1835.] 



Table of the Scantlings of Beams for Roofs. 



227 



XI. — A Table of the Scantlings of Beams of Teak or Saul Wood, to sus- 

 tain a Terrace Roof not exceeding seven inches in thickness ; the de. 

 flexion not to exceed one-fortieth of an inch for each foot of length. 



Distance a- 











part of Beams, 



LENGTH OF BEARING IN FEET 







one foot from 

 centre to cen- 

 tre. 











6 



j- CO 



8 



10 



■5 k 



12 



j- in 



14 j 16 



18 



t- Ul 



■S <u 



20 



B.J3 



22 



24 



a>! 



26 



■5 "& 



28 



30 



&J3 



32 



£ '& 



34 



Breadth. 



Q>5 



Inches. 



M 



8- 



10- 



<u i 

 12- 



1> jj &> " 



14- 16- 



18- 



Q5 

 20- 



22- 



3g 

 24- 



— 1 







4 a 



S7y 



OJ 



6. 





1 



476 



6-35 



7-94 



9-53 



1M 12-7 



14-3 



15-9 



175 



19-1 



207 ' 



22-2 



23-8 







U 



41(i 



5-54 



6-92 



8-32 



9-7 11-1 



12-5 



13-9 



15-3 



167 



18-1 



19-4 



20 8 



9.2-9. 



23-r; 



2 



378 



5-04- 



6-3 



7*56 



8-82 10-1 11-3 



12-6 



13-14 15-1 



16-4 i 



17-6 



18-9 



20-1 



21-4 



2i 



3-5 



4 67 



5-85 



7-02 



8-18 9-35 10-6 j 11 -7 



12-10 



14- 



15-2 i 



16-4 



17-6 



18-7 



19-9 



3 



3-3 



4-4 



5-5 



6-6 



77 



8-81 



9-91 11- 



12-8 



13 2 



14-3 ' 



15-4 



16-5 



17-6, 



18-8 



3J 



— 



4-18 



5-23 



6-27 



7-32 



8-36 



9-41 



10-5 



11-5 



12-6 



13-6 



14-6 



15-7.16-7| 17-8 



4 



— 



4. 



5* 



6- 



7* 



8- 



9- 



10- 



11- 



12- 



13- 



14- 



15- | 16 , 17- 



H 



— 







4-81 



5-77 



fi-73 



7-C9 



8-65 



9-61 



lo-G 



11-5 



12-5 



13-5 



14-5 15-4 1 16- 



5 



— 







557 



6'5 



7-42 



8-35 



9-28 



10-2 



11-1 



12 1 



13- 



13-9*14-8 15-8 



5J 



— 











6 29 



7-19 



8-09 



8-99 



9-89 



10-8 



117 



12-6 



13-5|l4-4! 15-3 



6 



— 



— 



— 



— 



6-11 



6-99 



7-86 



8-73 



9-61 



10-5 



11-3 



12-2 



13-1 



14- 



14-9 



Burgahs require to be made six times stiffer than beams, in order to prevent 

 cracks in the terrace roof; and as they are invariably placed one foot aparf, and 

 have a breadth of three inches, they should be as many inches in depth as they 

 are feet in length of bearing between the beams. 



Explanation of the Table, with Examples of its use. 



The table shews on inspeotion the scantlings of beams to support 

 roofs not exceeding 80 lbs. per square foot, including the weight of the 

 timber. It has been calculated, according to the rule in Tredgold's 

 Carpentry, Section II. par. 90, the value of the constant quantity, 

 a being taken at "01. The scantlings given in the table are measured 

 in the middle of the beam ; the lower side is supposed to be cut straight, 

 and the upper side with a curve of one or two inches, versed sine, for 

 each 10 feet in length of the beam. 



As the stiffest beam that can be cut out of a round timber has its 

 breadth to its depth in the proportion of • 6 to 1 nearly, the proportion 

 of the breadth to the diameter will be as '5 to 1, or the breadth will 

 be | the depth. 



As the cost of timber is partly proportioned to its contents, the deep- 

 er the beams are made, the cheaper the roof will be within certain 

 limits ; and as the cutting of timbers through the heart or centre of 

 the wood is supposed to render the beams more durable, all the tim- 

 bers should be cut into two beams, particularly as the strength of the 

 timber is not at all reduced by this measure. 



There is, however, a proportion between the depth and breadth 

 which cannot be exceeded without the risk of the beam breaking side- 

 ways. Tredgold's rule is, (Sec. II. par. 82,) " the breadth in inches 

 should not be less than six-tenths of the length in feet, divided by the 

 square root of the depth, in inches." 



