How do I figure out how many Rafters I need?

Rafter is a structural component of the roof and it is a very important component of the roof. It is generally used in slopped roofs. If you are planning to construct the slopped roof you will need the rafters. So you have to know how many rafters I need. So in this article, we will learn how to figure out how many rafters I need.

Rafters are made up of Iron, steel, LVL beams, and lumber. But most commonly rafter used is made up of lumber and LVL beam.

How do figure out how many rafters I need?

Roof rafter calculation formula = [ { (Roof length × 12) ÷ rafter spacing} +1] 2

To figure out the total number of roof rafters you have to know rafter to rafter spacing distance. According to the general rule, rafter-to-rafter spacing should be 16 inches or 24 inches. You can find the total number of rafters for your project by these 4 simple steps.

1. First of you have to decide the spacing distance between the 2 rafters.
2. Then convert the roof length into inches from feet.
3. After that, you have to divide the roof length by the spacing distance (which you have chosen).
4. Then add 1 to the result and multiply by 2, this value is your rafter number.

Let us understand it with some examples.

How many rafters do I need for 10-foot span?

For 10-foot span, the number of rafters depends on the rafter-to-rafter spacing. So here we will take both spacing 16 inches or 24 inches.

Roof length = 10 foot
Rafter spacing = 16 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (10 × 12) ÷ 16} +1] 2
= [ {120 ÷ 16} +1] 2 = [7.5+1] 2
8 × 2 = 16 Rafters

For 10 foot-long roof, you will need 16 rafters, on one side 8 rafters and the other side 8 rafters. But it is when the rafter-to-rafter spacing is 16 inches.

Roof length = 10 foot
Rafter spacing = 24 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (10 × 12) ÷ 24} +1] 2
= [ {120 ÷ 24} +1] 2 = [5+1] 2
6 × 2 = 12 Rafters

For 10 foot-long roof, you will need 12 rafters, on one side 6 rafters and the other side 6 rafters, when the rafter-to-rafter spacing is 24 inches.

How many rafters do I need for a 12-foot span?

The number of rafters for a 12-foot span depends on the rafter-to-rafter spacing. So here we will take both spacing 16 inches or 24 inches.

Roof length = 12 foot
Rafter spacing = 16 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (12 × 12) ÷ 16} +1] 2
= [ {144 ÷ 16} +1] 2 = [ 9+1 ] 2
10 × 2 = 20 Rafters

For 12 foot-long roof, you will need 20 rafters, on one side 10 rafters and the other side 10 rafters. But it is when the rafter-to-rafter spacing is 16 inches.

Roof length = 12 foot
Rafter spacing = 24 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (12 × 12) ÷ 24} +1] 2
= [ {144 ÷ 24} +1] 2 = [ 6+1 ] 2
Total rafters = 7 × 2 = 14 Rafters.

For 12 foot-long span roof, you will need 14 rafters, on one side 7 rafters and the other side 7 rafters, when rafter-to-rafter spacing is 24 inches.

How many rafters do I need for a 14-foot span?

The number of rafters for a 14-foot span depends on the rafter-to-rafter spacing. So here we will take both spacing 16 inches or 24 inches.

Roof length = 14 foot
Rafter spacing = 16 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (14 × 12) ÷ 16} +1] 2
= [ {168 ÷ 16} +1] 2 = [ 10.5+1 ] 2
11 × 2 = 22 Rafters

For 14 foot-long roof, you will need 22 rafters, on one side 11 rafters and the other side 11 rafters. But it is when the rafter-to-rafter spacing is 16 inches.

Roof length = 14 foot
Rafter spacing = 24 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (14 × 12) ÷ 24} +1] 2
= [ {168 ÷ 24} +1] 2 = [ 7+1 ] 2
Total rafters = 8 × 2 = 16 Rafters.

For 14 foot-long span roof, you will need 16 rafters, on one side 8 rafters and the other side 8 rafters, when rafter-to-rafter spacing is 24 inches.

How many rafters do I need for a 16-foot span?

The number of rafters for a 16-foot span depends on the rafter-to-rafter spacing. So here we will take both spacing 16 inches or 24 inches.

Roof length = 16 foot
Rafter spacing = 16 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (16 × 12) ÷ 16} +1] 2
= [ {192 ÷ 16} +1] 2 = [ 12+1 ] 2
13 × 2 = 26 Rafters

For 16 foot-long roof, you will need 26 rafters, on one side 13 rafters and the other side 13 rafters. But it is when the rafter-to-rafter spacing is 16 inches.

Roof length = 16 foot
Rafter spacing = 24 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (16 × 12) ÷ 24} +1] 2
= [ {192 ÷ 24} +1] 2 = [ 8+1 ] 2
Total rafters = 9 × 2 = 18 Rafters.

For 16 foot-long span roof, you will need 18 rafters, on one side 9 rafters and the other side 9 rafters, when rafter-to-rafter spacing is 24 inches.

How many rafters do I need for an 18-foot span?

The number of rafters for an 18-foot span depends on the rafter-to-rafter spacing. So here we will take both spacing 16 inches or 24 inches.

