screwing joists into old oak beam

[BigLebowski]

hi there

I need to screw down new thick oak joists onto a load-bearing late medieval beam to stop it creeping.
The beam is at head-height and supports an external elevation, ie the side of the house.

I am stuck between using stainless steel coach screws or coach bolts. I am worried screws might
split or crack the old timber which is hard and brittle. Using coach bolts will be a bit more fiddly requiring
steel plates to accept the square slot head. I'm also unsure re. bolt sizes, eg M10 or M12 and if I
use screws, should they be recessed into the new oak or all the way through?

Are bolts or screws generally used in these circumstances?

Best
Dude

 

[middi]

Use M12 coach BOLTS used in conjunction with heavy duty square washers, coach screws are not recommended if the oak joist need to act as a support beam.

Coach screws are usefull only if the oak plates need to be stabilised from lateral movement or " sliding" action, then again, if oak beams/plates need to be stabilised from lateral movement it may be easier to use oak pegs.

Middi

 

[Penners]

Seconded! (Not that Middi needs my endorsement!)

 

[robgil]

I would use oak over stainless if I could, stainless can be brittle.

 

[Moo]

Many of us can testify that oak pegs can last for hundreds of years.
The jury's still out on metal substitutes.

 

[BigLebowski]

Thanks guys

I need to stop lateral movement. The lintel at head height is creeping slowing into the house, a few mm/year. There's a door under it which is becoming difficult to open. The frame is twisting as one side moves more than the other.

The existing ancient peg holes are 20mm dia. So do I auger a 20mm hole? How do I make the plugs to bash back in? I am not a chippie unfortunately.

I've found ready-made dowels as follows:

http://www.uk-sleepers.co.uk/dowels-18mm-long-p-813.html

These look good but dO taper them and how do I prepare the hole?

Best
Dude

 

[middi]

Traditionally tapered oak pegs ( made from oak hartwood) would have been longer than the hole depth, about 5 mm smaller than the hole at the tapered end and about 5 to 8 mm bigger at the thicker end. The peg hole would have been drilled straight through a mortice but the tenon hole would have been drilled a few mm off centre , therefore when the peg was " hammered" in the hole (s) the two wood sections tendency is to become tighter with every hammer blow.
Because oak shrinks considerably when " cured" and this can lead to loose joints, it was normal practice to make a peg longer than " adequate" so that it could be hammered again as the joints became loose. This system worked brilliantly and especially effective when the pegs were hand made with rough exagonal to octagonal features. Due to this feature the peg would not only pull the " off centre" hole close together bu it would also " shape itself to the hole " as it was hammered in position.
No need for pecision engineering here.

After all this "waffling" I regret to inform you that this tapered peg " off centre" approach is totally useless for your application. You might as well drill a relativelly large hole and " wack in " a straight peg with no taper but slightly larger than the hole with, approximately, an octagonal shape.

( I have used old oak rake handles " shaped" into pegs to overcome lateral movement on some large beams )

Middi

Forgot. You prepare the hole with an auger and an electric drill ( B & Q if you like), unless you want to use an handle aperated auger drill of course.

 

[Penners]

I need to stop lateral movement. The lintel at head height is creeping slowing into the house, a few mm/year. There's a door under it which is becoming difficult to open. The frame is twisting as one side moves more than the other.

BL - I'm having some difficulty visualising the exact nature of your problem. As you'll appreciate, only certain types of joint are suitable for a pegged solution, and I'm slightly concerned that your explanations, "screw down new thick oak joists onto a load-bearing late medieval beam" and "lateral movement", might suggest that a peg would be inappropriate.

Would it be possible for you to post some pictures of the problem and your proposed solution?

 

[BigLebowski]

hi there,

You can see the situation here:

http://195.238.172.11/~green/beams/beams.JPG

I have limited access to the joists from above and can screw down or drill down and peg into the old plate.

