Moisture & Wood

EFFECT OF SHRINKAGE ON A STRUCTURE

          Moisture effects on lumber within a structure can cause problems such as

  • Warping
  • Buckling or cracking of other components such as drywall, siding, or plaster
  • Added stress on connections
  • Additionally, biological attack and decay are of greater concern at higher moisture levels.

 

  • Below is a demonstration of the structural effects of shrinkage on a two story exterior wall section.

 Demo 3: Global Effects of Shrinkage

HOW SHRINKAGE IS CALCULATED

          Below the FSP, there is a linear relationship between changes in MC% and the decrease and increase of lumber dimensions. Table M-2 lists typical values for the amount of shrinkage (%) which occurs as lumber MC% goes from FSP to oven-dry. Values are provided for the radial and tangential shrinkage of various wood species. Longitudinal shrinkage is not listed because it is so small, between 0.1% and 0.2% for all species.

 

Table M-2: Typical Shrinkage from FSP to Oven-dry
(% based on green dimensions)

Species

Radial (%)

Tangential (%)

Ash, white

4.9

7.8

Birch, yellow

7.3

9.5

Douglas fir, coast

4.8

7.6

Douglas fir, interior west

4.8

7.5

Hemlock, western

4.2

7.8

Larch, western

4.5

9.1

Pine, loblolly

4.8

7.4

Pine, longleaf

5.1

7.5

Pine, red

3.8

7.2

Spruce, red

3.8

7.8

Spruce, Sitka

4.3

7.5

 

          Linear interpolation is used to calculate the amount of shrinkage that occurs when MC% changes but does not go all the way from FSP to oven-dry. The following example shows how shrinkage is calculated.

          The moisture content of wood is not always constant.  As seen in the demonstration above, moisture content can change while the wood is in use.  The moisture content of in service lumber (and lumber in general) will change with environmental conditions.  Prolonged exposure to a level of relative humidity and temperature will cause the wood to change its moisture content.   When wood in in service it is not possible to determine the moisture content by getting an oven dry sample; you would not want to cut specimens out of beams and studs in your house would you?.  Therefore, the following table has been developed that related temperature and relative humidity to moisture content.

Temperature

Moisture content (%) at various relative humidity values

oC

oF

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

55%

60%

65%

70%

75%

80%

85%

90%

95%

-1.1

30

1.4

2.6

3.7

4.6

5.5

6.3

7.1

7.9

8.7

9.5

10.4

11.3

12.4

13.5

14.9

16.5

18.5

21.0

24.3

4.4

40

1.4

2.6

3.7

4.6

5.5

6.3

7.1

7.9

8.7

9.5

10.4

11.3

12.3

13.5

14.9

16.5

18.5

21.0

24.3

10.0

50

1.4

2.6

3.6

4.6

5.5

6.3

71.0

7.9

8.7

9.5

10.3

11.2

12.3

13.4

14.8

16.4

18.4

20.9

24.3

15.6

60

1.3

2.5

3.6

4.6

5.4

6.2

7.0

7.8

8.6

9.4

10.2

11.1

12.1

13.3

14.6

16.2

18.2

20.7

24.1

21.2

70

1.3

2.5

3.5

4.5

5.4

6.2

6.9

7.7

8.5

9.2

10.1

11.0

12.0

13.1

14.4

16.0

17.9

20.5

23.9

26.8

80

1.3

2.4

3.5

4.4

5.3

6.1

6.8

7.6

8.3

9.1

9.9

10.8

11.7

12.9

14.2

15.7

17.7

20.2

23.6

32.2

90

1.2

2.3

3.4

4.3

5.1

5.9

6.7

7.4

8.1

8.9

9.7

10.5

11.5

12.6

13.9

15.4

17.3

19.8

23.3

37.8

100

1.2

2.3

3.3

4.2

5.0

5.8

6.5

7.2

7.9

8.7

9.5

10.3

11.2

12.3

13.6

15.1

17.0

19.5

22.9

43.4

110

1.1

2.2

3.2

4.0

4.9

5.6

6.3

7.0

7.7

8.4

9.2

10.0

11.0

12.0

13.2

14.7

16.6

19.1

22.4

49.0

120

1.1

2.1

3.0

3.9

4.7

5.4

6.1

6.8

7.5

8.2

8.9

9.7

10.6

11.7

12.9

14.4

16.2

18.6

22.0

54.6

130

1.0

2.0

2.9

3.7

4.5

5.2

5.9

6.6

7.2

7.9

8.7

9.4

10.3

11.3

12.5

14.0

15.8

18.2

21.5

60.0

140

0.9

1.9

2.8

3.6

4.3

5.0

5.7

6.3

7.0

7.7

8.4

9.1

10.0

11.0

12.1

13.6

15.3

17.7

21.0

65.5

150

0.9

1.8

2.6

3.4

4.1

4.8

5.5

6.1

6.7

7.4

8.1

8.8

9.7

10.6

11.8

13.1

14.9

17.2

20.4

71.1

160

0.8

1.6

2.4

3.2

3.9

4.6

5.2

5.8

6.4

7.1

7.8

8.5

9.3

10.3

11.4

12.7

14.4

16.7

19.9

76.7

170

0.7

1.5

2.3

3.0

3.7

4.3

4.9

5.6

6.2

6.8

7.4

8.2

9.0

9.9

11.0

12.3

14.0

16.2

19.3

82.2

180

0.7

1.4

2.1

2.8

3.5

4.1

4.7

5.3

5.9

6.5

7.1

7.8

8.6

9.5

10.5

11.8

13.5

15.7

18.7

87.8

190

0.6

1.3

1.9

2.6

3.2

3.8

4.4

5.0

5.5

6.1

6.8

7.5

8.2

9.1

10.1

11.4

13.0

15.1

18.1

93.3

200

0.5

1.1

1.7

2.4

3.0

3.5

4.1

4.6

5.2

5.8

6.4

7.1

7.8

8.7

9.7

10.9

12.5

14.6

17.5

98.9

210

0.5

1.0

1.6

2.1

2.7

3.2

3.8

4.3

4.9

5.4

6.0

6.7

7.4

8.3

9.2

10.4

12.0

14.0

16.9

104.4

220

0.4

0.9

1.4

1.9

2.4

2.9

3.4

3.9

4.5

5.0

5.6

6.3

7.0

7.8

8.8

9.9

110.0

230

0.3

0.8

1.2

1.6

2.1

2.6

3.1

3.6

5.2

4.7

5.3

6.0

6.7

115.6

240

0.3

0.6

0.9

1.3

1.7

2.1

2.6

3.1

3.5

4.1

4.6

121.1

250

0.2

0.4

0.7

1.0

1.3

1.7

2.1

2.5

2.9

126.7

260

0.2

0.3

0.5

0.7

0.9

1.1

1.4

132.3

270

1.0

0.1

0.2

0.3

0.4

0.4

 This table is adopted from Table 3-4 of the Wood Handbook published by the Forest Products Society.

 


EXAMPLE

How much tangential shrinkage will occur if Douglas fir lumber is dried from FSP to 19%MC?

Solution

FSP for Douglas fir: 26% (Table M-1)
Tangential shrinkage from FSP to oven dry: 7.5% (D. fir, interior west) (Table M-2)

  • Use linear interpolation to calculate % tangential shrinkage at 19% MC:

  • The lumber will shrink 2.0% in the tangential direction from FSP to 19% MC.

 

 

 

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Updated: 09/12/00