Here is the file header format as of release 6.22.06. It is never compressed.

If END, SeekFree, or SeekInfo are located past the 32 bit file limit (> 2000000000) then these fields will be 8 instead of 4 bytes and 1000000 is added to the file format version.

Byte Range	Record Name	Description
0...3	"root"	Identifies this file as a ROOT file
4...7	Version	File format version	TFile::fVersion (10000major+100minor+cycle (e.g. 62206 for 6.22.06))
8...11	BEGIN	Byte offset of first data record (100)	TFile::fBEGIN
12...15 [12...19]	END	Pointer to first free word at the EOF	TFile::fEND (will be == to file size in bytes)
16...19 [20...27]	SeekFree	Byte offset of FreeSegments record	TFile::fSeekFree
20...23 [28...31]	NbytesFree	Number of bytes in FreeSegments record	TFile::fNBytesFree
24...27 [32...35]	nfree	Number of free data records
28...31 [36...39]	NbytesName	Number of bytes in TKey+TNamed for TFile at creation	TDirectory::fNbytesName
32...32 [40...40]	Units	Number of bytes for file pointers (4)	TFile::fUnits
33...36 [41...44]	Compress	Zip compression level (i.e. 0-9)	TFile::fCompress
37...40 [45...52]	SeekInfo	Byte offset of StreamerInfo record	TFile::fSeekInfo
41...44 [53...56]	NbytesInfo	Number of bytes in StreamerInfo record	TFile::fNbytesInfo
45...46 [57...58]	UUID vers	TUUID class version identifier	TUUID::Class_Version()
47...62 [59...74]	UUID	Universally Unique Identifier	TUUID::fTimeLow through fNode[6]
63...99 [75...99]		Extra space to allow END, SeekFree, or SeekInfo to become 64 bit without moving this header

Here is the file header format as of release 3.02.06. It is never compressed.

Byte Range	Record Name	Description
0->3	"root"	Identifies this file as a ROOT file
4->7	Version	File format version	TFile::fVersion (10000major+100minor+cycle (e.g. 30203 for 3.2.3))
8->11	BEGIN	Byte offset of first data record (64)	TFile::fBEGIN
12...15	END	Pointer to first free word at the EOF	TFile::fEND (will be == to file size in bytes)
16...19	SeekFree	Byte offset of FreeSegments record	TFile::fSeekFree
20...23	NbytesFree	Number of bytes in FreeSegments record	TFile::fNBytesFree
24...27	nfree	Number of free data records
28...31	NbytesName	Number of bytes in TKey+TNamed for TFile at creation	TDirectory::fNbytesName
32...32	Units	Number of bytes for file pointers (4)	TFile::fUnits
33...36	Compress	Zip compression level (i.e. 0-9)	TFile::fCompress
37...40	SeekInfo	Byte offset of StreamerInfo record	TFile::fSeekInfo
41...44	NbytesInfo	Number of bytes in StreamerInfo record	TFile::fNbytesInfo
45...63		Unused??