In a house build to American building code standards, you would never have 2x4 joists. In a storage shed, the joists might be 2x4 but they are just there for stiffening the structure. They are not meant to support any weight. I personally wouldn't put more than 20 pounds on a single 2x4 joist that is say, 10 ft long.
2x4 11'-6" 2x6 17'-0" 2x8 21'-8" 2x10 25'-7" 24" O.C.
For average storage use, 2x4 16" O/C is a standard floor joists with good strength. For customers looking to increase their load capacity with standard 2x4 floor joists, then consider spacing your floor joists 12" O/C.
Wooden joists can support 50 to 100 pounds per square foot.
Ceiling joists shall be in accordance with the maximum spans for common species of solid sawn ceiling joists specified in Tables 3.25A-B, and shall ... Ceiling joists shall be continuous across the structure or securely joined where they meet over interior partitions in accordance with Table R802.
For a 2×4 used in wall framing (such as studs), the IRC recommends that a 2×4 can typically span up to 10 feet for non-load bearing walls, but for load-bearing walls, additional support or larger framing materials may be necessary.
It depends on how long it is. The longer the board, the less weight it can hold. In a house build to American building code standards, you would never have 2x4 joists.
The solution is simple... sister on an 18 gauge 1-5/8"x6 steel stud material to act as a joist running plate to plate. Screw the metal joist to the existing wood ceiling joist and to each plate and you have very effectively reduced the load deflection of any stored attic loads for little cost.
In general the US and Canadian code agreed giving a capacity of around 300 kg for a European 2 by 4 loaded at mid-span.
Is there a simple way to size floor joists? A. Carl Hagstrom responds: The rule of thumb that I like to use when sizing uniformly loaded residential floor joists is "half the span plus two." First, round the clear span of the floor joist up to the nearest foot, and divide by two. Then add two to the answer.
A 2×4 in a vertical position can support significant compressive loads, making it ideal for this application. However, the load capacity is also dependent on the spacing of the studs and the type of load (e.g., dead load or live load) placed on the wall.
When builders need to increase the load-bearing capacity of floor joists, they often use a process known as “sistering.” This process involves attaching a new joist alongside an existing joist to increase its strength.
If you consider a 2x4 (of Eastern White Pine, straight grained, no knots) as being a shelf that is 1.5" wide and 3.5" thick (in other words using it on edge), and using 4' spans, and putting a load of 500 lbs on it at the center, the sag would total . 1".
In almost all cases, ceiling joists and hanging joists are in place to stabilise and support the ceiling only. On their own, they are unable to support the full load of a pitched roof. If, on the other hand, you have a flat-roofed house, your ceiling joists may be load-bearing.
4x2 can span around 1.91mtrs, 5x2 span 2.56mtrs.
When a joist is weight-bearing, meaning it holds up a wall or has extra weight attached to it, it's sometimes necessary to sister multiple pieces of new material to the old one. This is usually done by “sandwiching” the old joist in the middle of the two new ones. Doing this gives additional support to the joist.
A rafter is a structural member that supports the roof deck. They are almost always not horizontal. A joist is a structural member that supports a structural portion of a building; usually a floor or a ceiling. They are almost always horizontal.
Rafters and ceiling joists having a depth-to-thickness ratio exceeding 6 to 1 based on nominal dimensions shall be supported laterally by solid blocking, diagonal bridging (wood or metal) or a continuous 1-inch by 3-inch (25 mm by 76 mm) wood strip nailed across the rafters or ceiling joists at intervals not exceeding ...
The maximum span that can be supported by 2x4 floor joists is 9'–10". The maximum span that can be supported by other types of floor joists includes: 2x6: 12'–6" 2x8: 20'–1"
Joists are the actual framing of the house, and they're very strong. You can hang really heavy weight weights from a ceiling joist with the right hardware—over 100 lbs.
But I would tell you that using 2x4s as joist is problematic. They are much weaker than 2x6s so you will have to have a much tighter joist spacing, certainly no more than 12" from center to center. Secondly you will have to have many, many deck blocks along the length of each 2x4.
Generally, a single 2x4 can support several thousand pounds of compressive load in the direction of its length. However, three 2x4s (especially if nailed together) will not triple this load-bearing capacity.
Typically ceiling joists are 2x6, 2x8, or 2x10, and all are 1.5" wide as mattimus1234 pointed out. You need to be careful when putting screws into the joists when you don't know what is above the ceiling ...