Structures Video Roof Loads

Today we’ll look at light frame construction – which is typical for building houses in the United States – and.


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!

23 thoughts on “Structures Video Roof Loads”

  1. 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.

  2. 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!!!!

  3. 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.

  4. 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.

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