Strengthening undersized joists

[Student]

There are two joists in my upstairs extension floor that are 7x1 instead of 7x2 like the others. It's a Victorian house and there used to be a chimney at that end of the room, which they've removed.

 

If I were to sister another 7x1 without going into the pockets the existing 7x1s are in (no room without knocking through from outside and sliding through), would this significantly increase its strength? If not is there something else that can be done? My current plan was to sister with some M12 bar, square washers and timber connectors. Happy to just screw if consensus is that M12 bar is overkill? My 900x900 shower will be sitting on the corner of the room with these small joists, so I wan't to make sure there is support there.

 

On a side note, I hope to tile this floor with electric matting underneath. I've bought 18mm wbp ply for it. Do you think this, with the existing 7x2 joists (I will add noggins), will resist deflection enough to handle tiles? I've not noticed any spring in the joists myself but I'm no expert. Floor is 2900 x 2200, so quite small. Would you insulate this floor? I don't think it's normal but seeing as the floor will be heated, perhaps it needs insulation to stop heat escaping downward? Thanks for any tips.

Edited June 17, 2019 by Student

[Mr Punter]

Screw in from both sides alternate 50mm from top and bottom. 200mm ctrs should work fine so 10 screws per metre. More won't hurt. Screws not fully threaded are better as they pull the pieces together.

[SteamyTea]

Doubling the breadth does not make much difference. But probably be enough for your needs.

Calculate it if in doubt.

https://www.engineeringtoolbox.com/beams-fixed-both-ends-support-loads-deflection-d_809.html

[Russell griffiths]

Cut the pockets out in the wall and get a proper joist in there 

cut the pocket deeper than you need

so chop out 150mm deep hole for 100mm bearing

cut joist and fit at an angle will be a squeeze but a good clumping with a lump hammer will get it in

pack it to the correct height and point it all around

noggin the crap out of it. 

[Carrerahill]

4 hours ago, Student said:

There are two joists in my upstairs extension floor that are 7x1 instead of 7x2 like the others. It's a Victorian house and there used to be a chimney at that end of the room, which they've removed.

 

If I were to sister another 7x1 without going into the pockets the existing 7x1s are in (no room without knocking through from outside and sliding through), would this significantly increase its strength? If not is there something else that can be done? My current plan was to sister with some M12 bar, square washers and timber connectors. Happy to just screw if consensus is that M12 bar is overkill? My 900x900 shower will be sitting on the corner of the room with these small joists, so I wan't to make sure there is support there.

 

On a side note, I hope to tile this floor with electric matting underneath. I've bought 18mm wbp ply for it. Do you think this, with the existing 7x2 joists (I will add noggins), will resist deflection enough to handle tiles? I've not noticed any spring in the joists myself but I'm no expert. Floor is 2900 x 2200, so quite small. Would you insulate this floor? I don't think it's normal but seeing as the floor will be heated, perhaps it needs insulation to stop heat escaping downward? Thanks for any tips.

Plenty of options given to you above, my input:

 

If fitting a new joist is not possible - i.e. ceiling attached beneath etc. then you need to make do and improve what is there.

 

I make up OSB or plywood laminate joists in these situations. Glue some 18mm ply in 2400mm strips along the joist, then sister it up with another one, glue it all, drill and coachbolt it all together. Solid as a rock and stronger than timber only. 

 

I have also used flitch beams. I get steel laser or water jet cut from a local place and they are not expensive. Even a 3mm mild steel plate sandwiched in between two timbers and coach bolted is seriously strong - 5mm is best, however it really depends on dead-load and expected live loads.

Edited June 17, 2019 by Carrerahill

[Student]

Thanks everyone. That's some good information to think about. I tried doing the calculations but the timbers are some what of an unknown, I don't know what they are, some sort of pine I guess. From the calcs I have done it seems like each 7x1 can take a point load of 70kg at 1/3 and 2/3 span (note that the joists are running between the smaller distance of 2200) The dead load shared across the two joists is a shower at one end (85kg) and a toilet the other end (weight unknown but guess 50kg with water). There will be tiles and floor surface and potential live weight of a person. I'd guess 200kg dead and up to 300 to 400kg total if being super cautious. I used this site with additional info from the site you recommended SteamyTea; http://www.timbertoolbox.com/Calcs/beamcalc.htm http://www.timbertoolbox.com/Calcs/2ptbeam.html

 

I'm thinking that if I sister without going into the pocket (as per Mr Punters advice) and then put noggins at 1/3 and 2/3 span in the whole floor, I'd be comfortable that it can take the load with minimal deflection. Please do say if you think this is risky though.

 

Absolute worst case, I would consider punching through to outside and sliding in from outside. I do already have some damage to the exterior render due to the timber 'lintels' (actually just a floorboard doubled up) having to be replaced with concrete lintels a few months ago.

[Student]

Another thought. How are first floor suspended floors ventilated? As I see it, there's no ventilation at all if sealed from above and below and I assumed I'd want to as it's a bathroom?

[PeterW]

No ventilation required and you want it watertight if it’s a bathroom ..!!!