Retrofit / Structural Sistering Audit

Purpose

This SOP outlines the correct steps for auditing Retrofit tickets involving structural sistering or reinforcement. Retrofits are required when engineering has identified rafters or trusses that require strengthening to support the additional loads of a PV array.
This SOP is paired with the Retrofit Training section below.

Scope

This procedure applies to all auditors reviewing retrofit work, whether completed by internal crews or external contractors.

References

• Engineering Plans (S-3 Roof Plan / S-4 Structural Details)
  
  

• Luke’s Retrofit FAQ Sheet
  
  

• AHJ and structural engineering requirements (found in Highline)
  
  

1. Pre-Audit Preparation

Before reviewing photos or inspecting the work, the auditor must:

1.1 Locate and open the stamped engineering plans

• Navigate to page S-3 (Roof Plan)
  
  

• Navigate to page S-4 (Structural Details)
  
  

1.2 Identify all retrofit callouts

On S-3 (Roof Plan):

• Light blue callouts indicate what roof sections the retrofit reinforcement is required. (see the Image example drop down for an example photo) 

  • For call outs that expand across multiple, will typically cover both roof sections. (see the Image example drop down for an example photo) 
    
    

• Each callout will include:
  
  

  • A detail reference number (top part of the call out) (see the Image example drop down for an example photo)  
    
    

  • A page number  (bottom part of the call out) (see the Image example drop down for an example photo)
    
    

  • Example: “4/S-4” = Detail 4 on page S-4
    
    

1.3 Determine the retrofit type

For each identified callout, confirm whether the engineer requires:

• Partial sistering
  
  

• Full-length sistering
  
  

• Lap-spliced sistering
  
  

• Other reinforcement (blocking, of supported rafters, etc.)
  
  

This information will be listed in the notes on S-4 Structural Details.

Understanding Retrofit Types

Retrofit reinforcement can include partial sistering, full-length sistering, lap-spliced sistering, or blocking.
Below are the definitions and audit requirements for each.

2.1 Partial Sistering (Partial-Length Reinforcement)

Definition

A short reinforcement member attached alongside the existing rafter, typically positioned directly beneath the PV array footprint.

Requirements

• Centered under the array location shown on S-3.

• Lumber size matches engineering.

• Nail pattern:

  • 6” O.C.

  • Staggered

• The length of the partial sisters needed is listed on the same page as the detail in the top left corner. (see the Image example drop down for an example photo) 

2.2 Full-Length Sistering

Definition

A reinforcement member installed the full length of the rafter, or lap-spliced sisters to achieve full-length reinforcement.

Requirements

• Full-length sister or lap splice per S-4 details.

• Nail pattern:

  • (3) nails grouped at each end

  • 6” O.C. staggered along the remainder

• Splice overlaps must meet the engineered minimum (commonly 24–48”, check plans).

2.3 Blocking (Truss Blocking Retrofit)

Definition

Blocking is installed between trusses to provide adequate lag embedment and to reinforce the attachment point where module mounts will be fastened.

Important Engineering Note

Blocking may not be required if there are already sufficient rafters beneath each PV array attachment to achieve a minimum of 2.5” lag embedment OR  the type of mounts that we are using.

Blocking Requirements

If blocking is required per engineering:

1. Install 2x6 blocking between box trusses under the areas receiving lag screws and must be installed before the attachments go into the roof. 

2. Fasten each block at both ends using one of the following approved methods:

   • (3) 16d sinker nails into the existing vertical truss member      OR

   • (1) Simpson A34 clip at each end of each block (installed per manufacturer specs)

3.  Also to keep in mind is that if they can get the lags to go into truss top chords, then we should be good to proceed without blocking!
   
   

What Photos You MUST See

• Overview of each retrofitted rafter

• Nail pattern details

• Splice overlap photos (if applicable) 

• Full completion photos

• For blocking that they are tight-fitted, level, and installed directly between truss webs. The 16d sinkers properly installed OR the Simpson A34 clip properly installed.

• No split lumber or overdriven fasteners.