Historic Exteriors: Preserving wood
Preservation experts from the National Park Service recommend the following steps in restoring any historic structure. First, identify the historically significant features. Second, protect the features that are historically significant. Third, repair damage to historically important features. Fourth, replace what cannot be repaired. Fifth, recreate missing features that were historically significant. And lastly, avoid new additions that alter the historical appearance inside, or outside. These steps are based on guidelines developed by the Department of the Interior.
Wood: an introductionBecause it can be easily shaped by sawing, planing, carving, and gouging, wood is used for architectural features such as clapboard, cornices, brackets, entablatures, shutters, columns and balustrades.
These wooden features, both functional and decorative, may be important in defining the historic character of the building and thus their retention, protection, and repair are important in rehabilitation projects. Wood has played a central role in American building during every period and in every style.
Whether as structural membering, exterior cladding, roofing, interior finishes, or decorative features, wood is frequently an essential component of historic and older buildings.Step 1: Identify, retain, preserve
Identify, retain, and preserve wood features that are important in defining the overall historic character of the building such as siding, cornices, brackets, window architraves, and doorway pediments; and their paints, finishes, and colors. The wood features on this porch were repaired and preserved during rehabilitation.
Removing or radically changing wood features which are important indefining the overall historic character of the building so that, as a result, the character is diminished.
- Removing a major portion of the historic wood from a facade instead of repairing or replacing only the deteriorated wood, then reconstructing the facade with new material in order to achieve a uniform or "improved" appearance.
- Radically changing the type of finish or its color or accent scheme so that the historic character of the exterior is diminished.
- Stripping historically painted surfaces to bare wood, then applying clear finishes or stains in order to create a "natural look." In the picture above, the wood features have been inappropriately stripped of traditional painted finish.
- Stripping paint or varnish to bare wood rather than repairing or reapplying a special finish, i.e., a grain finish to an exterior wood feature such as a front door.
Step 2: Protect and maintain historically significant features
Protect and maintain wood features by providing proper drainage so that water is not allowed to stand on flat, horizontal surfaces or accumulate in decorative features.
Apply chemical preservatives to wood features such as beam ends or outriggers that are exposed to decay hazards and are traditionally unpainted.
Retain coatings such as paint that help protect the wood from moisture and ultraviolet light. Paint removal should be considered only where there is paint surface deterioration and as part of an overall maintenance program which involves repainting or applying other appropriate protective coatings.
Inspect painted wood surfaces to determine whether repainting is necessary or if cleaning is all that is required.
Remove damaged or deteriorated paint to the next sound layer using the gentlest method possible (hand scraping and hand-sanding), then repainting.
Use electric hot-air guns carefully on decorative wood features and electric heat plates on flat wood surfaces when paint is so deteriorated that total removal is necessary prior to repainting.
Use chemical strippers primarily to supplement other methods such as hand scraping, hand sanding and the above-recommended thermal devices. Detachable wooden elements such as shutters, doors, and columns may--with the proper safeguards--be chemically dip-stripped.
Apply compatible paint coating systems following proper surface preparation.
Repaint with colors that are appropriate to the historic building and district.
Evaluate the overall condition of the wood to determine whether more than protection and maintenance are required, that is, if repairs to wood features will be necessary.
Failing to identify, evaluate, and treat the causes of wood deterioration, including faulty flashing, leaking gutters, cracks and holes in siding, deteriorated caulking in joints and seams, plant material growing too close to wood surfaces, or insect or fungus infestation. In the picture at right, moss on wood shingles indicates damaging moisture retention.
- Using chemical preservatives such as creosote which can change the appearance of wood features unless they were used historically.
- Stripping paint or other coatings to reveal bare wood, thus exposing historically coated surfaces to the effects of accelerated weathering.
- Removing paint that is firmly adhering to, and thus, protecting wood surfaces.
- Using destructive paint removal methods such as a propane or butane torches, sandblasting or water-blasting. These methods can irreversibly damage historic woodwork.
- Using thermal devices improperly so that the historic woodwork is scorched.
- Failing to neutralize the wood thoroughly after using chemicals so that new paint does not adhere.
- Allowing detachable wood features to soak too long in a caustic solution so that the wood grain is raised and the surface roughened.
- Failing to follow manufacturers' product and application instructions when repainting exterior woodwork.
- Using new colors that are inappropriate to the historic building or district.
- Failing to undertake adequate measures to assure the protection of wood features.
Step 3: Repair damaged historical features
Repair wood features by patching, piecing-in, consolidating, or otherwise reinforcing the wood using recognized preservation methods.
Repair may also include the limited replacement in kind--or with compatible substitute material--of those extensively deteriorated or missing parts of features where there are surviving prototypes such as brackets, molding, or sections of siding. Limited replacement-in-kind of deteriorated wood clapboards is shown at left.
- Replacing an entire wood feature such as a cornice or wall when repair of the wood and limited replacement of deteriorated or missing parts are appropriate.
- Using substitute material for the replacement part that does not convey the visual appearance of the surviving parts of the wood feature or that is physically or chemically incompatible.
Step 4: Replace what cannot be repaired
Replace in kind an entire wood feature that is too deteriorated to repair--if the overall form and detailing are still evident--using the physical evidence as a model to reproduce the feature. Examples of wood features include a cornice, entablature or balustrade.
If using the same kind of material is not technically or economically feasible, then a compatible substitute material may be considered. At right, a rotted wood column base has been replaced with new wood.
- Removing a feature that is not repairable and not replacing it; or replacing it with a new feature that does not convey the same visual appearance.
Step 5: Recreate missing features
Design for missing historic features represents the particularly complex technical or design aspects of rehabilitation projects. It should only be considered after the preservation concerns listed above have been addressed.
Design and install a new wood feature such as a cornice or doorway when the historic feature is completely missing. It may be an accurate restoration using historical, pictorial, and physical documentation; or be a new design that is compatible with the size, scale, material, and color of the historic building.
- Creating a false historical appearance because the replaced wood feature is based on insufficient historical, pictorial, and physical documentation.
- Introducing a new wood feature that is incompatible in size, scale, material and color.
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