Joist replacement (woodworm)

[Adam Orr]

I've discovered a pretty severe woodworm problem when I went to fit a new carpet in the bedroom upstairs. It appears two of the joists have born the brunt of it and I think they need replacing. I'm going to attempt it myself as well because I'm struggling to get hold of a joiner due to them being busy. Can anyone give me some advice as to how best to approach the repair? See attached pics for extent of damage.

The house is a victorian terrace, joists are 2"x7", room is about 280cm wide, beneath is the bathroom and there's a stud wall installed perpendicularly about 3/4 of the way across.

My first thought was to reinforce the joists with 18mm hardwood ply, but I think the joists may be too far gone, and the damage goes right the way across. I also considered putting in some additional joists on hangers but the current joists are installed in pockets in the brickwork and not level with the brick line.

Do you think a total replacement is the way to go? If I do replace, any guidance on how to do it? Will I have to cut the new joist into two (or three) pieces and sister them in order to get them in? Any rules & regs for grades of timber, treatment, bolt spec & pitches etc.?

Any help is appreciated on this because I am almost a total novice. Thanks.

 

[Mottie]

Could you not, after treating the affected joists, just bolt new joists to them as extra support?

 

[Adam Orr]

Possibly, but they're so rotten that I could easily break some big chunks off with my bare hands, one of them I could probably split in two. I wasn't sure if it was really capable of being supported.

Is there any way of knowing when they're too far gone, or can you generally just treat and add extra support?

 

[Mottie]

I had exactly the same in my first house - 2 or 3 woodworm infested floor joists. Luckily I was able to remove them, pass the end of the new joists under the dividing bedroom wall and then slide them back into the cut outs in the wall, resting them on the wooden joists that were used to support the bedroom wall. All a long and distant memory as that was back in 1984. I then replaced the whole floor with interlocking chipboard flooring, screwing them down by hand with a Yankee screwdriver giving me a massive blister on the palm of my hand and all three days before my wedding!

 

[Diver Fred]

Hopefully you can get the skirting boards off. Then the last floor boards. Existing joists need treating (e.g. sovereign chemicals Aqueous woodworm killer) then sistering with pressure treated joists (6x2) set level with the top of existing joists, the joists need to be the tightest for you can get. Two 12mms bolts at each end and one in middle with Toothed Washers between the new and and old joists. You will probably have to recover and replace the electrics and piping. Once that is done then you can replace the floor boarding.

 

[Adam Orr]

Thanks wgt52, I'll give that a go.

Just to be clear:

• When you say two bolts at each end and one in the middle, do you mean so there are 5 bolts in total?
• Would there be any benefit to adding additional bolts along the length, in case one has run through a weak spot?
• Do I sister each joist with two joists, one either side?
• Does it matter what sort of torque I run them up to, as long as they're pretty tight?

Thanks!

 

[Diver Fred]

Thanks wgt52, I'll give that a go.

Just to be clear:

• When you say two bolts at each end and one in the middle, do you mean so there are 5 bolts in total?

• yes - per joist. bolts need to be about 6 to 10 inches from the end of the joist and the next one about 18 to 24 inches in.

• [*]Would there be any benefit to adding additional bolts along the length, in case one has run through a weak spot?
  

  • No I'd say not, the Toothed Washer will do a better job
    

  [*]Do I sister each joist with two joists, one either side?
  

  • No, just the one sister as you need working space.
    

  [*]Does it matter what sort of torque I run them up to, as long as they're pretty tight?
  

  • you need to tighten up so the toothed washer is fully embedded in new and old joists
    

Thanks!

 

[Adam Orr]

Thanks again wgt52.

Would it work if I just sistered a joist over around 2.1m? That would prevent a lot of the messy work and the joists are in a lot better condition on one side - they feel pretty much solid from about halfway.

Also when I replace the floor boards is it worth putting new ones in? I'm just thinking because I've cut quite a few of them to gain access so they may not spread the loads as well.

Cheers

 

[Diver Fred]

Thanks again wgt52.

Would it work if I just sistered a joist over around 2.1m? That would prevent a lot of the messy work and the joists are in a lot better condition on one side - they feel pretty much solid from about halfway.

You could - just make sure the overlap is around 3 feet.

Also when I replace the floor boards is it worth putting new ones in? I'm just thinking because I've cut quite a few of them to gain access so they may not spread the loads as well.

Cheers

Yes.

Don't forget the sistering joists should be pressure treated (or at worst - liberally treated with woodworm killer).

 

[Deleted member 174758]

Did anyone ask if the woodwork is still active? The evidence would be frass (powdered timber expelled by the larvae) left on the ceiling below. A lot of old timbers show signs of boring beetle in the form of flight holes (where the emerging beetle borrows out of the timber) but have no activity, so it's worth checking if the pests are still active.

