall framing.
Floor squeak
Creaking floors are caused by the relative motion of wood on wood or nails rubbing against diaphragms, joists, or
Acoustics of Floors in Condominiums 39