Heel blocking is a commonly used method to transfer in-plane lateral loads from the braced wall panel to the roof diaphragm. Heel blocking is installed between roof truss heels at the top of the exterior wall. Structural engineers may have questions regarding the prescriptive code requirements or the capacity of commonly used heel blocking to achieve a desired load transfer. This guide reviews the basics of determining whether a heel block is structurally adequate. 

Step-by-Step

Intro

Introduction

  • Heel blocking (also known as bird blocking) is one of several methods for transferring lateral loads from the roof diaphragm to the shear walls.

  • The heel blocking is installed between roof truss heels at the top of the exterior wall.

Step 1

Vented or Unvented?

  • Attics or roofs can be designed and constructed to be either vented or un-vented in any climate
  • When vents are added to heel blocking, the structural capacity is reduced
  • Heel Blocking must be designed to account for removed material

Step 2A

Bird Block Design Example A: Slot

  • The truss manufacturer uses 2x6 beveled blocks
  • The block has a 2″x10″ letterbox type ventilation hole as shown

  • Imagine a horizontal plane cutting through the block at the location with the least material.
  • Calculate the area of the remaining material along that plane.
  • In this case it’s a total of 18.75 sq. in.
  • This is the area of block left to resist the shear forces being transferred from the roof diaphragm to the wall below.
  • To be conservative, we will use a value of Fv = 110 psi, which is for “Northern Species” lumber

  • The only adjustment factor to consider is load duration factor (Table 2.3.2 of the NDS).
  • We will use 1.6, since these forces are either caused by wind or seismic events.

  • Shear Capacity of the block = Fv' x Area

  • A 22.5″ block with 3300 lb of shear capacity would have the following shear load in pounds per linear foot (plf)

Step 2B

Bird Block Design Example B: Holes

  • Bird blocks can also be designed with holes instead of a slot.
  • The effective area is given by:

    Area = (L*B) - (n*d*B)

    where:

    L = Length of block
    B = Breadth of the block (thickness)
    d = Diameter of the hole
    n = Number of holes

  • A 22.5″ long block with (3) 3″ diameter holes, has the following effective area:

  • Total shear capacity will be:

  • Shear capacity in plf:

Step 2C

Bird Block Design Value Check

  • According to the APA’s Introduction to Lateral Design, the highest recommended load listed is 820 plf for roof diaphragms and 870 plf for shear walls.
  • Therefore, even a low grade bird block with a large horizontal ventilation opening or ventilation holes is adequate, provided the building designer properly details the roof-to-block and the block-to-wall connections.

Step 3

Protect Attic Space

  • Blocks with ventilation holes typically have wire mesh on one side to prevent animals from entering the attic space.