If the structural timbers are so weak that they break away in the hand then surely it would be better to cut away the affected areas (which need to be burned) and replace them in their entirety as they don't offer any strength but can potentially harbour live infestation

We work on sistering joists with a minimum 1 metre overlap and bolts set at no more than 600mm centres, at least 100mm in from the ends of the timbers - but unless they are going through healthy timber they will serve no purpose. Where new timbers go into existing or new pockets in the masonry, and that masonry is an exterior wall, they should be envelope wrapped with something like roofing felt to reduce moisture take-up

 

[Adam Orr]

@JobAndKnock thanks for your reply. The woodworm is still active - I pulled up the floorboards several weeks ago and cleaned everything up and there is fresh frass that has appeared now, although I have only noticed it in the worst affected areas.

Regarding the state of the timber, it's not fully crumbling to bits as of yet but there are some spongy areas and some hefty chunks missing. The father in law seems to think it has plenty of strength left in it, although he has only seen pictures and he was a little too dismissive in my opinion.

I'm weary about cutting or removing the existing timber although I appreciate that may be the best way to go. The joists are a funny size as well (approx. 60mm x 162mm) which I can't seem to find off the shelf and I haven't the means to cut them to size properly. Regardless I'd definitely like to make sure I do the job properly because I suspect I am going to find similar issues in the other 3 timber floored rooms.

If I were to cut the rotten wood out:

1. Would I need to support the ceiling underneath? (Or would it be easier to just remove the ceiling and install from underneath - that would save me messing up the bedroom wall and possibly removing the last floorboard, assuming I can cut through the nails)
2. How tightly wedged is the current joist likely to be in the masonry and will I have to mortar around the new joist when I've installed it?
3. How much wiggle room will I have in the pocket? I'm envisaging problems with timber being cut a few millimetres too long/short, or just having difficulty getting the new piece in while half of the old one is still in situ.
4. Could I use a different size joist to replace it, say 47mm x 150mm, and just pack out the pockets with some small bits of timber/mortar?

Thanks!

 

[Deleted member 174758]

The woodworm is still active - I pulled up the floorboards several weeks ago and cleaned everything up and there is fresh frass that has appeared now, although I have only noticed it in the worst affected areas.

I'd say that before you do anything else you need to treat the infestation. Some of the best treatments contain permethrin (e.g Permagard), which kills a lot of insects and their larvae including wood borders, but whilst it is considered safe for humans (it is used in topical applications for lice infestation for example), it is not good for cats - so if you have cats, keep them well away from it until after the floor boards are down again. We are now in the period (April to September) when the larvae having pupated, have turned into beetles then break out and try to mate. The material from where they break out is actually the frass. It has to be said that the most common beetles have a preference for sap wood over heart wood.

Once treated I think I'd remove the punky timber with a stiff wire brush and depending on how bad it is I'd probably treat the better areas with wood hardener.

Your joists are indeed an odd size - 6-1/2 x 2-1/2in - dso if you are sistering or replacing them I'd look for something like 7 x 3in C16 treated (which will come in at something like 170 x 73) and have it sawn to 160 x 73 by the merchant. Any halfvdecent merchant should be able to do this for you - it is a standard trade service

If I were to cut the rotten wood out:

1. Would I need to support the ceiling underneath? (Or would it be easier to just remove the ceiling and install from underneath - that would save me messing up the bedroom wall and possibly removing the last floorboard, assuming I can cut through the nails)

Is this a lath and plaster ceiling or just plasterboard? If it is lath you are pretty much limited to treating the infestation, removing the punky stuff, using wood hardener, enlarging the masonry pockets and then sistering the joists. You may need to have the joists reduced to about 150mm to clear the "snots" on the top side of the laths, in which case go for a 6 x 3in (145 x 73mm) C24 treated timber. C24 is stiffer than C16 so that should be OK for the span. Taking out lath and plaster is a dirty, thankless task and should be avoided if at all possible

Always work on one joist at a time. In most cases support shouldn't be necessary so long as you avoid nailing stuff as far as possible

If working on boarded ceilings you can match the joist sizes exactly, but again work on one joist at a time.

Only take out large sections of ceiling if the joists are completely shot and need replacing

2. How tightly wedged is the current joist likely to be in the masonry and will I have to mortar around the new joist when I've installed it?

Generally not very, but it varies enormously. You ideally want to pack new/sistered joists onto something like slate shims then pack brick slips and mortar around the joist to keep it in place (this is done after bolting together). Joists going into an exterior wall or below ground level should always be treated timber. Ideally the joist ends should be additionally protected by using roofing felt or the like to do an envelope wrapping

3.How much wiggle room will I have in the pocket? I'm envisaging problems with timber being cut a few millimetres too long/short, or just having difficulty getting the new piece in while half of the old one is still in situ.

We aim for 100mm bearing ,,(timber to masonry) if possible, but never less than 70mm. It may be necessary to make one pocket deeper and/or wider to accommodate this or to allow you to get the timber in, then rebrick afterwards

4. Could I use a different size joist to replace it, say 47mm x 150mm

If you go shallower your new joists need to be stiffer (I.e.C24 instead if C16) and ideally should be thicker (e.g.73mm not 47mm)

The only other issue is bounciness - if the floor is bouncy I'd highly recommend installing a row of solid strutting across the room to reduce joist movement.