DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION (Policies)	6 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“US GAAP”) for interim financial information, and the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, these financial statements do not include all the information and footnotes required by US GAAP for complete financial statements. As such, they should be read together with Griffon’s Annual Report on Form 10-K for the year ended September 30, 2020, which provides a more complete explanation of Griffon’s accounting policies, financial position, operating results, business, properties and other matters. In the opinion of management, these financial statements reflect all adjustments considered necessary for a fair statement of interim results. Griffon’s CPP operations are seasonal; for this and other reasons, the financial results of the Company for any interim period are not necessarily indicative of the results for the full year. The condensed consolidated balance sheet information at September 30, 2020 was derived from the audited financial statements included in Griffon’s Annual Report on Form 10-K for the year ended September 30, 2020. The condensed consolidated financial statements include the accounts of Griffon and all subsidiaries. Intercompany accounts and transactions have been eliminated in consolidation. The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenue and expenses during the reporting periods. These estimates may be adjusted due to changes in economic, industry or customer financial conditions, as well as changes in technology or demand. Significant estimates include allowances for doubtful accounts receivable and returns, net realizable value of inventories, restructuring reserves, valuation of goodwill and intangible assets, sales, profits and loss recognition for performance obligations satisfied over time, assumptions associated with pension benefit obligations and income or expenses, useful lives associated with depreciation and amortization of intangible and fixed assets, warranty reserves, sales incentive accruals, assumption associated with stock based compensation valuation, income taxes and tax valuation reserves, environmental reserves, legal reserves, insurance reserves, the valuation of assets and liabilities of discontinued operations, assumptions associated with valuation of acquired assets and assumed liabilities of acquired companies and the accompanying disclosures. These estimates are based on management’s best knowledge of current events and actions Griffon may undertake in the future. Actual results may ultimately differ from these estimates. Certain amounts in the prior year have been reclassified to conform to current year presentation.
Fair Value Measurements	The carrying values of cash and equivalents, accounts receivable, accounts and notes payable, and revolving credit and variable interest rate debt approximate fair value due to either the short-term nature of such instruments or the fact that the interest rate of the revolving credit and variable rate debt is based upon current market rates. Applicable accounting guidance establishes a fair value hierarchy requiring the Company to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. A financial instrument’s categorization within the hierarchy is based on the lowest level of input that is significant to the fair value measurement. The accounting guidance establishes three levels of inputs that may be used to measure fair value, as follows: •Level 1 inputs are measured and recorded at fair value based upon quoted prices in active markets for identical assets. •Level 2 inputs include inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices in active markets for similar assets and liabilities, quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of assets or liabilities. •Level 3 inputs are unobservable inputs in which little or no market data exists, therefore requiring an entity to develop its own assumptions.
Inventories	Inventories are stated at the lower of cost (first-in, first-out or average cost) or market.
Issued but not yet effective accounting pronouncements and New Accounting Standards Implemented	Issued but not yet effective accounting pronouncements In August 2018, the Financial Accounting Standards Board ("FASB") issued guidance to clarify disclosure requirements related to defined benefit pension and other post-retirement plans. The guidance is effective for fiscal years beginning after December 15, 2020, with early adoption permitted, and is effective for the Company in our fiscal year beginning in October 1, 2021. We are currently evaluating the effects that the adoption of this guidance will have on our the related pension disclosures. In December 2019, the FASB issued guidance on simplifying the accounting for income taxes by clarifying and amending existing guidance related to the recognition of franchise tax, the evaluation of a step up in the tax basis of goodwill, and the effects of enacted changes in tax laws or rates in the effective tax rate computation, among other clarifications. Our effective date for adoption of this Accounting Standards Update ("ASU") is our fiscal year beginning October 1, 2021 with early adoption permitted. We are currently evaluating the effects that the adoption of this guidance will have on our consolidated financial statements and the related disclosures. New Accounting Standards Implemented In April 2019, the FASB issued guidance relating to accounting for credit losses on financial instruments, including trade receivables, and derivatives and hedging. This guidance is effective for the Company beginning in fiscal 2021. Adoption of this standard did not have a material impact on our consolidated financial statements and the related disclosures. In August 2018, the FASB issued guidance which modifies the disclosures on fair value measurements by removing the requirement to disclose the amount and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy and the policy for timing of such transfers. This guidance expands the disclosure requirements for Level 3 fair value measurements, primarily focused on changes in unrealized gains and losses included in other comprehensive income (loss). This guidance is effective for the Company beginning in fiscal 2021. Adoption of this standard did not have a material impact on our consolidated financial statements and the related disclosures. In March 2020, the SEC adopted amendments to the financial disclosure requirements for guarantors and issuers of guaranteed securities registered or being registered in Rule 3-10 of Regulation S-X, and affiliates whose securities collateralize securities registered or being registered in Rule 3-16 of Regulation S-X (SEC Release No. 33-10762). The amendment replaces the requirement to present condensed consolidating financial statements, comprised of balance sheets and statements of operations, comprehensive income and cash flows for all periods presented, with summarized financial information of the guarantor only for the most recently completed fiscal year and any subsequent interim period. We adopted the amendments to the disclosure requirements during the first quarter of fiscal 2021. This amendment did not have an impact on our consolidated financial statements as this amendment simplifies the financial disclosures required in our guarantor and non-guarantor financial information. See Part I, Item 2, Management’s Discussion and Analysis of Financial Condition and Results of Operations—Supplemental Guarantor Financial Information. The Company has implemented all new accounting pronouncements that are in effect and that may impact its financial statements, and does not believe that there are any other new accounting pronouncements that have been issued that might have a material impact on its financial position or results of operations.

Disclosure of inventory accounting policy for inventory classes, including, but not limited to, basis for determining inventory amounts, methods by which amounts are added and removed from inventory classes, loss recognition on impairment of inventories, and situations in which inventories are stated above cost.