Today we’ll look at light frame construction –

which is typical for building houses in the United States – and the loads it must

withstand. Light frame construction consists of repetitive

members like wood rafters or trusses for the roof

structure wood studs for walls and wood joists for floors. We use a shorthand to describe the combined

loads a certain weight per square foot called a “Uniform Load”. The calculation is load in pounds, times one

square foot, equals uniform load in pounds per square foot. Both roofs and floors carry Live Loads and Dead

Loads. Live loads are temporary like snow, furniture, and people. Dead Loads are permanent, like the weight of

building materials. Uniform Loads area a combination of Live Loads

and Dead Loads. Think of Uniform Load as a weight, like a sand

bag sitting on each square foot of roof. It’s called a uniform load because the repetitive

members and the load they carry are evenly

distributed across the roof Loads start at the roof. Structural repetitive

members at the roof, rafters or trusses, transfer horizontal loads to

beams or walls. Additional loads occur at floors, and repetitive

structural members (floor joists) transfer the loads to beams, walls or columns. All of these loads are eventually transferred to

the foundation system. The equation we use to calculate total loads for

horizontal surfaces like roofs and floors, is the uniform load in

pounds per square foot times the Tributary Area in square feet which equals Total Uniform Load

in pounds. For the area, we need both the width and the

length. However, these uniform loads can also be

expressed as pounds per linear foot which is how we measure loads on walls and

beams. To get this number we need to identify the

Tributary Width. When we multiple units in pounds per square

feet by units in feet, we simplify by deleting the feet from the equation and the resulting units are in feet. Tributary width is the horizontal measure

between support points. This means that the width measured is the

same whether it’s flat like a floor or sloped like a

roof, and the slope can be a little, or a lot! The tributary width remains the same. To determine tributary width for roofs, we first

need to consider the type of roof structure and

how it transfers it’s loads The three types of roof structure are trusses, rafters with a ridge board, rafters with a ridge beam. Both trusses and rafters with a ridge board

transfer roof loads in the same way so we’ll look

at them together. It’s important to remember that the ridge board

is nonstructural and merely provides lateral

stabilization to the rafters at their peak. To calculate tributary width for trusses and rafters which transfer loads to exterior walls we measure horizontally from the ridge to the edge of the roof. Note that we measure half of the roof width

because half the roof transfers its load to one support wall and the other half transfers its

load to the other support wall. We include eaves in this calculation. Later, we’ll look at eaves separately. This example has a roof that is 19 feet wide. Half the roof, from ridge to eave, is 9′-6″ linear feet. The uniform load can be looked up in the Building Code. If this house is in Oregon, the live load (snow

load) would be 25 pounds per square foot. The dead load is 15 pounds per square foot. Remember live plus dead equals uniform load! Remember live plus dead equals uniform load! for this example, in Oregon, 25 pounds per square foot plus 15 pounds per

square foot equals 40 pounds per square foot

uniform load. Our uniform load per linear foot will be our uniform load times our tributary width. Our load is 40 pounds per square foot and our width is 9′-6″, which equals 380 pounds

per linear foot on each supporting wall. Here’s an example of the same house with

rafters and a load bearing ridge beam. Each rafter transfers half of its uniform load to

the structurally significant ridge beam which sits

on columns or end walls and the other half to an exterior wall. Our loads are supported by more elements in

this type of structure. First let’s establish tributary width for the load on

the beam. Measure horizontally from the beam to halfway

to the exterior wall. Note that it’s 1/4 of the width from outside wall

to outside wall. In this case 1/4 of the width

equals 4 feet. Measure again on the opposite side of the ridge

beam to a point halfway to the exterior wall and

add the two numbers to get the tributary width. Our uniform load per linear foot will be our uniform load times our tributary width. Our load is 40 pounds per square foot and our width is 8 feet which equals 320 pounds per linear foot on the

beam. Now let’s establish tributary width for the load on

the two walls. Measure horizontally, from halfway between the ridge and the wall to

the exterior wall. We noticed before that it’s 1/4 of the width from

outside wall to outside wall and in this case 1/4

of the length equals 4 feet. But what about the eave overhang – the portion

of the rafter outside the wall? We need to

include it because it rests on the wall as well. So measure horizontally again, from the outside wall to the edge of the rafter. Our overhang is 1′-6″. Add the two numbers together to get the

tributary width. 4 feet plus 1 foot six is 5 foot six. Our uniform load per linear foot on the wall will be our uniform load times our

tributary width. Our load is the same as before, 40 pounds per

square foot, and our width is now 5 foot six. 40 pounds per square foot times 5 foot six is 220 pounds per linear foot on each supporting

wall. Let’s revisit Tributary Area before we end our

discussion on roofs. Remember the equation Remember the equation we use to calculate

total loads for horizontal surfaces, like roofs and

floors, is the uniform load in pounds per square foot, times the Tributary

Area in square feet, which equals total uniform

load in pounds. in pounds per square foot, times the Tributary

Area in square feet, which equals total uniform

load in pounds. For the area we need both a tributary width and

a tributary length. The example with trusses or rafters the ridge

board has two tributary widths, therefore two

tributary areas. Each area is 9′-6″ (tributary width) multiplied by the 19 foot length for total of 180.5 square feet. Multiply the square feet by the uniform load for a

total of 7220 pounds. This is the load of half the roof that rests on one

exterior wall. The total roof load on the small house is 14,400

lbs. This is true for both a truss roof system and rafters with a ridge board. For the example of the rafters with the ridge

beam there are three tributary widths and

therefore three tributary areas. The ridge beam’s tributary width is 1/2 total span

or 8 feet the length of the roof is 19 feet the tributary area is 104.5 square feet the ridge load is 6080 pounds one wall supports 4480 pounds and so does the other. The combined load on the small house is once again 14,400 lbs We hope you learned something today! And thanks for watching!

thanks

Thanks, Michelle!

This really helped. Thanks a lot!

love it!!! thank you so much! 🙂

thanks for the great video with clear explanation

I don't get why ridge beam would carry so much more load than the ridge board. In the picture they both have horizontal bracing.

psf or pounds per square foot is equal to load/Area not multiplication

I would like to calculate how big a ridge beam has to be. Any tips? Thanks.

Michelle!!!! I don't know if you'll ever see this comment. I've had you sub before in Residential codes and you were fantastic. THis video with our new Structures one instructor is difficult so far. THis video saved my homework life!!!!! Thank you!!!!

Great simple illustrations giving the basics. Makes it easy to understand

great video…expecting from subject from u …

great video…expecting more subject from u ..thanku

Great Video! It worths my subscribtion. Thanks

she have a stuffy nose?

Excellent explanation!!! Wish you had more on the topic of structural load analysis like this. Great job!!

Really like the sketchup model as a visual aid. Great explanation.

Do you guys have a video of wind uplift and the needed tie-down PSF required?

I didn't understand the purpose of such roof calculation usage in construction.

Minor correction at 9:50 – 8×19=152, and at 9:54 it should say 152 x 40 = 6,080.

I guess that's why teachers always tell us to show our work.

well explained, thank you

I'm trying to build a screened in porch on top of an existing deck and trying to calculate the linear load on the joist to know if I need to add deck beams. This was helpful but it would be great if it showed the assumed material sizes i.e. 2×6 or 2×8.

10 people don't like load paths? lol

She sounds like a Libriviox audiobook reader 😀

No offense