- 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.
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
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)
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)
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:
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.
Protect Attic Space
- Blocks with ventilation holes typically have wire mesh on one side to prevent animals from entering the attic space.