You should not let n architect or engineer design the floor joist layout for a house, it’s a job for the carpenter and technician.


You select the depth and breadth of joists required to span the longest run, typically the lounge, along with appropriate spacing to suit both the load and the length of the plasterboard on the ceiling along with the floor sheets.


So in a typical house you will have floor joists over the lounge  at 400 mm centres, you don’t want steps in the floor or ceiling levels of 50 mm  or so as you go from room to room. Therefore it is general practice that whatever depth of joist is used across the longest span is used throughout the house to avoid steps in the floor and ceiling levels.


If you just carried on with the same joist specification across the whole house the joists will be oversized, possibly massively so in some areas of the house. You don’t want to vary the height, so you can reduce the width of each joists or you could increase the spacing to 600 mm reducing the amount of timber used, but still suitable for 2400 mm sheets of plasterboard and chipboard. But often the same spacing is retained to give structural strength by using the joists to tie the building together.


So as an electrician you need to be careful drilling the joists over the longest span as those joists are designed to just do their job, but drilling that short span under the landing is unlikely to cause issues as the joists are probably well oversized.


I have seen NHBC inspectors make electricians strip out the cables in badly placed holes in joists with long spans, then the electricians have been contra charged for the carpenter repairing the joists with plywood gussets before the electricians reinstalled their cables.


I have never seen anyone get worried about holes in the short joist spans, as they are generally oversized.


Andy Betteridge.

You should not let n architect or engineer design the floor joist layout for a house, it’s a job for the carpenter and technician.


You select the depth and breadth of joists required to span the longest run, typically the lounge, along with appropriate spacing to suit both the load and the length of the plasterboard on the ceiling along with the floor sheets.


So in a typical house you will have floor joists over the lounge  at 400 mm centres, you don’t want steps in the floor or ceiling levels of 50 mm  or so as you go from room to room. Therefore it is general practice that whatever depth of joist is used across the longest span is used throughout the house to avoid steps in the floor and ceiling levels.


If you just carried on with the same joist specification across the whole house the joists will be oversized, possibly massively so in some areas of the house. You don’t want to vary the height, so you can reduce the width of each joists or you could increase the spacing to 600 mm reducing the amount of timber used, but still suitable for 2400 mm sheets of plasterboard and chipboard. But often the same spacing is retained to give structural strength by using the joists to tie the building together.


So as an electrician you need to be careful drilling the joists over the longest span as those joists are designed to just do their job, but drilling that short span under the landing is unlikely to cause issues as the joists are probably well oversized.


I have seen NHBC inspectors make electricians strip out the cables in badly placed holes in joists with long spans, then the electricians have been contra charged for the carpenter repairing the joists with plywood gussets before the electricians reinstalled their cables.


I have never seen anyone get worried about holes in the short joist spans, as they are generally oversized.


Andy Betteridge.