Timber-frame Construction:The Agents of Timber Decay and the Solutions for Timber Repair
Erika Marks
Sunday, April 15, 2007
Introduction
During the first semester in the Master of Preservation Studies program, I focused my primary paper on the subject of timber-frame construction by examining the origins of vernacular building techniques used in several parts of the United States—specifically, the Northeast and the Southern region of New Orleans and its outlying parishes.
Having established how and why certain timber-frame structures are built, I will now continue my investigation of wood construction by addressing the reasons for why these same structures deteriorate and decay, and what methods can be used to not only repair them, but to provide additional strength to aid in their continued preservation.
This presentation will consist of two parts: The first part will provide a brief overview of the basic components of wood frame construction before addressing the key reasons for the ultimate compromise and decay of their structural integrity, using a variety of vernacular examples. These examples will offer several strong examples of structural disintegration that will be relevant to this exploration and which will lend themselves to a proposal for their remedy.
The second part of this discussion will focus on the possible treatments for the restoration and preservation of weakened wooden structures, addressing the concerns for both reversible additions, as well as more permanent material alterations.
Part One: The Construction of Wood-frame Buildings and the Reasons for Their Eventual Decay
The technique of building a structure out of wooden parts is one of the oldest construction methods known to man. As a building material, wood has many benefits. It is easier to conform than, say, stone; wood can be joined without the aid of additional materials; and, conveniently enough, for much of mankind’s building history (before the planet was depleted of much of its forests), wood was a readily available material.
But, of course, wood is not without its detriments. As a moving material that it is in a constant state of readjustment to the conditions of its environment (expanding in moister, more humid seasons, then constricting itself in dryer, colder temperatures), such seasonal reconfigurations can ultimately threaten its strength. For once wooden members are unified in an interlocking structure by a series of vital joints, all subsequent movement of the material can potentially offer an opportunity for a variety of deteriorating agents to compromise its structural integrity.
The Main Systems of Wood-frame Construction
Until the late 19th century, wooden buildings were erected using a timber-frame system. But with advancement of the industrial revolution and the ability to mass produce many building components, specifically nails, a new type of wood-building construction was introduced: balloon framing. Where once craftsmen were required to assemble members with intricate wooden joints, now the relatively unskilled laborer could erect a home with full-length studs jointed by nails. Today, much of the construction world works with yet another evolution in building: platform framing. Unlike balloon framing, which consists of the installation of floor to ceiling studs, platform framing requires a floor-by-floor erection, each floor being capped off as it is completed.
Of the three types of wood frame construction, the preservationist is most often concerned with the two earliest types, timber-frame and balloon frame. This discussion will address examples of these two types, but it of interest to know how modern construction has developed to incorporate a third technique.
Wooden Joints Can Invite Proponents of Decay
The first example of a deteriorating agent occurs during the seasonal movement within the joined members of a wooden structure. Even in the case of a rudimentary mortise-and-tenon joint, which is a joint consisting of a hollow at one end of a member accepting the tenon of the other member (see figure 2), constriction of the material can cause once-snug joints to reduce in size and that reduction will often leave gaps in the accompanying mortise—gaps that invite a variety of dangers.
The first threat is that of pooled water. Gaps in the mortise are targets for moisture like rainwater, and the influx of moisture will leave the exposed wood wet and inviting to rot and decay-inducing fungi. This situation will invariably invite a second threat: that of infestation. Bugs and beetles, drawn to the now warm, moist environs of the mortise hole will infiltrate it and lay eggs.
These factors may seem minute in relation to the scale of a large structure, but in the case of wood-frame construction, particularly that of timber-frame buildings, a building is only as strong as the joints that connect its members. And once the strength of even a single key joint is compromised by decay or infestation, the overall structure is jeopardized.
The Danger of Alterations in Structural Systems
Environmental factors are not the only causes of deterioration in a wooden building’s structural integrity. Sometimes the advances of technology can result in some equally compromising adaptations to a structure.
Very often new technology will be added to older buildings, for example, plumbing or electrical wiring. However, one must keep in mind that when updating older building to accommodate newer technology, the original structural members were not designed for such additions.
To illustrate this point, let us examine the structure at 7820 St. Charles Avenue. Now a rental property housing several units, it is evident that the building has grown over the years since its initial erection. As is common in the modification of a building for rental units, the addition of supplemental plumbing and electrical services have been installed in a haphazard fashion, damaging both the structural integrity and the envelope of the building itself.
