[ahearsum]

I'm creating a first floor wet room and pulled up the floorboards and found joists in mixed condition. Short question is should I replace them? If so, am I right in thinking 6x2 at 400 centres should suffice in this room which is 2m x 1.7m. According to the tables I should be well within the deadload limit.
Additional benefits of doing it would be the opportunity to level the floor as the 200year old beam the joists dovetail into is quite bent (although solid otherwise). I'm assuming I should use joist hangers at the beam end with the recommended nails (would rather use screws if that is acceptable?). I'm not sure what to do at the other end. The current joists pass over and rest on a brick wall and carry on to another wall. If I saw off these joists this side of the supporting wall would that affect the structural integrity of the rest of those joists?

 

[carpdad]

Don't cut out the old joist. Just add new to the old. This is called sistering. Floor leveling can be done with sistering but must have the finish floor level in mind. Looks like there is at least 1.5" difference between joists and the other room floor. You don't have to get the new joist on that brick floor, as long as old joist ends are not damaged. I'd just use joist hangers for 4x4 lumber or look for hangers made for true 4x (full 4" and not 3.5) lumbers. I bought some. Use thin screws made for assembling decks or 16d nails but predrill.

Not sure how to say this. Post in electrical for more comments. Your electrical wires should not be draped over the joist edges. And if too close to the edge, covered with thick metal plates so they don't get punctured by nails and such. If mine, i'd try to find their origins and rewire as much as possible. You must NOT cut them in middle and bury the cuts permanently.

 

[ahearsum]

Don't cut out the old joist. Just add new to the old. This is called sistering. Floor leveling can be done with sistering but must have the finish floor level in mind. Looks like there is at least 1.5" difference between joists and the other room floor. You don't have to get the new joist on that brick floor, as long as old joist ends are not damaged. I'd just use joist hangers for 4x4 lumber or look for hangers made for true 4x (full 4" and not 3.5) lumbers. I bought some. Use thin screws made for assembling decks or 16d nails but predrill.

Not sure how to say this. Post in electrical for more comments. Your electrical wires should not be draped over the joist edges. And if too close to the edge, covered with thick metal plates so they don't get punctured by nails and such. If mine, i'd try to find their origins and rewire as much as possible. You must NOT cut them in middle and bury the cuts permanently.

Thanks for your advice. Yes the electrics is next on the list. I've inherited quite a mess.
Regarding the sistering. Should I coachbolt a sister joist to each of the ones that have have poor dovetail joints (like in the attached picture)? Should I then get a joist hanger big enough to accommodate the doubled up joists?
As you can see from the bottom of the studwall at the top of the picture the beam that joist dovetail in to has sagged quite a bit. I would be tempted to follow the position of the current joists when sistering then level up the joists with additional timber battens on top. Is that bad practice do you think? (Its what I uncovered when I took the flooring up). I suspect there is a longstanding levelling up of the whole of the first floor due to 200 year sagging; all the floors are raised up as much as 2 inches in places.

 

[Half-fast eddie]

You say the room is 1.7x2 meters. You might not be able to size the fllor joists using those numbers, unless that’s the total distance from bearing to bearing.

 

[ahearsum]

You say the room is 1.7x2 meters. You might not be able to size the fllor joists using those numbers, unless that’s the total distance from bearing to bearing.

1.7 is the current span of the joists. I'm not sure what you mean by "size the floor joists".. Do you mean space the joists out evenly at 400mm centres across the width of the room? I'm an amateur as you may be able to tell.

 

[Half-fast eddie]

I mean determine what size timber is required for that application. Some people will say their bathroom is only 5x8 ft so they only need small joists ... they don’t realize that the floor joists have to reach all the way to the supports which are another 10 ft away.

1.7x2 m is not large, it’s a really short span. Apparently the way your house is built there are supports pretty close together.

 

[Nealtw]

You can install the hangers first and then sister on the new joists. if you are levelling the sister it wouldn't sit in the hanger anyway.

 

[ahearsum]

I mean determine what size timber is required for that application. Some people will say their bathroom is only 5x8 ft so they only need small joists ... they don’t realize that the floor joists have to reach all the way to the supports which are another 10 ft away.

1.7x2 m is not large, it’s a really short span. Apparently the way your house is built there are supports pretty close together.

Thanks for your reply. I was looking up this website for allowable spans for certain timber sizes at various centres. Floor joist span tables for surveyors - Floor construction | Right Survey It seemed like I was in the ballpark of what was acceptable.

 

[ahearsum]

You can install the hangers first and then sister on the new joists. if you are levelling the sister it wouldn't sit in the hanger anyway.

Thanks for your reply. I was worrying about the ends of the old joists being a bit rotten. It they are then hangers won't help me much I suppose. What I had been considering was bolting (sistering) a new joist to the old one and either "joist hangering" the doubled up end or chopping the end off the old joist and "joist hangering" the new sistered joist end. Am I mad? I would then level up the floor with battens on top of the joist?

 

[Nealtw]

Thanks for your reply. I was worrying about the ends of the old joists being a bit rotten. It they are then hangers won't help me much I suppose. What I had been considering was bolting (sistering) a new joist to the old one and either "joist hangering" the doubled up end or chopping the end off the old joist and "joist hangering" the new sistered joist end. Am I mad? I would then level up the floor with battens on top of the joist?

A bit rotten? the rot has to be killed, then a hanger that will take them both would be in order. You may have to fill the hanger below the new sister.

 

[jeffnc]

I haven't seen that type of carpentry before. Why would you dovetail a joist in place? That main thing that seems to do is weaken the joist. A dovetail primarily secures that laterally, not vertically. Joists don't need lateral support beyond just a nail. They need vertical support, and keeping the joist at its full dimension is the strongest way to do that, beyond obviously supporting it fully. If they wanted to use a slot, it seems it would have been way better to simply drop the joist into a dado slot.

 

[ahearsum]

Thanks for your reply jeffnc. It's a 200 year old house maybe they had different ideas then. Actually , I had to look up a dado joint (not being a carpenter) and I think it may well be a dovetail joint I have not a dado joint. Maybe it's not obvious from the photographs. Does that make a difference to your opinion. In either case the joints aren't in great condition. Do you have any thoughts about what I should do about it?

 

[jeffnc]

Thanks for your reply jeffnc. It's a 200 year old house maybe they had different ideas then. Do you have any thoughts about what I should do about it though?

Sorry, I said "dado" when I meant "dovetail". I will edit my original post. I would have dadoed instead of dovetailed, or not done any fancy joint.

I would sister whenever possible, as long as the joists are strong vertically. I use these screws for that sort of thing

FastenMaster 3/16 in. 2-7/8 in. HeadLok Star Drive, Flat Head Wood Screw (12- Pack) FMHLGM278-12

HeadLOK is a heavy-duty wood screw that does it all with an internal drive and flat head. HeadLOK offers higher design shear than 3/8 in. lag screws and no predrilling is needed! HeadLOK zips right in

www.homedepot.com

I also use Simpson StrongTie joist hangers when I need to attach joists to ledgers, and I use Simpson's fasteners that go with them.

Simpson Strong-Tie #9 x 1-1/2 in. 1/4-Hex Drive, Strong-Drive SD Connector Screw (100-Pack) SD9112R100

The Strong-Drive SD Connector screw is specifically designed to replace nails in certain Simpson Strong-Tie connectors and is the only screw approved for that application. The load-rated SD screw has been tested and approved for use in many popular Simpson Strong-Tie products. In certain...

www.homedepot.com

 

[ahearsum]

Great, thanks.