How to determine joist and rafter sizes

This on-line span calculator will size floor and deck joists and roof rafters for most conditions.

A very good Span Calculator from the American Wood Council. Open this link in another window and then follow the directions below to learn how to use it.

   The calculator screen

First, choose a species of wood - when the other information is input and you click calculate the program will give you the grade you need for the span. Second, input the working span of your joist or rafter. This is measured as the horizontal distance between supports.


If you can get joists to span over a beam or center support wall this will stiffen the floor but can not be calculated by this simple program (just put in the simple span - the calculator will give you a more conservative span length).


The deflection limit determines the "springiness" of the floor or roof. It is expressed as a ratio. L = length divided by a number. The larger the bottom number the stiffer the member will be. If you set a floor joist to L/360 (a common value) it will be allowed to deflect (bow or bounce) a maximum of 1" for every 360" (30') of span. For a 14' span that would be less than 1/2". Long span joists and engineered I-joists usually recommend 1/480 for deflection, but 1/360 is normal code minimum. None of this is a question of strength, only bounce - and that is a bit of a personal opinion.

The live loads are people and furniture, dead loads are the weight of the structural materials themselves. When you put up a tile roof you are increasing the dead load on the roof rafters. I have included some guidelines but you will want to check locally (building department, forest service, product specs, etc.) for material weights, snow loads and special conditions. Usually 20 psf is a minimum roof live load for non-snow areas.

Outside decks are usually designed for higher live loads than the interior of the house (think party!) 60 psf is common for these situations but check locally to confirm.

The Exterior Exposure options can be used for deck joist sizing.

You can also click the Maximum Span Calculator button to look at this somewhat differently. Perhaps you already have a joist or rafter of a certain type and size. This calculator will give you the maximum span for that member.

You will find this calculator to be a great help in figuring out the basic framing for your design.

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