The first example of this can be found on the exterior (see figure 3). In order to accommodate additional plumbing needs, pipes have been installed through a door. Regardless of the esthetic offense of this modification, the main concern lies in the effect this change will have on the building envelope. The penetrations made for the pipes are rough and lack any kind of flange sealant to keep moisture and infestation out of the gaps. Additionally, the pipes, which will fluctuate in temperature over the course of each day, may impact the wood of the door, causing it to have to absorb additional heat and moisture.
Figure 4 offers a similar concern. This photograph, taken of an air conditioning unit to the left of the aforementioned door, displays another detriment to the overall structure. Air conditioning units expel a great amount of moisture when operated. Often times, this moisture has no where to drain and simply runs down the outer wall beneath it, as is obviously the case in this example. The discoloration is only a cosmetic issue, but the real concern lies beneath the paint. The wood siding cannot absorb the extent of the moisture that the unit is expelling. That concern is compounded by the demands of the cantilevered weight on a windowsill that was not likely installed to withstand such weight. And finally, a third contributor to any ultimate structural deterioration is that the overhang of the unit and the subsequent shade produced by it prohibits the constantly-soaked wood from having any sun exposure which might add in its absorption of the run-off. Ultimately, between the weight of the unit and the deterioration of the wood fibers from rot, the structure will begin to fail, unable to support its own needs.
Another example of structural compromise can be found inside the building (see figure 5). When a leak caused the ceiling plaster of this second floor unit to fall, its hidden members could be observed, and the evolutionary tale began to be revealed. It is obvious that the portion of the joist was removed to accommodate a new element, such as the pipe above it. While it is likely that the bath tub above the pipe and poor drainage resulting in moisture absorption and subsequent rot caused the plaster and lathe to deteriorate and fall, it is without question an unfortunate sight to see a key supporting joist altered, when that alteration can have a negative impact on its ability to maintain its strength. Too often eager plumbers, electricians and do-it-yourself homeowners do not account for structural integrity before they begin hacking away at obtrusive structural members in order to add piping or wiring to update their buildings. It is never a good idea to impair a key structural member. But this practice can have potentially devastating effects. Holes cut out of key support members only serve to weaken to strength of the wood by taking away even a modicum of the vital mass that is required to sustain the weight being thrust down upon it. Not only does a void in the material diminish its strength immediately, but over time, that void can be impacted by rot and infestation, just as was explained earlier in the discussion of joint movement.
Part Two: Solutions for Repair and Continued Maintenance of Wood-frame Buildings
In preservation technology, there is always more at stake than the mere resolution of a material’s repair. In accordance with many established treatises, repair must not, ideally, impair the material in any irreversible way. However, very often synthetic materials may provide the strongest repair to an endangered timber which is why it is important to establish the variety of repair/restoration/preservation technologies that are available before assessing the implications of their use.
Dutchman joints (Using like materials)
In a perfect world, a compromised timber would be satisfied by repair with natural materials, such as a Dutchman joint wherein a new piece of wood of the same species, and ideally age, replaces the weakened portion. This concept works much in the same way that a dentist first extracts the portion of the spoiled tooth and fills it with synthetic material to reinstate its strength.
There is no question that this type of repair may be the ideal one in the eyes of Preservation Technology, for it does not require the use of synthetic materials, and is therefore considered a reversible procedure. However, regardless of its ethical benefits, is it a strong repair? If done correctly, yes. Due to the fluctuating nature of wood fibers, and the varying absorption properties and ring strengths of different wood species, the matched material must be compatible. Much like an organ transplanted into a man’s body, this new organ, be it a heart or a lung or a kidney, must compliment the recipient’s existing system or else it will be rejected. A similar case can be made of a Dutchman repair.
Say, for example, that an oak timber required a Dutchman joint to replace a damaged portion of its fibers and the restorer decided to use a piece of pine to perform the repair. The result would be ineffective at best, and ultimately fatal at worst. Oak is a hardwood with a moisture content of, approximately, 80% versus pine which can have a moisture content of 30 to 40% (Hoadly 115). This discrepancy means that the two species will react to moisture differently, and thus, their shape will change differently seasonally. Therefore, the spliced piece might inadvertently weaken the member instead of strengthening it. Ideally, the new material should be of the same species and from a similar cut of the original tree. Additionally, the grain orientation of the new piece must be consistent with the existing member.
On the subject of a timber’s age and that of its replacement material, in his book Conserving Buildings, Martin E. Weaver gives the reader a comprehensive listing of wood species and their common structural uses, as well as their durability ratings which he categorizes in three ways: D for durable, M for moderately durable, and N for normally nondurable. Additionally, he cautions that:
…when these historic uses occurred and when the durability ratings were assessed, the timbers in question were first growth, mature specimens and their durability may far exceed that of many timbers available today (Weaver 15).