Best
Dude

 

[robgil]

Mine are nailed with iron nails through the joist support beam and into the joist, been fine since 1760 +/-.

 

[Penners]

OK, thanks for the picture. A few more questions, I'm afraid!

* You describe the mobile beam as a "plate" in the side elevation of the house - do I take it that the beam is therefore a plate resting on top of the ground floor studs and supporting the first floor studs?

* Is the door therefore an entrance to the house?

* You describe the mobile beam as "twisting into the house". By this, do you mean that the whole thing appears to be rotating on its longitudinal axis, or is moving inwards? See my sketch below, showing a cross-section of the wall, for (I hope!) clarification. (If you can't see the full height of the sketch, just click on it).

I'm not sure the plate looks "puny"! As for its age, the surface may appear dry and brittle, but in general such oak members tend to be hard as iron below the surface.

So what exactly is your proposed solution? Where and how are you intending to fix a reinforcing member?

 

[robgil]

Its not a support beam fixed to the studs is it?

 

[BigLebowski]

Hi Penners, Rob, many thanks for coming back to me.

Yes, the beam is a plate resting on top of the ground floor brick elevation, supporting the first floor stud wall and thence roof. The ground floor elevation was historically oak struts with wattle and daub infill but is now 9" brick and lime.

NB: the beam/plate simply "sits" on top of the brick ground floor elevation. There is no fixing apparent historically but I don't think this is significant.

Yes, the door is a main entrance to the house.

The beam/cill plate is moving inwards, along the left hand arrow of your picture. I presume this is because when the builders replaced the rotten joists they fixed the new joists to the old cill plate using 2mm wood screws which have, unsuprisingly, bent. I found a late-medieval long thick oak nail still in place in an old hole in the cill plate. Presumably this was one of several used at some stage to fix the joists in place.

My intended solution is to fix the joists firmly to the plate, paying particular attention to the joists around the door frame. I believe this will stop the cill plate creeping into the room. The fixings can be one or perhaps two steel or oak screws, bolts, dowels or nails inserted through the joist from above into the plate. Alternatively, "L" brackets can be placed on either side of the joists, to allow securing the joist to the top of the cill.

I am unsure about whether to use M8, M10 or M12 stainless steel coach bolts or screws or to augur a 10-20mm hole and wack in a very long (250mm) oak dowel or "nail". My feeling is that screws may crack the old oak which as you can see is already split in places. I am also concerned about drilling a relatively large 20mm hole in the old plate. This again may provide an opportunity for existing cracks to join up or enlarge. A further consideration is whether to repair the cracks before work commences using an epoxy filler such as Timbafill.

Best
Dude

 

[Penners]

BL - thanks for that further explanation.

A few questions remain. Is the plate moving sideways into the room by sliding across the top of the ground floor brickwork, or is it pulling the brickwork with it? How quickly is this happening? What is the exact evidence of the movement, other than the binding door?

My principle concern is to try and discover why this is happening, and how fast. It's difficult to understand the reason - one that might suggest itself is that the new joists (being screwed to the top of the plate) are shrinking and pulling the plate inwards. But the shrinkage of green oak is almost entirely across the grain - shrinkage along the grain is negligible. So I'm dubious about that possibility.

You mention that the screws used by the installers of the new joists have bent. What does the direction of the bend tell you about the direction of movement that has taken place?

I'm sorry to be bombarding you with so many questions, but I'm concerned to try and get to the root of this problem. That way we can ensure that your proposed solution is actually going to solve it. I regard this as very important, given the structural movement of the house.

By the way, I wouldn't be too concerned about longitudinal cracks in your oak beams. These are very common, and rarely affect the strength of the member to any significant degree. And it's perhaps worth mentioning that Timbafil won't have any material effect on the strength of the beam. It's just a cross-linked polyester filler, mainly used for cosmetic filling.

Middi - if you're following this thread I'd welcome your views. Am I making too much of this?

