Connection Detailing

As with any structural material, proper connection detailing is essential to assure the structural performance of the member. This is particularly true with glulam since an improperly designed and installed connection detail can lead to a serious failure. The designer must consider the effects of moisture changes in the glulam member, proper positioning of the mechanical fasteners, and the number of fasteners required to carry the loads to develop an adequate connection detail.

Based on many years of experience, the glulam industry has developed typical details for most connection situations. These details illustrate the right and wrong way to make these connections and indicate the consequences of an incorrect detail. Click here for illustrated details and descriptions. (.pdf download - 248 KB)

Rosboro Glulam

Glulam Guidelines and Specifications

Guidelines for Drilling Holes

For years there has been confusion in the field about whether holes can be safely drilled into or through glulam beams. It's true, you can damage a beam to the point of reducing its structural integrity by improperly drilling holes in the wrong places. But by adhering to Rosboro's Allowable Horizontal Hole Chart, holes can indeed be safely drilled into or through glulam beam products.

Zones where small horizontal holes are permitted in a uniformly loaded, simple supported beam

Allowable Horizontal Hole Chart


  1. The above diagram applies to horizontal holes and beams properly sized using APA or Rosboro uniform load tables. For concentrated load conditions, contact Rosboro Technical Support.
  2. Field holes should be drilled for access only (small diameter wires, conduit, cable and other lightweight materials) and not for load bearing hardware attachments unless designed specifically by the project engineer. Square and rectangular holes are not allowed.
  3. These field drilled holes should meet the following guidelines:
    • Hole size: The hole diameter should not exceed 1-1/2" or 1/10 the beam depth, whichever is smaller.
    • Hole location: The hole should have a minimum clear distance, as measured from the edge of the hole to the nearest edge of the beam, of 4 hole diameters to the top or bottom of the beam and 8 hole diameters from the end of the beam. Otherwise as shown in the shaded area above.
    • Hole spacing: The minimum clear spacing between adjacent holes, as measured between the nearest edge of the holes, should be 8 hole diameters based on the largest diameter of any adjacent hole in the beam.
    • Number of holes: The maximum number of holes should not exceed 1 hole per every 5 feet of beam length. In other words, the maximum number of holes should not exceed 4 for a 20-foot long beam. The hole spacing limitation, as given above, should be satisfied separately.
  4. For glulam members that have been over-sized, these guidelines may be relaxed based on an engineering analysis.
  5. Holes in cantilevered beams require additional analysis, contact Rosboro Technical Support.

The above illustration provides guidance for drilling horizontal holes without compromising the structural integrity of the glulam. For drilling vertical holes, the structural capacity of the glulam will be reduced. The following formula may be used to determine the reduction in the beam's load-carrying capacity: 1-1/2x (hole diameter) / (beam width) = the reduction. For instance, a 1/2" hole in the middle of a 5-1/2" ridge beam for a ceiling fan wire would cause a loss in carrying capacity at that point in the beam of 14% [1.5 x .5 / 5.5 = .136 = 14%].

For more in-depth information on field drilling (and notching) of glulams, consult the APA Technical Note EWS S560H.PDF. Even using these simple guidelines, remember: if you are not sure, don't drill. Call Rosboro Technical Support for guidance.