In other words, similar species and grain orientation are not the sole considerations when replacing like materials in timbers. A timber’s age clearly will have great bearing on its ultimate strength. Therefore, as Weaver indicates, it is often almost impossible to duplicate the strength of historic timbers with modern timbers, since we of the contemporary world, are not as fortunate to readily find such aged lumber as early builders had at their disposal. This quandary might be a good argument for use of epoxies in lieu of natural materials, since epoxies can often offer a strength that natural materials cannot.
Impregnation With Synthetic Resins
Having discussed repair with natural materials, one must also investigate the benefits of synthetic repairs. Modern technology is always producing newer, stronger epoxies so to debate contemporary epoxy brands is a seemingly futile exercise, however their applications can be discussed.
WER Method (Wood Epoxy Reinforcement)
The WER method was created by Paul Stumes, the restoration engineer of Parks Canada. This process involves the removal of the compromised material within a timber and replacement of it with a suitable epoxy (see Figure 6). Based on an earlier discussion in this paper, it would seem that this solution would be ideal if decay had compromised a section of a timber and the impaired material could simply be removed.
But what if the decay has evolved at the ends of a timber and there isn’t sufficient intact material existent from which to simply splice in new material? In that case, the BETA method might be a possible solution.
Consolidation using resins and steel or plastic rods (BETA systems)
Similar to the WER method, the BETA system of restoration of decayed timber requires that reinforced plastic rods be inserted into pre-drilled holes in the member. A form is then constructed around the timber and an epoxy mortar poured into the holes.
In the example of figure 7, a timber’s end has decayed and requires immediate repair. Once the timber has been shored with supports, the damaged material is removed. Since this timber was mortised into stonework, repair will also ultimately require resetting of surrounding stonework. But that can’t happen until the timber end is replaced. Doing so first requires that glass fiber reinforced plastic rods be inserted into pre-drilled holes into the remaining timber and then forms be constructed around them. An epoxy mortar is then poured into the forms. Once the mortar sets, the forms are removed and the stonework surrounding the new material is replaced.
According to the Adobe Architecture Conservation Handbook that is produced by Cornerstones Community Partnerships of Santa Fe, New Mexico, a similar restoration technique is employed when splicing new tails on the rotted end of an exposed viga beam on an adobe structure. The deteriorated end is removed, leaving a flat, smooth surface. A threaded fiberglass rod is inserted into the existing beam and its surround is filled with an epoxy resin that will harden and create a threaded shaft when the rod is removed. This threaded shaft will accept the new viga tail and the surrounding plaster will be replaced, completing the restoration.
Assessing the Criteria of the Repair
Before any type of repair is initiated to a compromised timber, it is imperative to answer several key questions that will help to determine the requirements of both the repair and the further maintenance of the building.
Question 1: Must this treatment of repair be reversible?
Very often a preservationist is confronted with the dilemma of priorities. In other words, is it more important to maintain a consistency and authenticity in materials than it is to promote future strength? The question of wood-frame repair provides a perfect example of this dilemma, because very often synthetic epoxies which can be used as fillers in members with deteriorated material can provide a stronger component than the replacement of those damaged fibers with the identical material, ie. a Dutchman joint of the same wood. However, if the goal of the repair is to maintain a reversible condition, then obviously the use of epoxy, although possibly the more durable method, would not be appropriate.
Within the world of preservationists, there are indeed treatises that dictate the criteria for such a condition. For example, Paul Stumes, inventor of the WER method, asserts the use of epoxy replacement of deteriorated material under the following conditions:
If the timber is painted
- If some historic event or person is intimately associated with the timber
- If the timber is significant because of its age or because it is an original part of a structure
- If it represents an important part in the development or history of the structure
- If the replacement of the element would unduly disturb the surrounding fabric
- If the replacement would be too expensive or impractical although otherwise justifiable
However, guidelines also accompany this method that discourage its use if:
- it interferes beyond reasonable limits with the visual integrity of the element or the resource
- it defaces important artistic works
- it destroys a large segment of the original fabric and hence significantly compromises the authenticity of the resource
- it does not meet a safety standard or code requirement
- the installation will adversely affect surrounding structures
Question 2: Is this member of key structural importance to the whole building, cosmetic or simply periphery?
Assessment of the affected timber’s role in the structural puzzle of the building is crucial in determining the best solution for repair. Such an assessment will help to determine which material would be best used for the repair and what measures must be taken in the interim to stabilize the member if it is indeed found to be of crucial support in the building’s construction.