Roof length = 18 foot
Rafter spacing = 16 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (18 × 12) ÷ 16} +1] 2
= [ {216 ÷ 16} +1] 2 = [ 13.5+1 ] 2
14 × 2 = 28 Rafters

For an 18-foot long roof, you will need 28 rafters, on one side 14 rafters and the other side 14 rafters. But it is when the rafter-to-rafter spacing is 16 inches.

Roof length = 18 foot
Rafter spacing = 24 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (16 × 12) ÷ 24} +1] 2
= [ {216 ÷ 24} +1] 2 = [ 9+1 ] 2
Total rafters = 10 × 2 = 20 Rafters.

For 18 foot-long span roof, you will need 20 rafters, on one side 10 rafters and the other side 10 rafters, when rafter-to-rafter spacing is 24 inches.

How many rafters do I need for a 20-foot span?

The number of rafters for 20-foot span depends on the rafter-to-rafter spacing. So here we will take both spacing 16 inches or 24 inches.

Roof length = 20 foot
Rafter spacing = 16 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (20 × 12) ÷ 16} +1] 2
= [ {240 ÷ 16} +1] 2 = [ 15+1 ] 2
16 × 2 = 32 Rafters

For 20 foot-long roof, you will need 32 rafters, on one side 16 rafters and the other side 16 rafters. But it is when the rafter-to-rafter spacing is 16 inches.

Roof length = 20 foot
Rafter spacing = 24 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (20 × 12) ÷ 24} +1] 2
= [ {240 ÷ 24} +1] 2 = [ 10+1 ] 2
Total rafters = 11 × 2 = 22 Rafters.

For 20 foot-long span roof, you will need 22 rafters, on one side 11 rafters and the other side 11 rafters, when rafter-to-rafter spacing is 24 inches.

How many rafters do I need for a 22-foot span?

The number of rafters for 22-foot span depends on the rafter-to-rafter spacing. So here we will take both spacing 16 inches or 24 inches.

Roof length = 22 foot
Rafter spacing = 16 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (22 × 12) ÷ 16} +1] 2
= [ {264 ÷ 16} +1] 2 = [ 16.5+1 ] 2
18 × 2 = 36 Rafters

For 22 foot-long roof, you will need 36 rafters, on one side 18 rafters and the other side 18 rafters. But it is when the rafter-to-rafter spacing is 16 inches.

Roof length = 22 foot
Rafter spacing = 24 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (22 × 12) ÷ 24} +1] 2
= [ {264 ÷ 24} +1] 2 = [ 11+1 ] 2
Total rafters = 12 × 2 = 24 Rafters.

For 22 foot-long span roof, you will need 24 rafters, on one side 12 rafters and the other side 12 rafters, when rafter-to-rafter spacing is 24 inches.

How many rafters do I need for a 24-foot span?

The number of rafters for 24-foot span depends on the rafter-to-rafter spacing. So here we will take both spacing 16 inches or 24 inches.

Roof length = 24 foot
Rafter spacing = 16 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (24 × 12) ÷ 16} +1] 2
= [ {288 ÷ 16} +1] 2 = [ 18+1 ] 2
19 × 2 = 38 Rafters

For 24 foot-long roof, you will need 38 rafters, on one side 19 rafters and the other side 19 rafters. But it is when the rafter-to-rafter spacing is 16 inches.

Roof length = 24 foot
Rafter spacing = 24 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (24 × 12) ÷ 24} +1] 2
= [ {288 ÷ 24} +1] 2 = [ 12+1 ] 2
Total rafters = 13 × 2 = 26 Rafters.

For 24 foot-long span roof, you will need 26 rafters, on one side 13 rafters and the other side 13 rafters, when rafter-to-rafter spacing is 24 inches.

How many rafters do I need for a 26-foot span?

The number of rafters for 26-foot span depends on the rafter-to-rafter spacing. So here we will take both spacing 16 inches or 24 inches.

Roof length = 26 foot
Rafter spacing = 16 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (26 × 12) ÷ 16} +1] 2
= [ {312 ÷ 16} +1] 2 = [ 19.5+1 ] 2
21 × 2 = 42 Rafters

For 26 foot-long roof, you will need 42 rafters, on one side 21 rafters and the other side 21 rafters. But it is when the rafter-to-rafter spacing is 16 inches.

Roof length = 26 foot
Rafter spacing = 24 inches.

Using the number of rafter calculation formula

Number of rafters = [ { (Roof length × 12) ÷ rafter spacing} +1] 2
= [ { (26 × 12) ÷ 24} +1] 2
= [ {312 ÷ 24} +1] 2 = [ 13+1 ] 2
Total rafters = 14 × 2 = 28 Rafters.

For 26 foot-long span roof, you will need 28 rafters, on one side 14 rafters and the other side 14 rafters, when rafter-to-rafter spacing is 24 inches.

Conclusion

In this article, We learned, how do I figure out how many rafters I need. After reading this article you can calculate the number of rafters for your desired span. I also provide a rafter calculation formula that is [ { (Roof length × 12) ÷ rafter spacing} +1] 2.