You may have seen that I am building a 4.4 x 6.8 workshop. The walls, 2000mt high, will be 4 x 2 filled with 90 PIR & a noggin midway up between 600mm studs. The outside will be 11mm OSB, glued and screwed, and weatherboard. The Inside will probably be 9mm ply. The 45deg pitched roof will be clay tiles - approx. 3500kgs. In simple terms, the static load on the walls will be 5000kg (tiles, joists, rafters and rounding up) / 20mts (length of walls, rounding down for ease) = load 250kg/mt. I intend to install a double glazed window which will be 1800mm long and therefore my idea was to construct a lintel 300 deep from a sandwich of 2x4 timber to form the perimeter/verticals between two layers of 18mm ply - all glued and screwed together and possibly injected with PU foam to form a mini SIP. Does this sound reasonable/sensible?