DEBT - ABL Credit Facility - Narrative (Details) - USD ($)	Sep. 29, 2025	Mar. 24, 2023	May 26, 2021	Sep. 30, 2025	Sep. 30, 2024
Senior Term Loan Facility Due September 29, 2032 | Secured Debt
Debt Instrument [Line Items]
Debt instrument, extended term, period prior to maturity date	91 days
Line of credit facility, term, outstanding obligations threshold amount	$ 100,000,000
Senior Term Loan Facility Due September 29, 2032 | Alternate Base Rate | Secured Debt
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.00%
Domestic Line of Credit | Amended ABL Credit Facility | LIBOR | Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			1.25%
Domestic Line of Credit | Amended ABL Credit Facility | LIBOR | Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			1.75%
Domestic Line of Credit | Amended ABL Credit Facility | Alternate Base Rate | Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			0.25%
Domestic Line of Credit | Amended ABL Credit Facility | Alternate Base Rate | Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			0.75%
Domestic Line of Credit | Amended ABL Credit Facility | Secured Overnight Financing Rate SOFR
Debt Instrument [Line Items]
Credit spread adjustment		0.10%
Domestic Line of Credit | Amended ABL Credit Facility | Secured Overnight Financing Rate SOFR | Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.25%
Domestic Line of Credit | Amended ABL Credit Facility | Secured Overnight Financing Rate SOFR | Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.75%
Foreign Line of Credit | Amended ABL Credit Facility | Line Of Credit Facility, Commitment Option One
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage			0.375%
Foreign Line of Credit | Amended ABL Credit Facility | Line Of Credit Facility, Commitment Option Two
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage			0.30%
Foreign Line of Credit | Amended ABL Credit Facility | Line Of Credit Facility, Commitment Option Three
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage			0.25%
Foreign Line of Credit | Amended ABL Credit Facility | Bankers Acceptance Rate | Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			1.25%
Foreign Line of Credit | Amended ABL Credit Facility | Bankers Acceptance Rate | Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			1.75%
Foreign Line of Credit | Amended ABL Credit Facility | Prime Rate | Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			0.25%
Foreign Line of Credit | Amended ABL Credit Facility | Prime Rate | Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate			0.75%
Line of Credit | Amended ABL Credit Facility | Secured Debt
Debt Instrument [Line Items]
Debt instrument, extended term, period from amendment date			5 years
Debt instrument, extended term, period prior to maturity date			91 days
Line of credit facility, term, outstanding obligations threshold amount			$ 100,000,000
Atkore International | Line of Credit | Amended ABL Credit Facility
Debt Instrument [Line Items]
Aggregate commitments				$ 325,000,000
Credit availability				$ 325,000,000	$ 325,000,000
Borrowing base percentage of eligible accounts receivable (%)				85.00%
Borrowing base percentage of eligible inventory (plus) (%)				80.00%
Debt instrument, covenant, borrowing base, percentage of inventory subject to certain limitations (%)				85.00%
Borrowings outstanding				$ 0	$ 0