Sizing joists and recommended method of fixing to wall

[Tony Bryer]

The old rule of thumb for floor joists was to halve the span (in feet) and add 1 to get the required joist depth in inches - i.e. for 10' you need 6"x2" [at 16" centres] - or 50x150 at 400c/s in new money. This assumes a superimposed load of 1.5kN/m2 or 30lb/ft2, so one of these joists is good for 30x10x16/12 400lbs. Do the sums using a well known program and you'll find that the calculated maximum span is about

3.1m. If all the load is at the centre of the joist the load it can take about 1.4kN or about 320lbs. Your washing machine probably weighs less than this and the floor deck will spread the load over several joists.

Given the curve of the blade, stitch drilling with a suitable drill bit might be better

Soft as in aerated concrete - like an Aero choc bar but grey? Assuming that the blocks are 100mm thick or more they are perfectly capable of holding your joists.

[don.g.irvine]

Is there a recommended way of working out the size of a joist (e.g.

50x150) required to span a 3m distance? It needs to be robust as it will be supporting a washing machine pretty much on its center point.

One of the walls that the joist will attach to is a conventional brick wall. To attach to this wall I was planning on cutting a grove with an angle grinder and then using joist hangers.

At the other end, the wall is soft breeze block - so soft that I can easily push a screwdriver blade into it. I'm assuming on this side that I'll need to support it from ground level rather than attaching to the breeze blocks - The new floor is only going to be about 2ft above an existing concrete garage floor, so this would be relatively straight forward as a solution.

Don

[Rick]

The building regs have a load of tables, which tell you the sizes. I don't have my copy handy, else I'd look for you. The type of wood effects the size.

If you can support from ground level all the way accross, you can make dwarf walls to hold the joits, which can now be much smaller.

Rick

[Ali Mac]

I've used joist hangers in lightweight block cut with an angle grinder for a 4m span and it worked fine.

Alistair