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An Overview Of Composite And Solid Plastic Decking Materials

By Harvey Freeman

Originally published in "Contractor's Desk Magazine"

   Several months ago, I happened to mention composite decking material to an associate. 

  This person said they knew of some benches at St. Mary's Boat Club which are still in excellent condition considering that they have been bolted to the deck for nine years and suffered the normal abuse that such public furniture undergoes as well as weathering for that period of time. 

  The benches are made from solid polyethylene lumber and the end frames are molded from the same material. HRM now has a policy of not using pressure treated wood where the public can normally make contact with it.  Staff are so happy with the durability of the solid plastic material that when the deck at this boat club is renewed this fall it will be done with polyethylene lumber.
There are a wide range of composite decking materials to choose from now.

Composition

   Composite decking, as the name would indicate, is made up of a
combination of materials.  One of those materials is always a plastic.
Several types of plastic are used, depending on the individual manufacturer.
Sometimes the plastic is virgin material but usually is recycled plastic.
The other component is typically wood in the form of fiber or sawdust.  The
usual plastic used is polyethylene from recycled pop bottles, and the like,
but one company uses polypropylene.

The wood used varies even more than the type of plastic.  The people who use
polypropylene mix it with oak.  Most others seem to just use wood fiber or
waste wood but some highlight that they use specific wood such as cedar.
I really wonder if anyone can tell what plastic and what wood are used in
any of them.

As well, there are companies making decking out of vinyl which obviously
would not be a composite but these get lumped into the whole collection of
man-made lumber products.  Each manufacturer will have their own take on
why their formulation is superior to everyone else's and that discussion is
beyond the scope of this article.

Profile

   Some composite decks are made as a solid piece of material that
more or less resembles a piece of wood.  These solid shapes can be worked in
the same way as a piece of wood, so ends can be cut in any shape desired and
a router can be used to do round overs and similar decorative work.  Some
solid material is prone to having air pockets in the interior of the piece
that can cause appearance problems if a piece is ripped and the cut faces
are to be exposed.  If your plans include this type of detailing, it may be
prudent to investigate this further.

   Other manufacturers make various hollow shapes either in rectangular form or
in channel shapes.  Typically, these profiles seem to have been created as
part of the method of fastening the material to the structure.  More on that
later.

   These products are usually coloured all the way through.  The advertising
varies in the claims as to how much the product will fade over time.
If you plan to use one of the hollow products, pay special attention to how
they treat the exposed ends.  Some have individual end caps, but these will
likely only work on square ends.  If you want an angled end, there may be no
cap available.  If you want to curve the ends for a special layout, you may
not be able to cap the individual ends at all and may have to resort to a
skirt board.  In some systems the ends are capped with a channel cover which
sure does not give the illusion of individual planks.

Spanning Capacity

   Composite decking material is not to be used as structural
framing but just as decking.  Structural framing must still be done using
wood.  When you read the literature from the various manufactures you will
discover that the spanning capabilities of composite decking materials vary
considerably.  One company says "strong enough to span 24 in".  That is
basically a meaningless statement.  Another manufacturer says a 40 lb/sq.
ft. live load can be achieved at 24" c/c joist spacing and a 100 lb/sq. ft.
live load can be achieved at a joist spacing of 16" c/c.  The live loading
for decks varies in the NBC based on the type of assembly, so these numbers
are important.

If you have an existing deck where the structure is in good condition and
you just want to replace the deck, you may discover that you have to install
a new joist between each two existing joist to deal with this situation.

Fastening Method

   The solid decking material is typically fastened through the face
with a nail or a screw.  That may be a hot dipped galvanized nail with the
head showing or a decking screw or stainless steel screw, also with the head
showing.  Others suggest using a trim head screw that is buried below the
surface of the deck.  One friend who used this type of screw with a Torx
recess head reported that often the recess stripped out and if this happened
it would be impossible to remove the screw without damaging the plank.  I
know that these screws are steel with some type of coating and wonder what
their expected life is before they rust out.

   In one hollow plank system, attachment is through over-sized holes drilled
in the top surface of the plank and those holes are then covered with plugs.
A dealer for this system said that they no longer used it because if snow
had to be shoveled off the deck, the shovel would remove the plugs.
It is obvious that most of the hollow plank systems were devised in part to
address the problem of trying to create an invisible fastening system to
secure the planks to the structure. 

   One dealer I spoke with said that many of the hollow plank systems had very
specific requirements for gaps at butt joints to allow for expansion and
contraction of the material.  Some instructions even give guidance as to the
space to leave depending on the ambient temperature on the day the material
is installed as well as the yearly temperature range of the place of
installation.  If these instructions are not properly followed, the deck
planks may do strange things including buckling and squeaking.

   Since January 1, 2004 all pressure treated wood in Canada, that is subject
to easy human contact, uses ACQ or copper quat.  Because these chemicals
attack regular steel fastenings at three times the speed of old CCA make
sure that all fasteners are real hot dipped galvanized or better still
stainless steel.  If you use any stainless steel hardware, such as deck
clips, the fasteners must be stainless steel nails or screws.  Failure to
follow this rule will result in corrosion of the stainless steel hardware.

   I also learned that coloured chalk, as used on a chalk line, will leave a
mark on real wood and many composite products and it will not totally
disappear.  The recommendation is to use white chalk or baby powder in the
chalk line.

Maintenance

   Many of the manufacturers include phrases like "little or no
maintenance" in their literature.  Others will go into great detail on all
the things that will affect the deck and what to do about them.  The most
extensive list I found included rust stains, oil and grease stains, berry
and wine stains, ink stains and mold.  Most of these products cannot be
sanded and great care must be used in using pressure washing equipment.  In
some cases, pressure washing voids the warranty.

Warranty

   Read beyond the first line of the warranty which states the
duration of the warranty.  A ten year warranty may give you more protection
than a twenty year warranty, but in my opinion there are far too many loop
holes in most of these warranties.  The best (or worst, depending on your
point of view) one I read in researching this article reads as follows: "Due
to factors over which ... has no control, such as weather conditions,
quality of installation or construction, the useful life of a given deck may
be more or less than the length of the limited warranty period applicable to
the products."

Conclusion

   I had an opportunity to discuss the deck selection process with
a friend who built one.  It was obvious he was
looking for a high end, low maintenance product that would look good for a
long time and was willing to spend the money to get it.  He was concerned
that the product look like wood and not scratch and not show traffic
patterns over time.  I said I thought there was no product on the market
that would do all this for him, especially when he has a young family who
will be playing on the deck.  He agreed that when he got the first ding on
the hardwood floor it was a traumatic experience.

I believe there is a place for composite decking materials but I really
wonder if marketing claims and consumer expectations are too high for the
realistic performance that can be expected from the product.

Harvey Freeman is an architect with the Halifax firm of Fowler
Bauld & Mitchell Ltd.  His areas of expertise include roofing design and
inspection as well as investigation of building envelop problems.  He also
presents sections of Dal Tech Continuing Education seminars on building
related issues.



Harvey Freeman
Halifax
Nova Scotia
Canada
The Home of the Precision 50
T-Square Table Saw Fence
http://www.chebucto.ns.ca/~ac258/Precision_50.html

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