The finish line

A Forest Products Laboratory finishing factsheet

Forest

Service

Forest

Products

Laboratory

United States

Department of

Agriculture

Is there any value to placing

exterior wood bark-side up, or

does the common practice

encourage “barking up the wrong

side?” The bark-side/pith-side

quandary arises when flat-grained

lumber—that is, lumber cut

tangential to the grain—is used

for decking or siding (Fig. 1).

The usual reason given for

placing deck boards bark-side up

or siding bark-side out is to

reduce cupping, a form of warp.

Cupping results from the shrinking

of the top and swelling of the

bottom of the wood, which are

caused by exposure to moisture

and drying by the sun. These

changes do not occur evenly

throughout the wood.

The amount of initial cupping

depends on the difference between the moisture content of the

wood at the time it is cut compared to the moisture content in

service. As the boards weather

and continue to experience

wetting and drying cycles, cupping continues to increase. The

boards take on increased “set.”

Even during periods of rain, when

the moisture content rises in the

surfaces, boards remain cupped

because the surfaces undergo a

permanent set. During periods of

increased moisture content, the

surface cells are slightly crushed;

during drying periods, small

cracks (checking) appear in the

wood because the surface cells

remain crushed.

Vertical-grained lumber tends to

have fewer problems with cupping

than does flat-grained lumber, but

both kinds of wood will cup when

the top or outside of the wood is

lower in moisture content than the

opposite side. In the case of

decking, the top of the board dries

much faster than the bottom; in

fact, water may be driven down by

the drying process. Water-repellent

treatment also helps to keep the

wood dry. In tests at the Forest

Products Laboratory, the bottom

of deck boards was about 2%

wetter than the top. The better the

water repellent, the greater the

difference.

Contrary to popular notion, there

is only a small advantage to

placing boards bark-side up with

regard to cupping. However, it is

advisable to place the bark-side

up for other reasons. The pith

side is more prone to a severe

type of raised grain, called

shelling, particularly in species

that have dense latewood growth

rings, such as southern pine

(Fig. 2).

The second advantage to placing

deck boards bark-side up relates

only to wood that has been

pressure-treated with preservatives and not incised. Pressure

treatment does not penetrate the

heartwood of unincised wood very

well. In a flat-grained board, the

bark side may consist of 100%

sapwood and therefore be fully

penetrated with treatment,

whereas the pith side may contain

a portion of poorly penetrated

heartwood. When the deck is

exposed to the weather, checking

of the heartwood surfaces can

expose the untreated heartwood

to attack by decay fungi. The

bottom of the deck is not prone to

checking because its moisture

Figure 1. The grain of wood is

determined by how the log is

sawn: (a) vertical- or edgegrained lumber; (b) flat-grained

lumber.

a

b

a

b

The Bark-Side/Pith-Side Debate
Figure 2. The pith side of lumber is prone to shelling, a severe

type of raised grain.

content is less changeable than

that of the surface.

These factors, although important,

can be overridden by a third

factor—the quality of the pith side

compared to that of the bark side.

If the pith side is clearly better,

place this side up.

Which side to place up is not an

issue when applying resawn bevel

siding. When a single board is

resawn into two pieces, each has

a rough side and a smooth side;

one half of the rough side will be

pith and the other, bark. This is

why paint often peels on only half

the siding on a building.

_________

R. Sam Williams and Mark Knaebe

are researchers in Wood Surface

Chemistry at the USDA Forest

Service, Forest Products Laboratory,

One Gifford Pinchot Drive,

Madison, WI 53705–2398

December 1995