SHINGLES AND SHAKES 369 



exposed to the weather, the thickness of the shingle, the method of laying, 

 and last, but very important, the fire hazard involved. 



The length of service varies considerably with the different species 

 of woods used for shingles. The following shows the approximate 

 service that the principal shingle woods should give under average con- 

 ditions: 



Species. Length of Life. 



Cedar (western red and northern and southern white) 1 5 to 30 years 



Cypress 15 to 30 



Southern yellow pine 6 to 12 



Redwood 12 to 25 



White pine 1 2 to 20 



Chestnut.' 15 to 25 



Western pine 8 to 1 2 



Hemlock 7 to 1 2 



Spruce 7 to 1 1 



Shakes, which, as a rule, are much thicker than shingles, will last 

 much longer than the periods given above. Split or cut shingles always 

 last longer than sawn shingles. Instances are on record of cedar, cypress, 

 and redwood shingles lasting for from thirty to fifty years or more, but 

 this is an unusual exception. Decay is caused chiefly by water, the 

 accumulation of moss and debris on the roof, splitting, warping, etc. 

 The use of preservatives has been widely introduced to prevent decay. 



The following methods, briefly enumerated, are the principal processes 

 of preventing decay. Along with the prevention of decay various 

 stains and preservatives are used to lend attractiveness to the appearance 

 of the structure when used with various coloring agents. 



1. Dipping. This is the most common method, the shingles being 

 merely dipped in the preservative, and nailed to the roof. The shingles 

 should be thoroughly air dried before dipping, and the preservative 

 should be applied warm or hot. The exposed part of the shingle only, is 

 dipped. They are usually given a final coating of preservative after 

 being laid. Preservatives used are creosote, carbolineum and various 

 patent forms. 



2. Brush treatment. This is a cheap and less efficient method in 

 which the shingles are merely painted with a preservative, after being 

 laid. Paint aids chiefly in keeping shingles flat and preventing leaks. 



3. Impregnation. This is the most efficient method, in which the 



