Finding first floor joists

[23vc]

Anyone know a clever way of finding the height of existing first floor joists, if the joists run parallel to the wall, with minimal damage to the inside which won't be knocked through until later? Done this before by just bashing a hole in the outside skin and seeing the joists, but obviously that only works if they run at right angles to the wall. This is to get the right height for my new extension joists which I'll joist hanger off a bearer bolted to the existing wall.
Cheers for any tips as I've not thought of any yet

 

[^woody^]

Knock a hole through and level it. You'll regret it later if you faff about trying other ways and end up with a slight step.

 

[23vc]

OK so the landing wall will suffer but I guess your right, ta. Or rather if you don't know another way Woody then there probably ain't one

 

[garyo]

If they're built in to the wall then you'll only have to remove some from the outer skin to see the joist ends. Otherwise you're a bit stuffed.

I've tried doing it the tidy way in the past by drilling a 10mm hole from the outside through both skins using a 400mm bit, then inserting a rod, checking it with a sprit level, and transposing the levels but ended up losing 5 or 10 mill or so along way... I wouldn't bother doing it that way again, but you could try it with a water level I suppose?

 

[23vc]

Haha, that's exactly what I was thinking of doing until you said that. Water level now that's not a bad shout, if I can find a way, it worked a treat with the dpc brickwork so would be nice to find another use for it

Can't see the joists from outside as they run parallel to wall hence the issue

 

[^woody^]

Although these joists run parallel, you must surely be doing something on that wall at some stage, so you can take the outer bricks out and then you may see a course of split blocks because joist heights don't normally work block courses.

Or you can carefully remove some block in the approximate location of the floor void and will see the first joist. You can do this without disturbing the plaster.

A water level is not the best option because you will need to expose the top or bottom of a joist, unless you want to guess from the plaster ceiling or floor boards - but it will still just be a guess.

 

[garyo]

The way I did it using the drill/rod was to make the hole around 100mm above floor level and then use a steel rule, which allowed me to slide it between floorboards to find the ceiling plasterboard (i.e. bottom of joist) or the top of the joist. I think most of the inaccuracy was due to it being so difficult to get a level hole, and then measuring/compensating for that.

 

[^woody^]

I think most of the inaccuracy was due to it being so difficult to get a level hole, and then measuring/compensating for that.

It's inherently inaccurate. The OP is extending on that wall and will be doing some or other work there at some stage, so why not just do it properly now and get a precise level.

 

[23vc]

On a related subject any recommendations for the type of fixing to use to fix the 6x2 bearer along the existing house wall? I've got some M10 bolts which I'll screw straight into the brick for now but not sure if that's suitable long term

 

[^woody^]

Those will do

 

[bernardgreen]

Chemical Anchor Systems are very good for fixing studding into brick, block and stone walls such as.

http://www.screwfix.com/p/easyfix-kem-psc-380-ef-polyester-resin-380ml/1245j

I use Anglia Fastenings ( Bedford ) own brand which is a lot cheaper ( it is made by http://zetapro.co.uk/ who could probably provide the name of an own brand supplier close to your location )

Prepare all the holes at the same time and have all the fixings ready as the mixed resin start to harden after about 10 minutes on a warm day. If anchoring studding then wipe resin into the thread of the studding before pushing the studding into the hole 1/2 filled with resin. It is well worth buying a few extra nozzles and doing only a couple of holes at a time and getting the studding in place before the resin begins to harden in the holes and the nozzle.

 

[^woody^]

Chemical Anchor Systems

Why is a chemical anchor required for that situation?

 

[bernardgreen]

Why is a chemical anchor required for that situation?

M10 bolts which I'll screw straight into the brick for now but not sure if that's suitable long term

Best option is studding anchored into the wall and washer and nut on the studding to press the timber to the wall.

 

[23vc]

Thanks Bernard I have used resin fixings before for things like floating shelves but do not intend to do so for this. However I will speak to my BCO and see what he says.

Last question on this I promise - my blockwork is set to end up about 50mm lower than the underside of the joists - the BCO says the joist should land on the block not a mortar bed - worth getting some coursing blocks to allow the joists to sit on a decent sized block, or just cut thin slivers of block and get them bedded at the right height? I guess I can't just use a tiny bit of cut block (eg. 50mm x 50mm) under each joist as that would be too flimsy, so should run a straight course all the way along level with the undersides of the joists? (I'm using those long celcons) anyway any tips on the better way to do this much appreciated

 

[Doggit]

If you have a water level that's long enough to get from inside and outside the wall, then that would give you a datum line. You then lift one of the floorboards, and that gives you the distance to the datum line, then you measure the same distance outside. Of course, that only gives you the height of the joists, so you then have to work downwards to get the distance to set the joists, and take into account the size of the joists you're using. But it'll give you the positioning for the bearer.