Contents:
Overview | Terms | Span Table | Values for Douglas Fir | Material dead weight | Links

Overview:
The information below is primarily for floor and ceiling joists.
My brother put too much stuff in the attic over his garage and cracked a beam.
I just put plywood on the floor of the attic of the California home where I'm moving,. (No basements in California) so thought I should see how much it would hold.
I thought I could go to the Internet and find a table tha given the type of joists e.g. 2x6s on 16" centers and span e.g.12 ft which would tell me how much weight it would hold.
No such luck. 6 hours later I think I have a rough idea.
The formula to compute weight from span and construction type is very complicated. There are no automatic calculators on the web.
I had to look at tables for modulus of elasticity for No. 2 Douglas Fir and things like that. Then the tables (e.g below) show maximum span for a given weight (10, 20, 30, ...Lbs) and acceptable sag (deflection) by type of lumber.

Of course it depends on how you place things. Placing heavy objects at the ends will not put as much stress on the joists as weight in the middle.

```TERMS
Furniture, appliance, pelople, wind, snow etc.

The weight of the structure
Construction materials, plumbing, etc.

Values for residential buildings
Floors
live load = 30-40 lbs/sf (psf)
Ceiling
Since it is possible to use the attic for storage, the live load of the attic floor
is set at 20 psf according to code.
```
Strength and stiffness are the main criteria in determining load capacity. If a floor is strong enough to avoid breaking but not stiff enough, it may flex under load causing a plaster ceiling below to crack.

Maximum deflection limits are set by building codes. They are expressed as a fraction; clear span in inches (L) or () over a given number. For example: a floor joist appropriately selected to span 10 feet with an L/360 limit will deflect no more than 120"/360 = 1/3 inch under maximum design loads.

```Examples of live load values are:
Living room floors L/360 & 40 psf
Bedrooms and habitable attic floors L/360 & 30 psf
Attic no storage 10 psf
Attic floors with limited storage L/240 & 20 psf.
```
E value or modulus of elasticity of the individual elements. E is a ratio that relates the amount a given load causes a material to deform. A material with a higher E value is stiffer. For example: No.2 grade eastern white pine has an E value of 1,100,000 and No.2 hem-fir has an E value of 1,300,000. Hem-fir is a stiffer material.
The local lumber company said a 50 year old home here in N. California probably used No 2 Douglas Fir.
He said it's possible the grade is stamped on the lumber.
No. 1 Douglas Fir has an E value 1,700,000 psi
No. 2 Douglas Fir has an E value 1,600,000 psi
See Allowable Property Values | West Coast Lumber Inspection Bureau
and

Fb value or extreme fiber stress in bending. Loads cause beams, joists and rafters to bend. As a beam bends the outermost (extreme) fibers are compressed along the top edge. And at the same time, fibers stretch along the bottom edge. The outermost (extreme) wood fibers on the top and bottom surfaces are stressed more than those fibers in the middle. An Fb value indicates design strength for those extreme fibers. The higher the Fb the stronger the wood.

No. 1 Douglas Fir has an Fb value value 1,300 psi
No. 2 Douglas Fir has an Fb value value 1,170 psi
See Span Tables for Joists and Rafters | American Wood Council (AWC)

Lumber grade. A higher grade of a given species has a higher strength rating (Fb) and often has a higher stiffness value (E) too.

Species of wood. All species are not created equal. For example southern pine is much stronger and stiffer than spruce.
No. 2 grade Douglas-fir is common in California construction.

