How Much Weight can a Beam or Post Hold

At its structural core, the style of building that we do consist of beams and columns. It was the ancient Romans, the same people who gave us arches, who 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.

Just to make sure we’re speaking 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 very useful, except perhaps as a large 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 up the space into usable areas, as well as to protect 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, by splitting them up into floor joists and the 2”x 4”s used in building walls. Nevertheless, the same construction principles apply, as it is the overall combined profile of the structural elements that provide the strength. Two 2”x 4”s aren’t quite as strong as a 4”x 4”, but in many circumstances they are close enough to work.

Wood Columns or Posts

Wood is strongest 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 gain running perpendicular to its length, simply because trees don’t grow that way. When that post is stood on end, the weight is supported along the length of the long grain, where it can support that load.

On one hand, posts and columns are the same thing, vertical structural elements, which are used to support weight. However, while all columns are posts, not all posts are columns. Columns are generally round and are more decorative than posts, while posts are intended to support the 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 actually stronger than square posts which are made of the same amount of material. This is because a circle has no corner 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 amount of weight. For construction, we are usually looking at some sort of conifer, like Douglas Fir, for which the characteristics are well known. 
  • Quality of the wood – We normally 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 obviously 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 don’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 enough, the Uniform Building Code (UBC) does not provide specific load rating information for posts, while it does for beams and joists. This is probably because the UBC deals with minimums and normal 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 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” as well.

SpeciesPost Length

No. 1 Lumber

No. 2 Lumber

Stud

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-Larch2’2039231356300041838428237210271070116233
4’173422994929194157932703620547983015809
6’117882696627586109362450419612794214971
8’7484221052484170002032618032563813541
10’5004168302098946941563515721393411514
12’354812648169023332118151303528409324
14’2636967513421247890621055021307530
Hem-Fir2’1833828213254941757827127172761004915266
4’155232691724770144772571316894912714823
6’10465241612332393432266716154715513935
8’6620197002084958101793514906494612416
10’4422149321743638541329013070341310337
12’31331119613922272798531090224528224
14’232885541100020237487886018346484

Please note that these numbers will vary with other species of wood. As it may be difficult to determine what species 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 keep in mind that pressure-treated lumber may have considerably different numbers, often lower due to the damage to the wood caused by pressure treating.

beams, columns, wooden
Wooden beams and columns, Ryo Chijiiwa

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 in order to support the required weight.

Wood beam requirements in the UBC aren’t based upon the failure point of the wood, but rather how much deflection the weight causes in the wood. Deflection can lead to failure, but it also makes a floor feel spongy when walking on it, a decidedly uncomfortable feeling. The maximum limitation allowed is calculated by taking the span and dividing it 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 are supporting floors being different from beams that are supporting roofs.

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

GradeNominal Size4’6’8’10’12’14’

Select Structural

2x6116962435222613685
2x81703992667363252182
2x1024671361846541376276
2x12347117881137723505371
2x14484922911420909631464
4x61210733412263159100
4x817611026716458318228
4x1025521407971689489351
4x12359018501246939649477
4x145015237015511153818601
6x61701886498319211132
6x826001505926593439322
6x103748205114121014705518
No. 12x694241923515110576
2x81509672378242168123
2x1022591002564361251184
2x1230321348758465336248
2x1437861682946606421309
4x6109848827517812290
4x81761549477308212156
4x1025521275717459319234
4x1235901730973623432318
4x14490721811227785545401
6x615947083399255177130
6x826001318741474329242
6x10374820511338856595437
No. 22x68483772121359469
2x81360604340218151111
2x1022591002564361251184
2x1230321348758485336248
2x1437861682946606421309
4x699044024715611081
4x81719764430275191140
4x1025831148646413287211
4x1235021557876560389296
4x14441619671104707491361
6x693041323314910375
6x81729768423277192141
6x1034691542867564385283
GradeNominal Size4’6’8’10’12’14’
Select Structural2x692356033019711572
2x81345783529338235165
2x1019481074741502351258
2x1227411412951679471347
4x695557938523113384
4x81391810571427297192
4x1020151111767586446299
4x1228341460983741595445
No. 12x692340823014710267
2x81345655368236164120
2x101948977550352245180
2x1227411314739473328241
4x695547626817211879
4x81391810466298207152
4x1020151111700448311228
4x1228341460949607422310
No. 22x68483772121359469
2x81360604340218151111
2x102029902507325225166
2x1227291213682437303223
4x693441523415010468
4x8139172240626018068
4x1020151064610390271199
4x1228341460827529368270

This however, does not answer the question of what size joists need to be used, according to the UBC, for 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 SizeSpacing O.C.Floor JoistsCeilingRafter-Ceiling Joist Combination with Drywall Below

Rafters

With Drywall Below

Light RoofTile RoofLight RoofTile Roof
2x4126’-9”12’-5”9’-6”8’-2”10’-10”9’-10”
166’-2”11’-3”8’-8”7’-5”9’-10”8’-6”
245’-4”9’-10”7’-4”6’-5”8’6’-11”
2x61210’-8”19’-6”15’-1”12’-11”16’-7”14’-4”
169’-8”17’-8”13’-3”11’-8”14’-4”12’-5”
248’-1”14’-10”10’-10”9’-6”11’-9”10’-2”
2x81214’-1”25’-8”19’-6”17’-2”21’18’-2”
1612’-7”23’16’-10”14’-10”18’-2”15’-9”
2410’-3”18’-9”13’-9”12’-1”14’-10”12’-10”
2x101217’-9”>26’23’-10”21’25’-8”22’-3”
1615’-4”>26’20’-8”18’-2”22’-3”19’-3”
2412’-6”22’-11”16’-10”14’-10”18’-2”15’-8”
2x121220’-7”>26’>26’24’-5”>26’25’-9”
1617’-10”>26’24’21’-1”25’-9”22’-4”
2414’-6”>26’19’-6”17’-2”21’18’-3”
2x141223’>26’>26’>26’>26’>26’
1619’-11”>26’>26’23’-7”>26’25’-1”
2416’-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.

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