Determining proper beam sizes
Posted by Doug Lethin
When determining beam sizes for patio covers there are several things to consider:
The easiest way to calculate how far a beam can reach is to take the depth of the beam and convert this to feet. For instance a 4x6 beam will reach 6’, a 4x8 beam will reach 8’, and so on. This is a simple rule of thumb, but there are many other things to consider when planning proper sizes and beam spans…
- What is the type of covering you will be using?
Are there any additional loads to consider?
- If the covering is polycarbonate/plastic or fiberglass the weight of these coverings is considered lighter weight.
- Plywood with composition roofing is much heavier than just a plastic covering material.
There are some very helpful web sites that have information for beam calculations. Sometimes these beam sizing calculations are for decks and/or floor loads. Decks and Floor loads are typically higher then what a roof load is. Caution should be used and always error on the stronger selection when choosing materials.
The people that work at the help desks at most contractor lumberyards or home centers have information that will help in determining beam sizes.
Of course, engineers, architects and design professionals will be able to help also for a fee.
For online tables, you might try the following:
The span tables will show various beam sizes and configurations.
Beam availability varies by region of the country. In the Northwest, where a lot of timber is grown and manufactured, we are familiar with dimensional beams like 4x’s and 6x’s etc. We use the term 4x but we do know that the finish measurement is actually 3-1/2”. So for instance a 4x6 beam is actually 3-1/2” x 5-1/2”. The reason for this is that when lumber was cut it started out rough cut at an actual 4” x 6” measurement, but after plaining and finishing the beam has been reduced in size.
Beams in many parts of the country are made by laminating or joining 2x materials together. For instance to create a beam someone might join two 2x6’s together.
There are also a variety of other beam choices like Glu-Lam beams and other manufactured beams, consult with your local building materials dealers.
The wood species of the lumber used also varies and different wood species has different load carrying capabilities. For instance a Douglas fir beam will be able to carry more weight than a Southern Yellow Pine beam.
We could hope to have a simple answer to the question: “What size beam do I use?” but the variety of projects, designs and material choices by region do not allow for a simple answer.
- Could there be snow? How much potential weight could there be based on your geographical area? Some local building jurisdictions have ways they have addressed snow loads and figured out the minimum standards for specific areas.
- How far will the rafters be spanning?