A typical span table (Source: UMass Amherst) (Douglas fir No. 2 2x6 highlighted)
Interpreting the table: The maximum span for Doug Fir with E=1,600,000 is 9' 9"
Fb = 1255 psi
 FLOOR JOISTS WITH L/360 DEFLECTION LIMITS DESIGN CRITERIA: Deflection – For 40 PSF live load. Limited to span in inches divided by 360. N/360 Strength – Live load of 40 psf plus dead load of 10 psf determines the required bending design value. Joist Size (in.) Spacing (in.) Modulus of Elasticity, E, in 1,000,000 psi 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 2×6 12.0 8-6 8-10 9-2 9-6 9-9 10-0 10-3 10-6 10-9 16.0 7-9 8-0 8-4 8-7 8-10 9-1 9-4 9-6 9-9 19.2 7-3 7-7 7-10 8-1 8-4 8-7 8-9 9-0 9-2 24.0 6-9 7-0 7-3 7-6 7-9 7-11 8-2 8-4 8-6 2×8 12.0 11-3 11-8 12-1 12-6 12-10 13-2 13-6 13-10 14-2 16.0 10-2 10-7 11-0 11-4 11-8 12-0 12-3 12-7 12-10 19.2 9-7 10-0 10-4 10-8 11-0 11-3 11-7 11-10 12-1 24.0 8-11 9-3 9-7 9-11 10-2 10-6 10-9 11-0 11-3 2×10 12.0 14-4 14-11 15-5 15-11 16-5 16-10 17-3 17-8 18-0 16.0 13-0 13-6 14-0 14-6 14-11 15-3 15-8 16-0 16-5 19.2 12-3 12-9 13-2 13-7 14-0 14-5 14-9 15-1 15-5 24.0 11-4 11-10 12-3 12-8 13-0 13-4 13-8 14-0 14-4 2×12 12.0 17-5 18-1 18-9 19-4 19-11 20-6 21-0 21-6 21-11 16.0 15-10 16-5 17-0 17-7 18-1 18-7 19-1 19-6 19-11 19.2 14-11 15-6 16-0 16-7 17-0 17-6 17-11 18-4 18-9 24.0 13-10 14-4 14-11 15-4 15-10 16-3 16-8 17-0 17-5 Fb Fb Fb Fb 12.0 718 777 833 888 941 993 1043 1092 1140 16.0 790 855 917 977 1036 1093 1148 1202 1255 19.2 840 909 975 1039 1101 1161 1220 1277 1333 24.0 905 979 1050 1119 1186 1251 1314 1376 1436 Note: The required bending design value, Fb, in pounds per square inch is shown at the bottom of each table and is applicable to all lumber sizes shown. Spans are shown in feet – inches and are limited to 26′ and less. Check sources of supply for availability of lumber in lengths greater than 20′. EXCERPTED FROM SPAN TABLES FOR JOISTS AND RAFTERS, Copyright © 1993 AMERICAN FOREST & PAPER ASSN., WASHINGTON, D.C.

```Other values for Douglas Fir No. 2 2x6 16" on center
per 2015 Span Table at AWC
Ceiling joists
10 psf live load L/240 17-8
20 psf live load L/240 14-1  (flex for 12' is 0.6 in)

30 psf live load L/360: 10-9  span
40 psf live load L/360: 9-9  span
50 psf live load L/360: 9-1 span

Simplified Span Table at West Coast Lumber Inspection Bureau
No. 2 Douglas Fir - Ceiling Joists - 10 PSF Dead Load / 20 PSF Live Load /
Drywall Ceiling / Limited Attic Storage / 16" on center

2x4 7 - 10

2x6 12 - 3

2x8 16 - 2

2x10 20 - 2

2x12 23 - 4

1/2" Sheetrock= 1.6 lbs/sf
1/2" plywood = 1.4 lbs/sf
Some Live Weight items
Stack of books 3" x 6" x 1'  43 lbs/sf
Material Strength - Forces - Structures
2015 Span Tables2 | American Wood Council (AWC)
Span Tables for Joists and Rafters | American Wood Council (AWC) | American Forest & Paper Association (AF&PA)
Wood beam calculator | Cornell
Understanding Loads and Using Span Tables | Building and Construction Technology | UMass Amherst
Calculating Loads on Headers and Beams | Building and Construction Technology | UMass Amherst
American Forest and Paper Association Home
How to Calculate Load Bearing Beams | Hunker

last updated 1 May 2017
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