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Existing floor joist depth


How would I determine which way the joists ran, again without actually lifting floor boards etc, would a stud finder do it?


4 Answers from MyBuilder Architectural Designers

Best Answer

I find a good place to measure is at the top of the staircase. There are normally 12 stair risers. 13 being landing level. At the side of the top stair is normally an accessible measuring point for the overall depth of the first floor. ie, landing level to the underside of the ceiling level below. This depth includes the thickness of floor covering (10mm for carpet), floor board thickness (15 mm), joist depth plus the thickness of the ceiling plasterboard + plaster skim (15mm) So, depth of floor joist is the measured overall depth minus 40mm. The answer will be close to either 6, 7, 8 or 9 inches depending on your particular span and joist centres. Round your answer up or down to the nearest inch and that will be your joist depth. This is normal and approx., but I find that I am usually right. Adjust your minus figure for floor covering, as it is obviously not always 10mm thick carpet.

You can determine which way the floor boards run by lifting a good size piece of carpet or floor covering at a corner of the room without lifting the floor boards. The direction and centres of floor joists will be apparent by looking for the lines and direction of the floor board nail heads on the floor surface.


Answered 13th Jan 2020

The floor joists always run perpendicular (90 degrees) to the floorboards, otherwise the floorboards would just fall through.


Answered 14th Jan 2020

As noted in the previous answer just check the line of nails in the boards. Joists tend to be at 300,450 or 600 centres to correspond to the floor board sizes. If the floor is a tounged and grooved board that's even easier...the boards will always run perpendicular to the joists


Answered 19th Jan 2020

The answer is simple - the floor joists will always be in the opposite direction to the floorboards.


Answered 19th Jan 2020

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