Hole Saw Size

[TudorCottage]

I’m refacing a section of our oak timber frame.

I’ve got a length of oak dowel which is 19mm in diameter.
Which brings me onto my question, is it best to use the same size as the dowel, smaller than the dowel or larger than the dowel? A quick look online has examples of all 3

Smaller suggests a really tight fit, the same suggests a snug fit and those in the larger hole saw camp suggest it will give room for the glue to spread. Although if a really snug fit is achieved would glue be necessay?

I was hoping somebody on here might have undertaken a similar procedure recently and could offer some practical experience.

If glue is used in this situation could anybody recommend the best glue for the job.

Many thanks.

 

[Feltwell]

Well, I'm no oak framer, but.....

I'd go same size, that's what I always do with beech dowels in my softwood or hardwood joinery repairs. Use a long auger bit as well, not a flat spade bit, they wander about too much and don't keep a straight line.

Glue or not? I don't know, I always do for repairs, I always use Titebond III. I don't suppose ye olde Quercus framers used any glue though on their pegs in structural joints.

So what are you doing, are you taking the decayed face off some timbers for a cosmetic repair with a replacement patch on the face, or have I got the wrong end of the stick?

 

[Cubist]

Timber framers would typically use a tapered peg, thin end first, and hammered into place. The taper has two functions; the first is to pass through the off-set holes in a mortice and tenon, say, and then to pull the two timbers together as the thicker part of the peg is hammered into place.

If I understand properly what you are trying to do this approach will not be appropriate and, like any ordinary wooden dowel, holes in the timbers should be equal to the dowels diameter and, for ease, glued in place. A strong, water-proof, expanding glue will do a good job if used sparingly. An alternative means of fixing the dowel in place is to cut a slot in the outer part of the dowel and, when its positioned in the hole, drive in a wedge to secure a 'force-fit'.

Remember to thoroughly seal the outer exposed faces of the dowels/pegs as rain-water will wick through the end grain. This, as well as ultimately rotting the peg, also a risks the peg swelling and perhaps fracturing/splitting the timbers.

 

[CliffordPope]

Pegs holding the top of the principal rafters in our old cottage are square at the thick end and gradually taper by having the points slowly planed off - 4 to 8 to nearly round. They are left with about an inch projecting at each end after hammering in. I'm sure there is no glue - surely the point is that if over time the wood shrank then the peg could be hammered in further?
I think round holes and dowels is more appropriate to fine furniture making, not to heavy building construction?

I've always understood that this was the original meaning of the expresion "a square peg in a round hole". The point was not that the peg didn't fit properly - the opposite in fact: it fitted very securely indeed.

 

[Cubist]

Yes. The 'square' shank of the pegs was a product of how the pegs were cut. They used a rather nifty guillotine like knife, one end of which was tethered to a wooden base (like a paper-guillotine), to shape the pegs from rough cut square sections. The shanks would typically be cut off a couple of inches from the surface of the beam/post to allow for just the situation you describe in terms of shrinkage. These days the pegs are turned but some framers keep the old practices going.

 

[Cubist]

Oh, an afterthought. If you are intending to let a new piece of oak into a frame its worthwhile letting the new piece of timber acclimate alongside the old section before making the final cuts if at all possible. The relative levels of moisture content can cause minor warping and different rates of expansion/contraction can cause cracking and splitting.

 

[Cubist]

Errrm.. reading your original post again, how thick do you intend to make the new timber splice and what is the corresponding thickness and age of the receiving timber. Oak has the nickname of 'Ironwood' for a very good reason, how do you intend to cut the rebate and what tools are you proposing to use?

 

[TudorCottage]

Dimensions in inches of the new oak are 17 in length, 4.5 in width and 1.5 in depth.
Not lightweight but not major construction.

It’s going onto I think original oak (500 years old) which I’ve got back to good solid wood and still has over 5 inches of depth.

l’ve read all about scarf joints but I’m not confident of a getting an accurate fitting on the existing oak. Is the pooling of water the potential issue of cutting straight edges?

I get the theory of tapered dowels but was hoping tight rounded would do the job in this instance. It’s in a convenient area and keeping an eye on things will be easy. i.e movement, damp areas and further rot.

