Filling holes in joists

[The3rd Earl Of Derby]

pmsl.

Knot that it will make any difference to beam strenght.

[JupiterJon]

There have been at least 3 possibly 4 different re-wires of my house, none of the old wiring has ever been removed and the joists are full of holes. Can I use a wood filler from something like a mastic gun to fill the holes and restore some strength? The closest I have found so far is

looks a bit pricey given the quantity of holes I need to fill. There is also this stuff cheaper would it be OK??

TIA Jon

[Bob Mannix]

Sorry, squirting stuff in the holes will fill them up but will never restore any strength. Wood is good as a structural material becasue of the continuous grain running through it. Forming joints properly, using wood glue and clamping can create very strong joints as the (designed for purpose) glue penetrates the fibres over a large mating area (large in relation to the beam cross section). A filled hole is just a filled hole - all the stress goes round it (increasing the stress in those areas)and will continue to do so. Don't waste your money!

[The3rd Earl Of Derby]

And if you are persistant to fill the holes :-) by a couple of brush poles and cut them up into 1" pieces and bang them into the holes either side of the joist.

Common sense prevails. ;-)

[dennis]

As long as their diameters are less than 20% of the joists depth and they aren't on top of each other they should not be a problem.

If the floor is sagging or there are any other structural failure symptoms then you need some help to fix it as you don't understand the properties of timber.

This may involve fitting reinforcing (steel, plywood, etc.) to the sides of the joists or even replacing them.

[Peter Taylor]

Agreed. Epoxy resin used in conjunction with stainless steel tie rods can restore a good amount of strength to a beam, but it's a specialist job suited to the restoration of old oak beams, and not the material advertised on these links.

If the holes are at mid-height of the joists then they're not significantly weakening them, especially if they're not at centre span or right against the end bearings. The top edge of a beam is in compression, the bottom edge in tension, so along the centre (called the "neutral axis") there is a line where there is virtually no stress. Deep notches in the top or bottom edges weaken joists much more than holes.

Peter

[JupiterJon]

You (and other respondees) are right about not understanding timber, hence the question! I merely wanted to try to rectify a bodge while it was easy to do it. I get the message though, nothing is sagging so leave alone.

Thanks Jon

[Phil L]

What good is knocking dowel into a hole? - if anything it is going to weaken the joist even further by putting more stress onto the surrounding timber

[The3rd Earl Of Derby]

>>> much cheaper would it be OK??

Do you have evidence to back that assumption up Phillip?

[Phil L]

Yes, try knocking a piece up your left nostril and see what happens - either your face will become solid or your nose will explode into a ball of snot and blood, my money is on the latter.

Seriously George, *you* must know what happens if you force an object into wood? - especially if you force two in, one from each side as you have suggested.

[jim]

Disagree, having had this problem.

Leaving alone might be OK if the holes are within the zones given in the B Regs, but my joists had nasty large holes for CH pipes knocked through close up to the support wall. My prediction was a collapse waiting to happen.

I filled the holes with timber rounds turned to size on a lathe and glued them in place with a PVA glue (Evostik W waterproof version). (Tight fit but not forced) Then I faced both sides of each joist with

12mm WPB plywood glued with PVA and screwed on with 25mm gauge 6 screws and clamped whilst the glue set. Note: ordinary PVA glue *may* be susceptible over the years to weakening due to absorbation of atmospheric moisture.

Joist span was longer than the plywood so the plywood butt joiints are staggered on each side & run diagonally down at around 30deg to the vertical. In a few cases where the joist wall sockets looked dobtful, I hacked out the wall and ran the plywood into the wall, but mostly I stopped at the wall edge.

If it is just a short length of joist peppered with holes it is proabably Ok to face over the holed zone plus say 300mm at each end.

Method produces extremely strong joists which should give no future problem. Slight snag is increased thickness & decreased inter joist gap which can make for more difficult drilling.

I treated the joists with Cuprinol (Green) before fixing the plywood to guard against future rot or worm problems. Incidentally it rather appeared from setting times that Green Cuprinol is a catalyst (or similar) for PVA glue, ie it speeds up setting.

Result is a good solid floor with far less bounce than ordinary joists sized to the B Regs give.

In some parts of the world, eg N America where wood is plentiful, composite beams formed in a similar way are widely used, for instance to provide long ceiling spans.

HTH