Framing is the process of building the structure of a project whether it be a home or a deck or a shed.

To build your project correctly so roofs can support snow and wind loads and so your floor system can support the extra weight of the whirlpool bath you installed it is important to choose the correct sized lumber for building with beams, floor joists, wall studs and roof structures.

So what is a span chart and why are they so important?

Architects, Carpenters and even do it yourself builders should have a good understanding of how to refer to Lumber Span Charts.

Lumber manufacturers have gone to extensive lengths to test and recommend the proper load rating of dimensional lumber used for framing.
Not only is the size of the board important but also the number of knots, cracks, and wain. Additionally different species of trees across the united states and other countries will have different load properties.

In the USA Spruce and Southern Yellow Pine are the two most common lumber types used for framing but there are times when other species such as redwood

Lets say you are building a home and it is 28 feet wide x 60 feet long

Obviously you will not be buying pieces of lumber for the floor joists that are 60′ long so you will want to set your joists to run the short dimension.

You want to use a 2×10″ board 16″ on center for your joists.

The Load that needs to be supported is 40 PSF Forty Pounds per Square foot for Live loads like people.

Lets look on the Southern Yellow Pine span chart to see our options

This Span Chart Table is available free from the Southern Pine Council
http://www.southernpine.com/spantables.shtml

From the chart above we can see that the size board we want to use will only reach 16″ feet 4″ inches if we install them at 16″ inches on Center.

This means that we will have to use a center support beam at the middle of the span that will support Two 16 foot Joists where they overlap.

That is pretty much all there is to it.
The lumber industry has taken most of the calculations out of the process all you need to do is understand what type and the placement of the board you need and the chart will give you the proper distance that the board can span based on the Load that it will be supporting.

If you are an Engineer or an Architect there is much more thought that goes into the selection process but if you are a builder or a do it yourself person then you can quickly double check the charts and get the information that you need.