This timber runs alongside the window which I guess has been changed for smaller windows over the years and the gap has been filled with a fillet of cement which I’m guessing has caused the rot to otherwise good solid oak.

 

[Cubist]

I was a little concerned that you were intending to add a fairly thin section that would be at risk of splitting when the pegs/dowels were hammered into place. That said a 1.5 inch thick piece is still at risk - be careful. As I now understand it you're just replacing some rotten material and provided you can get a clean enough contact between the old and new timber that should be reasonably straight-forward. In effect you would be splicing the new material into a rebate and It would be worthwhile considering the use of a strong water-proof and expanding adhesive between the two mating surfaces - after they have been treated to prevent future decay. Be aware though that if the two timbers are not very firmly clamped/pinned together theres a risk that an expanding glue will push them apart. Round dowels alone would probably not resist the pressure from the curing of the glue and the results could get ugly.

I would not be considering any kind of scarf joint for this kind of fix - but I may if I were replacing a whole section of the beam in question.

Some flat edges/ledges on external timbers are almost unavoidable and they do contribute to water ingress into joints or pooling that in turn contributes to decay. Often the only fix possible is a sloped fillet of burnt-sand mastic. That said, if theres an opportunity to include/make a slope in a new timber section, then that would be of some small help.

 

[TudorCottage]

I did think of making the new piece of oak slightly thicker and sloping the top edge. This is still possible as I’ve been given a lot of leftover oak from a friends finished project

Thanks for all the suggestions so far.

 

[Cubist]

I did think of making the new piece of oak slightly thicker and sloping the top edge. This is still possible as I’ve been given a lot of leftover oak from a friends finished project

Thanks for all the suggestions so far.

Coo left over oak. Doesn't last long round here, I had to start growing my own but most of my trees are 10 or less years old so not even enough to cart me away in.

 

[Cubist]

And... another after-thought (damn this aging (and raging) business).

You may like to think about using a 'hidden dowel' technique. Mark the positions on the beam where the dowels will be inserted and drill the receiving holes and drive in/glue the dowels to leave about half/three quarters of the thickness of the new timber exposed. Mark the drill holes on the inner face of the timber you will splice to the beam and drill the holes to say a half/three quarters of the timbers thickness. Check alignment and depths and when you're ready fit the spliced timber in place as previously discussed.

Not much extra work for a neater, and perhaps more weather-proof, finish.

Oh, and remember, as I just did, that you will need to cut slots in the sides of the dowels to allow air/glue trapped behind the heads to escape when you press/hammer them and the new timber into place.

 

[a twig]

Coo left over oak. Doesn't last long round here, I had to start growing my own but most of my trees are 10 or less years old so not even enough to cart me away in.

We have 8 large (4 stories high) oak trees surrounding our garden. Considering felling them and building a 16 gun sloop

 

[malcolm]

Don't use glue. That would be bad.

The principle for dowels (at least on my new timber frame) was to drill holes such that when a dowel was pressed through it would tighten the joint. The hole in the bit of wood with the tenon was drilled just a little short of where it needed to be and the dowel would tighten the joint.

For repairs it's good to make the joint at an angle that sloped down so water will be less likely to make it's way into the joint.

 

[Cubist]

We have 8 large (4 stories high) oak trees surrounding our garden. Considering felling them and building a 16 gun sloop

And you still won't cure the squirrel problem.

 

[Cubist]

Don't use glue. That would be bad.

The principle for dowels (at least on my new timber frame) was to drill holes such that when a dowel was pressed through it would tighten the joint. The hole in the bit of wood with the tenon was drilled just a little short of where it needed to be and the dowel would tighten the joint.

For repairs it's good to make the joint at an angle that sloped down so water will be less likely to make it's way into the joint.

Ordinarily I would agree but I don't think TC has much choice Malcolm. He's just applying a smallish patch after removing some deep surface rot from a beam and there would be an on-going risk of rain penetration behind the splice.

 

[malcolm]

I couldn't find prep photos but this repair is what I was thinking. The existing wood was prepared at an angle so any water would fall and not get into the joint when the seal fails. I used linseed putty to seal the joints. Then plugged an oak dowel in for effect and sealed that with linseed putty. This was not structural. Seems to work 7 years later.