Replacing oak floor joists/beams...couple of questions and request for any advice

[Minkeycat]

Hi All

I'm replacing three oak beams which some may refer to as floor joists. I've got the new ones and have removed the old ones and I have a couple of questions...

The beams are pretty much 10cm x 10cm (4" x 4") and 250cm (8ft-ish) long

1. Do oak beams expand and contract much with temperature or atmospheric moisture changes? If so, how much movement can one reasonably expect over an 8ft/250cm joist/beam?

2. Is it better to alter the cuts in the central beam and wall beams into which the new beams will be fitted or is it better to cut the new joists so that they fit the existing cutouts in the existing beams or doesn't it matter? Having done a few rough measurements, I don't need to make a cut of more 1cm from any beam/joist.

3. Does anyone have any other hints/tips or advice that I can use to get the job done? I'm lucky in that we have very low ceiling height and I can lift each beam above my head without machinery. Additionally the wall beam into which they sit juts out into the room so I can just drop the new joists into them. This does make the job significantly easier.

One tip I can pass on to anyone else working on old beams is that if you're working on your own, you can't accurately tell by looking how much they're flexing/moving when you're on top of them [so it IS worth listening to your wife!]. I thought I was being sensible by sticking to the wall side of the beams and not the middle until I discovered the hard way that one of the old joists was completely eaten at the wall end where it was impossible to see so when I leaned on that end to remove a nail from the next joist, it gave way with a big crack and I came crashing through the ceiling. It was very silly but I thought I'd been quite careful and didn't consider I had been taking undue risks. Fortunately the low ceiling height in our house was a big advantage and I was able to grab the next joist and lower myself down where my foot found faithful Henry hoover who enabled me to step to safety so I was fine, the 'remove ceiling project' was suddenly well ahead of schedule and apart from more cleaning up than planned, no harm was done. (Henry Hoover is far more helpful than the dog who had decided to sleep next to the radiator in that room and opened one eye, gave me a kind of withering look and went back to sleep.)

 

[Cubist]

Hi Minkeycat,

The short answer to your first question is yes it does expand and shrink BUT the degree depends on how dry the new joist timbers are right now. Timber Shrinkage is measured on three axies; Longitudinal, Radial and Tangential, where Radial is the Narrowest and Tangential is the Widest face of the timber section. Shrinkage in each of these can be between 0.1% and 0.2% on the longitudinal, 3.0% to 5.0% for the radial and 3.0% to 12.0% for the tangential face BUT these figures reflect what can be expected for shrinkage from the timbers green state (freshly cut) and the air or kiln dried state. Hopefully the timbers you have purchased are air or kiln dried and future shrinkage will therefore be minimal. Unless you have a very wet room expansion should not be a problem you need to concern yourself with, just make sure that any joints are not too tight - you need a slight interference fit. If you can it is worthwhile storing the new timbers in the room they ae intended for use in for a couple of weeks to acclimate but this is not absolutely essential.

Regarding your second question; it is never a good idea to try to modify the housings in the existing support timbers if it can be avoided. If it becomes essential for any reason make sure not to cut deeper than one third of the thickness (tangential) of the support beams. Equally you need to make sure that any tenons or dovetails you cut into the new joists do not exceed 50% of the thickness of the new joist; 33% is about the ideal.

As to your third question. In my opinion this is a job best done from above to make sure you get the positioning right, to check levels and to ensure that the new joists are seated properly before they are secure. Use wooden pegs if you can to secure them but if necessary good non ferous or coated screws can be used; I prefer not to use hammer and nails for these jobs - it can be amazing how much collateral damage you can do to nearby plaster and other stuff without missing the nail head. As you have already discovered its a good idea to have a good sized piece of ply or particle board to hand to spread your weight across the existing and new joists while at work.

Sounds like you're having some fun and I hope this helps.

Cheers,
Cubist

P.S. When you have cleaned out the pockets in the support beams best to make sure the timber is strong enough to support the joist, they may need to be strengthened. It may also be an idea to treat the new and old timber with a suitable woodworm treatment.

 

[MikeG]

Firstly, at 4x4 they're not beams, they're joists. Common joists, in fact.

If these are of green (ie unseasoned) oak then you can expect shrinkage across the grain of approx. 8%, so some 8mm. They will not shrink along their length, although if they warp, you might expect a little pulling out of their mortises as they take on a banana shape. They will not expand at all if they are green, but once they have dried (about 4 years time), you can expect some minor seasonal movement which would be unnoticeable in floor joists.

The new timbers should be jointed to fit the existing mortises. The joint at the bridging joist (the central beam half way across the room) is one of the most sophisticated and evolved pieces of traditional carpentry, and you should make every effort to replicate it. The extended bare-faced tenon will sit on the centre-line of the bridging joist where the beam is in neither tension nor compression, and the haunching above it resists twisting in the common joist.....and the entirety of the joint is designed to take the least possible strength out of the bridging joist.

Finally, replacing joists which have rotten ends isn't the only way of proceeding. If this was a listed building then you might be asked to make a repair rather than a replacement, by "sistering" the joist with another bolted alongside. I trust you have already dealt with the issues which led to the outside end rotting, because this is obviously the most important part of this job.

 

[Minkeycat]

Many thanks Cubist and Mike G, you guys ensure that this is such a fantastic, informative resource.

I have been assured by the very helpful wood yard that the joists (thanks for the clarification!) are "bone dry" to the point where I had to wait until the weather was clear to collect them so I'm confident that any movement will be minimal. We've cured the issue that was causing the rot and I'm ready to go with the cutting of the joints in the joists currently measuring about 10 times to cut once!

Thank you so much for your extremely informative responses.
All the best

 

[MikeG]

........ I'm ready to go with the cutting of the joints in the joists currently measuring about 10 times to cut once! ......

Cut a template for the joints. Make it out of a cornflakes packet, or some thin hardboard or MDF. That way you can offer it up repeatedly until you've got it right, and then mark your joist with certainty.

 

[skier-hughes]

I'm not sure Cubists idea of making a platform is the best one, I'd go for a mattress under your working area
Ours are so low, if you managed to grab a neighbouring one you would be touching the floor with your feet if you measured 5' 6" or more, I know, I can!!!

 

[Cubist]

I'm not sure Cubists idea of making a platform is the best one, I'd go for a mattress under your working area
Ours are so low, if you managed to grab a neighbouring one you would be touching the floor with your feet if you measured 5' 6" or more, I know, I can!!!

Being one of those people who is a trifle perturbed by heights my preference is for anything that stops me falling in the first place. That said though softening the bump is also a good strategy but in low ceilinged homes make sure not to use a box spring which may produce the same result as a trampoline. :lol:

........ I'm ready to go with the cutting of the joints in the joists currently measuring about 10 times to cut once! ......

Cut a template for the joints. Make it out of a cornflakes packet, or some thin hardboard or MDF. That way you can offer it up repeatedly until you've got it right, and then mark your joist with certainty.

Just a reminder Minkeycat, and to add a little to Mikes suggested template, always make sure to measure from one end only of the joist when marking up for the cuts. Sad to say that many people forget this essential technique and end up wasting a timber.