How Much Weight can a Beam or Post Hold

The style of building we do consists of beams and columns at its structural core. The ancient Romans, the same people who gave us arches, came up with the idea of using timbers as structural elements in construction. The first lattice truss bridges were built by the Romans using wood timbers with metal fasteners. Since then, the use of wood in structural construction has increased exponentially to the point where it is the primary structural material used in homes and other smaller buildings.

To ensure we speak the same language, a beam is a horizontal structural element, and a column or post is a vertical one. An entire structure can be built out of beams and columns, but it wouldn’t be beneficial, except perhaps as a giant jungle gym. Those beams and columns will support the weight, but they need floors and walls to hold the weight and transfer it to the beams and columns. They need walls to break the space into usable areas and protect it from inclement weather.

We don’t typically see these structural elements as they are hidden inside walls, floors, and ceilings unless they are intentionally left exposed as design elements. But in many cases, those beams and columns are hidden in another way, splitting them into floor joists and the 2”x 4”s used in building walls. Nevertheless, the same construction principles apply, as the overall combined profile of the structural elements provides the strength. Two 2”x 4”s aren’t quite as strong as a 4”x 4”, but they are close enough to work in many circumstances.

Wood Columns or Posts

Wood is most robust when the load is parallel to the long grain. That also happens to fit the way that wood grows and is cut. There’s no such thing as a 4”x 4” post with the grain running perpendicular to its length simply because trees don’t grow that way. When that post is on end, the weight is supported along the size of the long grain, which can help that load.

On the one hand, posts and columns are the same—vertical structural elements are used to support the weight. However, while all columns are posts, not all posts are columns. Columns are generally round and more decorative than posts, while posts are intended to support weight without providing any aesthetics to the overall structure.

Another way of looking at this is that posts are square or rectangular, while columns are round. Being round, columns are more substantial than square posts made of the same amount of material. This is because a circle has no corners to be weak points. Additionally, the circle places more material at a greater distance from the center than a square does.

How much weight that post can handle is dependent on four factors:

• The type of wood – Not all woods have the same density or grain structure; therefore, they don’t all support the same weight. For construction, we usually look at some conifers, like Douglas Fir, for which the characteristics are well known. 
• Quality of the wood – We usually think of wood grading as affecting the appearance of a board, but those knotholes also affect the strength of the wood. 
• The dimensions of the post’s cross-section – A larger cross-section will support more weight. 
• The unbraced length of the post—Wood is flexible, and too much weight can cause it to flex, putting undue stress on the grain and leading to buckling. Bracing the post doesn’t add any additional strength to the post itself but can help prevent it from flexing, thereby allowing it to hold more weight safely. This becomes more important as longer posts are used. 

Interestingly, the Uniform Building Code (UBC) does not provide specific load rating information for posts, but it does for beams and joists. This is probably because the UBC deals with minimums, and standard construction techniques cover those minimums. If it were to provide that information, it would probably define the minimum number of posts to be used in a particular-sized area. Dealing with the spans for posts and joists more than covers those minimums.

Most posts in home construction are either 4”x 4” or two 2”x 4” studs put together to make a 4”x 4”. However, there are some situations in the home building where larger posts might be used. Therefore, the table below also covers the maximum weight in pounds for 4”x 6” and 6”x 6”.

Species	Post Length	No. 1 Lumber			No. 2 Lumber			Stud Check the price on Amazon
		4”x 4”	4”x 6”	6”x 6”	4”x 4”	4”x 6”	6”x 6”	4”x 4”	4”x 6”
Douglas Fir-Larch	2’	20392	31356	30004	18384	28237	21027	10701	16233
	4’	17342	29949	29194	15793	27036	20547	9830	15809
	6’	11788	26966	27586	10936	24504	19612	7942	14971
	8’	7484	22105	24841	7000	20326	18032	5638	13541
	10’	5004	16830	20989	4694	15635	15721	3934	11514
	12’	3548	12648	16902	3332	11815	13035	2840	9324
	14’	2636	9675	13421	2478	9062	10550	2130	7530
Hem-Fir	2’	18338	28213	25494	17578	27127	17276	10049	15266
	4’	15523	26917	24770	14477	25713	16894	9127	14823
	6’	10465	24161	23323	9343	22667	16154	7155	13935
	8’	6620	19700	20849	5810	17935	14906	4946	12416
	10’	4422	14932	17436	3854	13290	13070	3413	10337
	12’	3133	11196	13922	2727	9853	10902	2452	8224
	14’	2328	8554	11000	2023	7487	8860	1834	6484

Please note that these numbers will vary with other species of wood. As it may be challenging to determine what species of construction-grade dimensional lumber purchased at the lumberyard is, prudence dictates that a lower weight rating should be assumed when the actual species is unknown. Also, remember that pressure-treated lumber may have considerably different numbers, often lower due to the damage to the wood caused by pressure treating.

Wood Beams

Determining the weight rating of wood beams is considerably more complex than determining that of columns. The prime difference between the two is that rather than the unbraced length of the post. We have to consider the span that the beam is covering between posts. The longer the span, the less weight the beam can support or the greater the cross-section of the beam required to support the required weight.

Wood beam requirements in the UBC aren’t based on the failure point of the wood but rather on how much deflection the weight causes in the wood. Deflection can lead to failure, but it makes a floor feel spongy when walking on it, a decidedly uncomfortable feeling. The maximum limitation allowed is calculated by dividing the span by 360, with a maximum allowable deflection of 20mm. It gets a bit more complicated than that in that the use affects what is required, with beams or joists that support floors being different from beams that support roofs.