 

[BigLebowski]

hi Penners

The plate is moving sideways into the room taking the brickwork with it. It is happening very slowly, a few mm each year. There is no evidence other than the progressively binding door and visible twisting in the door frame at the top where it meets the beam. The brick elevation looks slightly slanted but has probably "lime-crept". The direction of the bend is not clear from the screws I suspect they bent during installation. They are puny enough.

The oak joists are not fixed in any meaningful sense to the plate. I suspect the movement is due to a slight imabalance in forces exerted from above. Perhaps the studding above is not transferring an entirely vertical force and there is an inward, twisting component?

I suspect it is reasonable to assume that the joists should be fixed to the plate and that will solve the problem; the other end of the joists is fixed into a meaty beam. All that remains is to reinstate the bond in the best way possible.

Best
Dude

 

[Penners]

Hmmm... OK, so long as you're satisfied that the proposed solution will indeed solve the problem.

I'm hoping that Middi will catch up with this thread - he's a real expert on timber framing.

 

[Moo]

BL - you've kept your whereabouts a mystery, which is a mistake: if you were to add a hint in your Profile there's a chance that one of the real experts on the forum lives quite near to you and would be only too happy to pop over and give you sound advice based on close observation.

 

[middi]

I can fully understand why Penners is " bombarding this thread with questions in search for answers to your problem.

It appears, from what you describe, that you may have a slightly more serious problem than just a couple of oak plates sliding across each other. If I understand correctly the top half of the building ( timber framed ) is, albeit very slowly, sliding across a brick wall that replaced the original oak frame in the lower section of the building.
This later modified construction is not unusual mainly due to the lower half of the building rotting away ( usually due to poor maintenance) and repaired with readily and cheaply available bricks, especially if there was no listing restrictions to adhere to. Ther are quite a number of examples timber frames " repaired" in this manner.

There is absolutely nothing wrong with this kind of repair and some modern half brick/timber buildings are still being built, although with stabilised treated timber and strict building regs followed in the manner which the top half of the building is secured to the brick built lower half, ie, one metre heavy duty straps on the cill plate bolted to the wall face.

It is a lot different in you case, the building (old oak presumably) was already well distorted when " repositioned " on the later added wall probably with little, if any, anchorage to the wall. This should not be cause for major concern but before you worry about stopping oak sling across itself it may be wise to engage the services of someone that understand and deals with these building to try and ascertain how serious the problem is and what would be the best solution to the problem.

Due to the " distortion" forces existing in the upper structure it is quite possible that this top half of the structure is trying to distort itself from a rigid wall hence slowly shifting bricks in the process.

Middi

 

[Penners]

it may be wise to engage the services of someone that understand and deals with these building to try and ascertain how serious the problem is and what would be the best solution to the problem.

Due to the " distortion" forces existing in the upper structure it is quite possible that this top half of the structure is trying to distort itself from a rigid wall hence slowly shifting bricks in the process.

Middi

I wholeheartedly agree. If I detected your problem in my building, BL, I'd want to get a qualified structural engineer to trace the root cause of the problem. You describe the movement as "only a few mm a year", but that's not insignificant.

Obviously it's your choice, but - as I said - I would be engaging the services of a structural engineer who is qualified in old buildings of this type.

 

[Keithj]

I endorse the warnings! If it's moving, there has to be a reason. Old timber doesn't suddenly start twisting or moving. Have a structural surveyor take a close look at it Our house has some seriously "out of true" timbers, but it's clear they haven't moved for a few centuries (confirmed by the surveyor).

We had some problems with the extension (also timber framed) when an error by someone (they all pointed at each other) meant the building developed a list to starboard. The fix was to screw on reinforcing braces using large coach screws (M10, if I remember right).

Which is when we discovered the wide range of torsional strengths available in coach screws. One of the original ones bought by the builder sheared under the strain of being screwed in, before it was even tight. It turned out that the shear strength of that batch was a few hundred pounds. The ones finally fitted had a strength of several tonnes.