(1) Lintels and arches over openings2..-The portion of a masonry or concrete wall above an opening shall be provided with a lintel constructed as an arch, and shall comply with requirements as set out in Chapter 3 or section 18.

(2) Chases and recesses.- Where horizontal chases or recesses are cut to a depth greater than one-fifth of the thickness of the wall, or leaf of the cavity-wall, concerned, or are built into the wall to a depth grater than one-third of such thickness, or where chases or recesses are cut or formed on both sides of particular length of wall, then such chases of section 40, and shall be bridged over accordingly.

(3) Decorative timber or timber nailing-strips may be set into masonry or concrete walls, piers or columns, but such timber is regarded as a recess if the depth of building is less than one-third of the thickness of the wall, and as an opening if it is more than one-third.

(4) No length of wall between openings which is less than 230 millimetres in length shall have any openings, recess or chase out into it unless supported to the approval of the local authority:

Provided that this provision shall not be constructed as preventing such an opening, recess or chase being built in as construction proceeds in such a length of wall.

(5) Every chase shall be filled solidly around services with mortar not weaker than class B, and the filling shall be allowed to harden before any plastering is done. The surface of the chase shall be wetted down and the mortar made dry around service-boxes shall be similarly filled with mortar not weaker than class B.

(6) Wall not to be endangered by openings, chases or recesses.- The number, size, position and manner of forming of openings, recesses and chases in any masonry or concrete wall such that such wall, pier or column is not, in the opinion of the local authority, thereby rendering unsafe.