The table below shows the maximum weight that various structural beams or joists can support based on their size, material grade, and span.

Grade	Nominal Size	4’	6’	8’	10’	12’	14’
Select Structural	2x6	1169	624	352	226	136	85
	2x8	1703	992	667	363	252	182
	2x10	2467	1361	846	541	376	276
	2x12	3471	1788	1137	723	505	371
	2x14	4849	2291	1420	909	631	464
	4x6	1210	733	412	263	159	100
	4x8	1761	1026	716	458	318	228
	4x10	2552	1407	971	689	489	351
	4x12	3590	1850	1246	939	649	477
	4x14	5015	2370	1551	1153	818	601
	6x6	1701	886	498	319	211	132
	6x8	2600	1505	926	593	439	322
	6x10	3748	2051	1412	1014	705	518
No. 1	2x6	942	419	235	151	105	76
	2x8	1509	672	378	242	168	123
	2x10	2259	1002	564	361	251	184
	2x12	3032	1348	758	465	336	248
	2x14	3786	1682	946	606	421	309
	4x6	1098	488	275	178	122	90
	4x8	1761	549	477	308	212	156
	4x10	2552	1275	717	459	319	234
	4x12	3590	1730	973	623	432	318
	4x14	4907	2181	1227	785	545	401
	6x6	1594	708	3399	255	177	130
	6x8	2600	1318	741	474	329	242
	6x10	3748	2051	1338	856	595	437
No. 2	2x6	848	377	212	135	94	69
	2x8	1360	604	340	218	151	111
	2x10	2259	1002	564	361	251	184
	2x12	3032	1348	758	485	336	248
	2x14	3786	1682	946	606	421	309
	4x6	990	440	247	156	110	81
	4x8	1719	764	430	275	191	140
	4x10	2583	1148	646	413	287	211
	4x12	3502	1557	876	560	389	296
	4x14	4416	1967	1104	707	491	361
	6x6	930	413	233	149	103	75
	6x8	1729	768	423	277	192	141
	6x10	3469	1542	867	564	385	283

Grade	Nominal Size	4’	6’	8’	10’	12’	14’
Select Structural	2x6	923	560	330	197	115	72
	2x8	1345	783	529	338	235	165
	2x10	1948	1074	741	502	351	258
	2x12	2741	1412	951	679	471	347
	4x6	955	579	385	231	133	84
	4x8	1391	810	571	427	297	192
	4x10	2015	1111	767	586	446	299
	4x12	2834	1460	983	741	595	445
No. 1	2x6	923	408	230	147	102	67
	2x8	1345	655	368	236	164	120
	2x10	1948	977	550	352	245	180
	2x12	2741	1314	739	473	328	241
	4x6	955	476	268	172	118	79
	4x8	1391	810	466	298	207	152
	4x10	2015	1111	700	448	311	228
	4x12	2834	1460	949	607	422	310
No. 2	2x6	848	377	212	135	94	69
	2x8	1360	604	340	218	151	111
	2x10	2029	902	507	325	225	166
	2x12	2729	1213	682	437	303	223
	4x6	934	415	234	150	104	68
	4x8	1391	722	406	260	180	68
	4x10	2015	1064	610	390	271	199
	4x12	2834	1460	827	529	368	270

This, however, does not answer the question of what size joists need to be used, according to the UBC, for the construction of a home. That depends largely upon how those joists are going to be used. The table below provides maximum spans, according to the UBC, for double 2” Douglas fir-Larch or better dimensional lumber joists and rafters.

Nominal Lumber Size	Spacing O.C.	Floor Joists	Ceiling	Rafter-Ceiling Joist Combination with Drywall Below		Rafters
		With Drywall Below Check the price on Amazon		Light Roof	Tile Roof	Light Roof	Tile Roof
2x4	12	6’-9”	12’-5”	9’-6”	8’-2”	10’-10”	9’-10”
	16	6’-2”	11’-3”	8’-8”	7’-5”	9’-10”	8’-6”
	24	5’-4”	9’-10”	7’-4”	6’-5”	8’	6’-11”
2x6	12	10’-8”	19’-6”	15’-1”	12’-11”	16’-7”	14’-4”
	16	9’-8”	17’-8”	13’-3”	11’-8”	14’-4”	12’-5”
	24	8’-1”	14’-10”	10’-10”	9’-6”	11’-9”	10’-2”
2x8	12	14’-1”	25’-8”	19’-6”	17’-2”	21’	18’-2”
	16	12’-7”	23’	16’-10”	14’-10”	18’-2”	15’-9”
	24	10’-3”	18’-9”	13’-9”	12’-1”	14’-10”	12’-10”
2x10	12	17’-9”	>26’	23’-10”	21’	25’-8”	22’-3”
	16	15’-4”	>26’	20’-8”	18’-2”	22’-3”	19’-3”
	24	12’-6”	22’-11”	16’-10”	14’-10”	18’-2”	15’-8”
2x12	12	20’-7”	>26’	>26’	24’-5”	>26’	25’-9”
	16	17’-10”	>26’	24’	21’-1”	25’-9”	22’-4”
	24	14’-6”	>26’	19’-6”	17’-2”	21’	18’-3”
2x14	12	23’	>26’	>26’	>26’	>26’	>26’
	16	19’-11”	>26’	>26’	23’-7”	>26’	25’-1”
	24	16’-3”	>26’	21’-10”	19’-3”	23’-7”	20’-5”

Please note that longer lengths of lumber, especially those over 20’ in length, may be hard to find. This table does not take that into account. Lengths over 26’ are not specified, as dimensional lumber is not sold in these lengths.