How To – Using Span Charts For Framing

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    Framing is the process of building the supporting 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, walls can withstand wind loads and your floor system can support the extra weight of the whirlpool bath you installed it is important to choose the correct sized lumber when building. For this reason architects and engineers take special care when they are selecting the sizes of  beams, floor joists, wall studs and roof structures.

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

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

    Span Charts are predefined ratings of lumber at nominal sizes.

    Manufacturers of wood products saw their wood in what is referred to as a Nominal Sized Lumber. For instance if a lumber mill is cutting 2×4 lumber they will cut the wood green and then it will shrink during the drying process. The resulting size of a 2×4 green board that has dried is about 1-1/2″ x 3-1/2″ but it can vary slightly.

    When the wood is dry and ready for sale the manufacturer will test the wood for strength. The wood will be put in a hydraulic press and loaded until it fails or snaps. They will then record the rating and stamp the batch of wood with a seal that lets the carpenter know it’s grade.

    Grades are ratings of the quality of the wood within its species. The boards will be inspected for knots, cracks and wain or bad cuts. A perfect board would be a #1 Grade because it can withstand the maximum loads that this type of tree can be rated at. If a board has lose knots or cracks it may be rated #2 or general purpose and will cost less. (remember don’t cheat on the grade because building inspectors look for the stamps and will fail you if you use weaker lumber to save a few dollars.)

    Along with manufacturers many associations have taken it as their duty to specify grades and strengths of different lumber that way carpenters and architects will know how to specify sizes of lumber to meet their needs.

    Not only is the size of the board important but also the number of knots, cracks, and wain.

    Additionally different species of trees will have different load properties.

    In the USA Spruce and Southern Yellow Pine are the two most common lumber types used for framing. After the wood is cut only an experienced carpenter can tell the difference between the two boards when they rest side by side.

    An inexperienced carpenter or do it yourselfer might not even check the species of wood and could just grab a load of 2″x10″ boards to build a deck frame. If the Architect specified #1 Grade 2×10 Southern Yellow Pine and you grabbed the #2 Grade Spruce you could end up with a deck that collapses under the weight of your guests or a roof that caves in during a hard winter storm.

    Now that we know that Grade Numbers and Species are just as important as the physical size of the wood being used lets look at how you choose the right board from a span chart.

    Lets say you are building a deck for your home and you want its dimension to be 20 feet x 14 feet.

    Now obviously you are not going to purchase boards that are 20 feet long to use for your floor joist even if you could find them. What you will want to do is use 14 foot long boards and space them at the normal 16″ on center but what size board will you need to use?

    2×4″s are very weak so you know you can’t use them
    2×6″ are stronger but are they strong enough?

    If you examine the chart below you will see that in order to use a 14 foot long board and space it 16″ apart to build your floor joist frame you will need to use 2×10″ inch boards for your joists.

    You will also notice that we are looking at the Southern Yellow Pine Table.

    If we were looking at Spruce the rating may have required a 2×12″ because spruce is a softer and weaker board.

    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 2×10 will reach up to 16″ feet 4″ inches long if we install them at 16″ inches on Center.

    If we wanted to use 2×8″ lumber we could only build our deck 13 feet 1 inches instead of the full 14 feet that we want to build it.

    A way around this and to still use 2×8 lumber would be to attache one side of the joist to the house and then place a beam under the 2×8 at the 12 foot mark. This would mean the final 2 feet of our 14 foot deck would cantilever 2 feet past the beam under it. This is acceptable too.

    And the final way to still use 2×8 lumber would be to decrease the span between the joists. If we used 2×8 boards placed 12 inches apart they would make it to 14 feet 5 inches.

    Always refer to an architect or engineer if you are having problems finding the right board for the right job. There are a number of new technologies such as Wood I-beams and laminate products that can be used in some situations. Conventional techniques of using steel are also useful.

    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.

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