I have some rotten joists in my downstairs room, the rot has spread a couple feet down the joists:
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'm trying to plan the repair. Assuming I treat/kill and the rot and stop the water ingress are these solutions sound structural fixes(joists are 5" x 2"):
a)
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back the rotten joists, a metre or so back from the rot. The I bolt on new timber on one side and sit them in a widened hole in the wall. New timber will be treated of course. If this method is sound just how far alongside the existing joist do I need to run the new timber?
b)
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cutting back the same amount, this time fixing a new piece into the original wall hole and sandwiching it to the old timber.
There's also enough room in the cavity to leave acro props permanently in place. Any advice much appreciated.
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Don't forget to make sure the under floor ventilation is adequate as well.
I would go for b). As for the length you could probably get away with relatively little but timber isn't expensive. I would go for at least 2 feet either side of the join. You can get spiked plates to go in the sandwich at each fixing but I don't know their proper name.
It might possibly be the easiest option but that is a bodge.
You can get a metal box in two halves to fit around the joists to add on an extra piece. I think BAT or one of the other steelwork/tie/joisthanger/etc manufacturers made them. This would give you B but using steel rather than timber joiners.
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(complete with DPC) is a recognised way of supporting suspended floors while avoiding burying the ends in what might be damp masonry so depending on your actual circumstances you might find that a better prospect than 2 end walls and a bridging beam which would have to be relatively massive to take the weight of the suspended floor.
Dear Mark My advice is as follows: If, as you say it is a Ground Floor room - simply go back to the next wall plate which if it is a 5" x 2" joist is unlikely to be more than
8' away. You can then do on one of two things a) cut the joist on the wall plated with a butt join in the middle (or better still longitudinally half-lap it) so it rests on the wall plate and run it back to the same bearing or b) cut the timber back to sound timber only (it is a complete myth that you need to cut back 1 m - all based on RTFs minimising risk and maximising profit) - use a hammer to determine what is sound and what is not and then neaten up with a saw
- check the endgrain! - and run a new timber alongside to a new bearing bricking up the old one or better still hanging on joist hangers (am suprised it is not on a wall plate at GF)
Use Tanalith timber with the treated ends in the joist hangers put a thin membrane (poly bag) over the end before fixing in the joist hanger and fit it nice and tight to the brick dip treat cut ends 60 mins in OS borne fungicide
IF it were on the first floor or if the structure has no wall plate and you HAVE to use a connection (as has been assumed by all other posts0 then the correct way of doing it is as follows
1) prop from oversite to defective joists back into sound timber
2) cut off defective timber only
3) dependent on the distance of the cut back (I am assuming your diagrams are to scale) you need a greater or lesser overlap (technical term is "spacing") between the sets of bolts. I would estimate your span on 5" by 2" not to exceed 10' so I would have an overlap of 3'
4) the holes you need to drill for the bolts and doublesided toothplate connectors are normally as a rectangle (not staggered as so many suggest UNLESS the wood is wet which as new timber it should not be as it it building regs for it to be 18% or less). BUT in a 5" timber the rectangle is not possible. Were it to be it would consist of two sets of two holes one above the other at each end of the joint. the other key dimensions are known as "edge" distance and "end" distance. In this size of timber your end distances are safe at
75mm and your edge at 45 mm.
What I suggest you do is drill one hole 5" from the end and the other
3" from the end one ""nearly" above the other Given that the overlap is (say) 36" that means that the spacing (distance between the sets of bolts) is at least a 36 - (2x5") =26" or just over 2 foot which is fine. I personally would put the 3" end distance holes at the bottom tension zone giving a spacing of 30" and the 5" in the top compression zone.
How to do this? Well you are best off with a rightangle attachment for your drill but you can do it without if your drill is small enough to fit between the joists or you can lift the joist such as to get at a right angle to drill. Put the joist end in place and loosely ~G cramp together with joist hanger etc all in place and check your levels for floorboard and skirtings etc Once happy, mark out the four drill hole points on your new timber take out of the cramps and drill a 1/2" hole exactly at right angles in all four places G cramp back in place and mark the old timber by putting a pencil in the drilled holes. either circle round as best you can or use a centre punch or drill to mark the old timber. Take off the new ones and IF you can drill the old timber at right angles - may need to have small drill bits (tiny augers are available) and right angle adaptor once drilled put in your bolts with 2" thick plate washers (not coach bolts!) and double sided toothplate connectors you will need to get some studding to draw the two pieces of timber together before the bolts will fit! put on a nut the other side once you have the two timbers biting into the connectors and tighten up hard till there is no gap between the two timbers but not so hard as to deform the washers. Bob is then veritably your uncle.
The traditional answer would be a scarf joint to join the two timbers. Whatever system you choose, I would add a dwarf wall to support the ends of both the old and new joists. I presume you have solved the problem that lead to the rot before starting repairs.
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