OBSERVATIONS
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Most old buildings have cracks in their interior finish. These cracks have many causes, but those resulting from differential settlement of one part of the building against another represent a particular cause for concern. These cracks, especially if they are new or large, may indicate the need for structural repairs. Settlement cracks most characteristically occur in wall plaster (Similar cracks may occur in ceilings, but they are due to a slightly different cause). They are almost always diagonal, and they point up to the area that has settled the most. Although this at first may seem odd, a basic understanding of geometry and the physical properties of plaster will make clear why it is so: Plaster, like most masonry materials, is relatively strong in compression (pushing), but relatively weak in tension (pulling). It is also brittle, which means that unlike an elastic material such as rubber, it does not change dimensions perceptibly under stress before it breaks. When part of a building settles, the rectangular plaster wall tries to change its shape into a parallelogram. The distance between one pair of diagonally opposite corners tries to shorten as the other tries to grow longer. Since plaster is neither very elastic nor very strong in tension, it breaks, across the pull, producing a diagonal crack that rises to the point of maximum settlement.
Whether a settlement crack is a cause for concern requires more information, including evaluation of its location,* age, and size, and investigation of the conditions actually responsible for the settlement, such as a failing timber under the floor. To answer these questions, we evaluate the crack for age (Are the broken surfaces clean and bright or dirt-stained and dark? Are there signs of previous repairs?) and size (Is it a hairline, the diameter of a pencil lead, the diameter of a pencilÉ?), and look for conditions that may explain it; all to try to determine whether it represents an active process requiring intervention or merely the artifact of a process that has already gone to completion. *) Foundation settlement, especially new foundation settlement, is normally a cause for concern, and calls for professional evaluation. Back
to the top One of the many tools that preservationists constantly wish they had is an inexpensive, safe, non-destructive, easy-to-use, and effective way to look inside a building's outer envelope. X-ray imaging is a useful tool, and can show considerable detail, but it has the drawbacks of cost and the inability to image areas larger than a foot or two square, to say nothing of the safety hazards attendant on the use of ionizing radiation. Since different parts of a building envelope transmit heat at different rates, thermal (infra-red) imaging would appear to offer a way to see into a building's exterior walls and roof without the problems of x-rays. In its crudest form, we have all observed thermal imaging when heat from the attic melts snow from a roof in a pattern that reveals the layout of the rafters below. Since placing snow on the roof and waiting for it to melt is an awkward way to create thermal images (and useless for walls, which generally are the elements into which we most need to see), we need something better. Infra-red sensitive photographic film seems promising at first ; it responds to temperature differences, and so should make the image that we're after. And so it can. A client once photographed his house on a bitterly cold morning using infra-red film, and was rewarded by a remarkably clear image of the principal structural members within the walls. Alternatively, the equipment used for thermal-energy audits can, under favorable circumstances, image structural members hidden inside building walls. Another client (a church) actually made a video using this technique. It was not sharp (thermally-imaged video images are fuzzy), but it revealed the locations of the primary structural elements in sufficient detail to assist an understanding of some distressing conditions in the walls and windows. There are serious problems with infra-red imaging, though. Whether photographic film or videotape is the medium, the current state of the art makes infra-red imaging feasible only when there is a large temperature differential between inside and out--in other words, in coldest winter, when the interior is heated. During warm weather, or when the building is unheated, useful images can not be obtained. Still, when it works, infra-red imaging can expose the construction of an exterior wall quickly and in useful detail. As for the rest of the time, the quest goes onÉ Back
to the top An earlier issue of Observations (#10) contained a piece about the impossibility of expecting to get something good and fast and cheap. Not long afterward, I encountered another view on the matter: The author--a carpenter-contractor writing about his own experience--described three similar aspects of a working relationship: The job, the service, and the price. As with "good, fast, and cheap," it is possible to choose any two, but not all three: If you get a low price for a great finished product, you'll probably notice that the service could have been better. Similarly, if you find super service at a rock-bottom price, you might have to accept a less-than-wonderful result. And if you're looking for an excellent end product backed by responsive service, it will not come at the lowest price around. In my own practice, I find the third option to be the one that leaves me and my clients most satisfied, and it's the way I try to practice my profession. Back
to the top From time to time we get calls from anxious building owners concerned about whether their buildings are moving: Joints in the trim around windows and doors seem to be opening up. Often, upon further investigation, this situation turns out to be the result of the wood trim responding to seasonal fluctuations in humidity, which cause the wood to swell and shrink. Almost invariably when this is the case, the trim joints turn out to be mitered. The process has to do with the fact that wood expands and contracts significantly across its grain as humidity rises and falls. It doesn't, however, change its length. When trim joints are mitered, as humidity rises and the wood expands across its width, the outer corners of the joint move away from each other, creating a triangular gap that widens from the inside of the corner to the outside. Similarly, as the humidity drops and the wood contracts, after initially closing, a new triangular gap in the joint may appear, running now in the other direction.
This action is intrinsic to wood; it can not be wholly prevented. A good paint (or varnish) film, by retarding vapor transmission, however, will slow the process down and reduce its effects. The worst thing to do is to fill the open joints with a rigid joint filler such as plaster, a wooden wedge, epoxy, or (perish the thought!) Plastic Wood. As the wood tries to move, this hard joint filler will cause permanent deformation of the wood, and the problem will get steadily worse, with the cracks never closing. Occasionally opening trim joints do indicate structural movement, but in those cases the open joints are all oriented differently, and there are usually other indications of movement, such as settlement cracks in nearby plaster, cracking window panes, or doors that are increasingly difficult to open (or to latch). Open joints caused by expansion and contraction, on the other hand, point consistently toward (or away from) the center of the door or window.
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to the top A piece in The Boston Gobe on the merits of building versus buying a house contained the following gem: "[A] not so obvious but important advantage, noted by anyone who has actually gone through this process, is that building [or extending] a home provides a uniquely effective test of a marriage or any other close relationship. Building [or altering] a home is as close as we come in this society to an ancient endurance ritual--If you and your spouse or significant other are still talking to each other, and still interested in sharing the same space, when the project is complete, then you can face the future with a high level of confidence about your prospects of spending many more happy years together." (Substitute "business partners," "vestry," or "museum board" for "spouse or significant other," as appropriate.) Later on, the article's author repeated some important wisdom: Construction projects, whether they consist of restoration, conservation, extension, or wholly new work, tend to cost more, take longer, and be more frustrating than ought to be the case in a more perfect world. This observation applies to well thought-out work designed by able and careful architects and executed by the most competent contractors, just as much as to the other kind. The best defense that you, the owner, have against this situation is to budget realistically, allow extra time, constantly remind yourself that it, too, shall pass, and cultivate a sense of humor. A realistic budget is one that doesn't stress your ability to pay to the limit, but leaves some room to accommodate the inevitable surprises. Allowing extra time can mean that when a delay occurs, you won't be forced to seek emergency alternative quarters. Reminding yourself that the project is finite, no matter how it feels in the heat of the moment, and that it will come to an end, will give you badly-needed perspective. And a sense of humor? It's worth all the other precautions rolled together (but is no substitute for any of them!). |
