SPECIFICATIONS
SOFFIT VENTILATION GUIDE
PROPER SOFFIT VENTILATION Properly installed soffit allows air to flow naturally. Moisture can collect under roof structures and on rafters causing rotting and decaying. ALSCO ® aluminum soffit is engineered so that air circulates properly to help eliminate moisture problems. ALSCO ® soffit panels are designed to provide optimal airflow. Following building codes while installing a proper ventilation system is critical to the structural performance of your house. The International Code Council (ICC) has provided general guidelines for determining proper ventilation for the home. The 1/300 and 1/150 ratios are for calculating ventilation requirements. Be sure to follow all applicable, local building codes.
ICC guidelines recommends that the free area of ventilation (in square inches) needed is the number at which the two columns intersect. The sum of the free area of all ceilings equal the total free area required. The total square feet of an attic space divided by 300 = the total net free area required for appropriate ventilation. Attics without vapor barriers must use the 1/150 ratio. Keep in mind that some climates or locations demand more net free area to prevent ice dams in cold climates and mold and mildew in hotter climates.
CEILING WIDTH (FT.) 20 22 24 26 28 30 32 34 36 40 42 44 46 48 58
20 192 211 230 250 269 288 307 326 346 384 403 422 441 46 480
22 211 232 253 275 296 317 338 359 380 422 444 465 485 506 528
24 230 253 276 300 323 346 369 392 415 461 484 507 530 553 576
26 250 275 300 324 349 374 399 424 449 499 524 549 574 599 624
28 269 296 323 349 376 403 430 457 484 538 564 591 618 645 662
30 288 317 346 374 403 432 461 490 518 576 605 634 662 691 720
32 307 338 369 399 430 461 492 522 553 614 645 675 706 737 768
CEILING LENGTH (FT.)
34 326 359 392 424 457 490 522 555 588 653 685 717 750 782 815
36 346 380 415 449 484 518 553 588 622 691 726 760 795 829 864
38 365 401 438 474 511 547 584 620 657 730 766 803 839 876 912
40 384 422 461 499 538 576 614 653 691 768 806 845 883 922 960
42 403 444 484 524 564 605 645 685 726 806 847 887 927 968 1008
44 422 465 507 549 591 634 676 718 760 845 887 929 971 1013 1056
46 442 486 530 574 618 662 707 751 795 883 927 972 1016 1060 1104
48 461 507 553 599 645 691 737 783 829 922 968 1014 1060 1106 1152
50 480 528 576 624 672 720 768 816 864 960 1008 1056 1104 1152 1200
Free area chart utilizes 1/300 ration. Double the values to obtain 1/150 ratio.
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