Likewise, one must also consider if the timber is exposed or hidden. Obviously, an exposed timber, like a viga, or log beam, in an adobe house would be better served by a repair using natural like materials to preserve its visual authenticity, while a rotted interior stud, hidden behind a wall of plaster, would allow for a partial or total synthetic replacement.
Question 3: Is this structural member still viable in part or in whole?
The choice of repair technique will also depend on the condition of the existing timber. Ideally, a damaged timber may only be suffering at its joinery, or at a single localized area that can be repaired with either a natural or a synthetic material. But very often a single timber may be too far gone for such a solution. For example, if more than 75% of the existing timber is impaired, it would not make sense to splice a new material into so much damaged fabric, when the ultimate strength of the timber has been irreparably compromised. A possible solution in this case might be simply to remove the entire timber and replace it with a concrete beam, possibly boxed out in a wood veneer, or to sister the damaged beam with new timbers or steel reinforcements. Again, the decision will depend on what the preservationist deems of the greatest priority, be it to preserve the remaining material or forsake it to return the greatest structural integrity to the member.
Question 4: Will this repair require the use of synthetic materials?
Once the degree of damage to the timber has been assessed, the next step must be to decide which solution will best suit the task. As was discussed earlier, the use of synthetic materials are often unallowable in preservation work. However, if the project will allow for synthetics, then the preservation technician might consider all of his or her options, including which materials to use in the repair. Very often, epoxies are stronger than the woods they are injected into, but that additional strength may not be the ideal solution in the long-term. A fair analogy, and one that is prevalent in New Orleans restoration work, is that of Portland cement and soft brick. Historically, in tomb repair, when the brick substrate of the tombs were exposed and their lime mortar deteriorated under the elements, Portland cement was used to re-point the missing mortar. It was, however, a poor marriage, since Portland cement was much harder than the soft brick it attempted to bond, and eventually the bricks separated from the new mortar, causing further deterioration. The same warning could apply to timber restoration and repair if hard epoxies are introduced into softer fibers and these epoxies do not, like the Portland cement, move in tandem with the surrounding wood.
Conclusion: It’s Fixed, Now What?—Technology to Ensure Timber Maintenance
Timber maintenance need not require an army of technicians. There are several basic components that must be tended to on a regular basis to ensure that wood members remain sound.
A Secure Building Envelope is the First Step
As was discussed earlier in this paper moisture can be a great hazard to timber. While wood is indeed a movable material that is constantly absorbing and releasing moisture, that characteristic should not be misinterpreted. Excessive moisture can cause great harm to a timber, no matter its absorption properties, inviting rot and bug infestation. Therefore, the first line of defense must be to keep timbers dry, and a sound building envelope is the key. The building envelope is a reference to the foundation, walls, windows, doors, and, most importantly, roof that will ensure the interior of the structure is kept safe and dry from the exterior elements. Devices such as moisture meters can help. Through the use of a prong inserted into the wood, or a sensor read simply by being placed atop the timber, moisture meters can read the moisture content of a wood member and determine whether the member is retaining a healthy amount of moisture.
Protection Against Infestation
As in epoxy technology, there are always advances being made on the war with bugs. From sprays to injectable rods, pesticides come in a variety of forms to combat the decaying effects of bug infestation. Again, the best defense is a consistently critical eye to make certain that uninfected timbers remain that way, by keeping them dry and, if desired, injected with pesticide treatments.
Keeping an Eye on the Foundation
New Orleans, like many cities in the world, was built on a swamp, and therefore its buildings are constantly struggling to maintain their structural integrity while the earth beneath them shifts, often dramatically, under the distress of so much unstable soil. Such moist soil can clearly impact the long-term soundness of a wood-frame structure for as its foundation shifts, its timbers and joints will have to shift as well to accommodate the ground’s movement, or settling. But this fact is not as dooming as it might seem for the benefit of differential settlement in the ground is that it is an evident alteration and once a building settles over time, cracks will develop to signal its movement, and once those cracks are evident, then reparations can be made to compensate.
Thus, in conclusion, the first line of defense to combating the environmental deterioration of timber-frame buildings must be a consistent analysis of the structure, a constant awareness, for deterioration is inevitable and the task should not be to fight this natural evolution towards decay, but rather to inhibit it to the best of our abilities. Much like the trend of facelifts and Bo-tox injections in humans, we may silently acknowledge the ultimate futility of our efforts to stave off aging, but we certainly do our best to try and exhibit control of it all the same.
