While the internet may be a fantastic tool for sharing
information, we must realize that good advice is transmitted as easily as
bad. Consider this recent dialogue from
a historic preservation message board in New England. An architect asked, “Has anyone had
experience with use of cellular PVC to ‘replicate’ architectural details on a
historic building?”
While the respondents were quick to cry nay! for that use, I was surprised by how many suggested that there are permissive uses of PVC composites. In fact, the chairman of a historical commission near Boston indicated that they had adopted a policy of allowing cellular PVC in place of ground-contact lumber (like bottom step risers and porch skirt boards) and rooftop balustrades (if traditional profiles are duplicated) in their local historic districts.
I was shocked.
In the limited instances where wood-meets-earth or the worst
precipitation (window sills, balustrades and stair treads), PVC composites may
appear to be an attractive alternative. But its use should be considered with
caution; new composite risers, fascia plates and sills will not show signs of
degradation, but they will conceal what's happening to wooden structural
members behind them. So, when it fails,
be prepared for wholesale failure of the system. ("Gee, the stairs looked great ... who knew
the stringers were rotted?")
Like many issues in restoration work, this one can be attributed
to a lack of informed sources. The
building owner typically knows only as much as the contractor has told
them. PVC composites are widely available
and spend a tremendous amount of money "educating" would-be consumers
through marketing. Consider the Secretary of the Interior's position on the
subject:
"If repair by stabilization, consolidation, and conservation
proves inadequate, the next level of intervention involves the limited replacement
in kind of extensively deteriorated or missing parts of features when there are
surviving prototypes (for example, brackets, dentils, steps, plaster, or
portions of slate or tile roofing). The replacement material needs to match
the old both physically and visually, i.e., wood with wood, etc.
Thus, with the exception of hidden structural reinforcement and new mechanical
system components, substitute materials are not appropriate in the treatment
Preservation."[1] (Underlining added for emphasis)
The use of PVC composites is an attempt to cut corners and remove maintenance
and upkeep from the equation. Consider
the use of Spanish cedar, mahogany, oak, and other hardwoods in these limited applications. You may be surprised how close the price is
to the composite materials.
Photo credits: Three accompanying photos, all courtesy of
the author, depicting the finials that will be replicated in the millwork shop
for the author’s upcoming project at Historic New England’s ‘Roseland Cottage’
in Woodstock, Connecticut.
[1]
Standards for Preservation and Guidelines for Preserving Historic Buildings
(http://www.cr.nps.gov/hps/tps/standguide/preserve/preserve_approach.htm)