The size of timbers needed for rafters or joists depends on factors like span, spacing, construction requirement, design, imposed & dead load, local building codes and regulation.

The International Residential Code (IRC) specifies that the timber used for joists will typically be placed at 400 mm centers but no more than 600 mm centers. The common sizes of timber used for rafters joists for different span include 38×120 mm, 38×145 mm, 47×170 mm, 47×195 mm, 47×220 mm, 75×220 mm, 75×245 mm and etc.

All roof timber like different joists (floor joist, ceiling joist, pitched rafters, flat roof joist or rafters), wall plates, blocking, strutting, battens, firrings and nogging must be treated with preservatives to secure its durability.

In general, the roof joist or rafter span is typically measured horizontally distance between two support end. The roof joists span may be either 2m, 2.5m, 3m, 3.5m, 4m, 5m and 6m and so on higher. In this article you know about what size timber or joists do i need for a specific span for roof that will help you in better understanding and to figure out or estimate of joist size and their depth according to span.

**Common timber sizes for a given span**

The size of timbers needed for rafters or joists depends on various factors. However, for general residential construction in wooden frame house, common sizes for joists or rafters may range from 38×120 mm for shorter spans to 75×240 mm for longer spans.

The required timber size for a given span depends on factors like load, species, and intended use. Here are the floor joist, ceiling joist, pitched roof rafters and roof joist size for flat roof required for different given span:-

**What size floor joist do i need for a given span**

In general, the typical size of timber needed for floor joist to span 2m got to be 47×120 @ 600c/c, while the size of floor joist to span 2.5m need to be 47×120 @ 600c/c. Likewise, the typical size of timber needed for floor joist to span 3m got to be 47×195 @ 400c/c, while the size of floor joist to span 3.5m need to be 47×195 @ 400c/c. In addition, the typical size of timber needed for floor joist to span 4m got to be 47×220 @ 400c/c, while the size of floor joist to span 5m need to be 75×220 @ 400c/c.

**What size pitched roof rafters do i need for a given span**

In general, the typical size of timber needed for pitched roof rafters to span 2m got to be 38×125 @ 600c/c, while the size of pitched roof rafters to span 2.5m need to be 38×150 @ 600c/c. Likewise, the typical size of timber needed for pitched roof rafters to span 3m got to be 47×150 @ 400c/c, while the size of pitched roof rafters to span 3.5m need to be 47×195 @ 600c/c. In addition, the typical size of timber needed for pitched roof rafters to span 4m to 6m purlin is needed.

**What size ceiling joist do i need for a given span**

In general, the typical size of timber needed for ceiling joist to span 2m got to be 38×120 @ 400c/c, while the size of ceiling joist to span 2.5m need to be 38×145 @ 400c/c. Likewise, the typical size of timber needed for ceiling joist to span 3m got to be 47×170 @ 600c/c, while the size of ceiling joist to span 3.5m need to be 47×170 @ 400c/c. In addition, the typical size of timber needed for ceiling joist to span 4m need to be 47×195 @ 400c/c, while the typical size of timber needed for ceiling joist to span 4.5m to 6m Binder is needed.

**What size roof joist for flat roof do i need for a given span**

In general, the typical size of timber needed for flat roof joist to span 2m got to be 38×120 @ 400c/c, while the size of flat roof joist to span 2.5m need to be 38×145 @ 400c/c. Likewise, the typical size of timber needed for flat roof joist to span 3m got to be 47×170 @ 600c/c, while the size of flat roof joist to span 3.5m need to be 47×195 @ 600c/c. In addition, the typical size of timber needed for flat roof joist to span 4m got to be 47×195 @ 400c/c, while the size of flat roof joist to span 5m need to be 75×220 @ 400c/c.

**What size timber do i need for 2m span**

The typical size of timber needed for a 2m span should be 47×120 @ 600c/c for floor joist, 38×120 @ 400c/c for ceiling joist, 38×125 @ 600c/c for pitched rafters, and 38×120 @ 400c/c for flat roof joist.

**What size timber do i need for 2.5m span**

The typical size of timber needed for a 2.5m span should be 47×145 @ 600c/c for floor joist, 38×145 @ 400c/c for ceiling joist, 38×150 @ 600c/c for pitched rafters, and 38×145 @ 400c/c for flat roof joist.

**What size timber do i need for 3m span**

The typical size of timber needed for a 3m span should be 47×170 @ 400c/c for floor joist, 47×170 @ 600c/c for ceiling joist, 47×150 @ 400c/c for pitched rafters, and 47×170 @ 600c/c for flat roof joist.

**What size timber do i need for 3.5m span**

The typical size of timber needed for a 3.5m span should be 47×195 @ 400c/c for floor joist, 47×170 @ 400c/c for ceiling joist, 47×195 @ 600c/c for pitched rafters, and 47×195 @ 600c/c for flat roof joist.

**What size timber do i need for 4m span**

The typical size of timber needed for a 4m span should be 47×220 @ 400c/c for floor joist, 47×195 @ 400c/c for ceiling joist, purlin needed for pitched rafters, and 47×195 @ 400c/c for flat roof joist.

**What size timber do i need for 4.5m span**

The typical size of timber needed for a 4.5m span should be 75×195 @ 400c/c for floor joist, Binder is needed for ceiling joist, purlin needed for pitched rafters, and 47×220 @ 400c/c for flat roof joist.

**What size timber do i need for 5m span**

The typical size of timber needed for a 5m span should be 75×220 @ 400c/c for floor joist, Binder is needed for ceiling joist, purlin needed for pitched rafters, and 75×220 @ 400c/c for flat roof joist.

**What size timber do i need for 6m span**

The typical size of timber needed for a 6m span should be 75×245 @ 400c/c for floor joist, Binder is needed for ceiling joist, purlin needed for pitched rafters, and 75×245 @ 400c/c for flat roof joist.

**Conclusions:-**

In general, the size of timber needed for a 5m should be 75×220 @ 400c/c, 75×245 timber for a 6m span, 47×195 timber for a 4.5m span, 47×220 timber for a 4m span, 47×170 timber for a 3m span, 47×145 timber for a 2.5m span, and 47×120 timber for a 2m span.