Should floor joists be 2X6 or 2X8?

There is a commonly used ,old fashion method, of ensuring that the proper size joists or headers are used. If the span is 6 feet or less, use 2X6's, if the span is 8 feet or less, use 2X8's, less than 10 feet, 2X10's, etc.

builder-questions.com - 2x8 vs 2x10 Floor Joists for Single Story Home - Builder-Questions.com

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Question: I was wondering would 2×10 floor joists 16 o.c. spanning 8′ on a single story home 32′ wide, be an overkill? Or should I use 2×8 joists? I want a sound floor but at the same time don’t want to over do it.

Answer:

2 X 8 joists spanning only 8 feet is more than enough. However, with this type of situation, the heavier the better, so you will have a very stable and secure platform with the use of 2 X 10’s. Depending on the length of the home and the number of floor joists, the additional cost for the heavier floor joists, may not be very substantial. There is a commonly used ,old fashion method, of ensuring that the proper size joists or headers are used. If the span is 6 feet or less, use 2X6’s, if the span is 8 feet or less, use 2X8’s, less than 10 feet, 2X10’s, etc. However it is important to understand, that all substantial spans, should be properly engineered, especially with the more contemporary structures with open spaces and longer spans. If you are truly looking to stabilize the floor, an additional layer of sub-floor will offer the most stability and strength. We like to go with a 5/8 ” tongue and groove plywood as an initial sub-floor, running perpendicular to the joists, and then a second sub-floor of plugged and sanded, 1/2″ tongue and groove plywood running in the opposite direction. This assembly will provide a stable floor that will not flex, squeak or move. In addition, there are many different synthetic floor decking products that have been developed over the years that will add to the floor strength. Commercially we also use a product known as gyp-crete which is a gypsum product that is actually poured over the sub-floor to soundproof, fireproof and provide additional density to the floor assembly.

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How far can a 2x4 span as a floor joist?

So, for 2x4 at 12” oc the maximum span is about 7'-6”, which is about the distance you need. (If you go to C24 it could be longer.)

diy.stackexchange.com - What is the max span that can be supported by 2x4 floor joists?

We had old 2x4 floor joists sitting in our bathroom that we ripped out as they were rotten and falling apart. 2x4 is ideally our limit of thickness, and I was thinking of having 4 or 5 2x4 joists going from one end of the wall to another (147cm / 58") bolted to a 2x4or 2x6 (not sure which one) bearer against all 4 walls. The bearers will be thunderbolted to the brick walls and joist hangers will be used for the 2 by 4s. I have a very poorly drawn picture from myself to illustrate the 2 by 4 joists. We're thinking about putting the joists 30cm (12") OC.