The purpose of a lintel over a window is to support the weight of the wall, floor and roof above the window opening. Modern window frames are not normally designed to carry such loads. The lintel acts as a support beam and transfers the load to the wall either side of the window.
The “bearing area” under each end of the lintel must be sufficient to stop the load crushing the bricks or blocks. In some circumstances a “pad stone” has to be inserted under the ends of the lintel to stop this.
If you have a cavity wall the top of the lintel may also be shaped/sloped so that any water running down the inside of the cavity is deflected to the outside. Otherwise it can pool on the flat top of the lintel and cause damp on the inside of the window and window opening. Alternatively a separate "Cavity tray" can be fitted above a flat lintel to perform the same function.
Calculating the correct size of a lintel or any other structural beam is a job for a professional like a Structural Engineer. In some countries Building Regulations/Codes require you to seek permission/approval before carrying out work on a lintel.
The “bearing area” under each end of the lintel must be sufficient to stop the load crushing the bricks or blocks. In some circumstances a “pad stone” has to be inserted under the ends of the lintel to stop this.
If you have a cavity wall the top of the lintel may also be shaped/sloped so that any water running down the inside of the cavity is deflected to the outside. Otherwise it can pool on the flat top of the lintel and cause damp on the inside of the window and window opening. Alternatively a separate "Cavity tray" can be fitted above a flat lintel to perform the same function.
Calculating the correct size of a lintel or any other structural beam is a job for a professional like a Structural Engineer. In some countries Building Regulations/Codes require you to seek permission/approval before carrying out work on a lintel.