Replacing/strengthening cill plate and wall beams

[Philip]

In replacing weatherboarding I have found a seriously rotted cill plate and the adjoing ends of wall beams are rotted also. (Not listed barn conversion)

These wall beams will be behind the internal wall surface so the repair is not an exercise in aesthetics.

I intend replacing the cill plate and ideally the beams with green oak of a similar dimension to the existing ones. I would like to cut tenons on the beams and mortice the cill to take the tennon but my dilemma is how to insert the tennon. The top end of the wall beam enters a beam supporting the roof.

The SPAB pamphlet on timber repairs indicates cutting a wedge shaped piece of timber from the cill sliding the the tennon in, from the side, and then replacing the wedge. Whilst I can make cuts vertically in the timber at an angle to the edge, how do you make the cut longitudinally in the cill to release the wedge.

Alternatively I could just replace the ends of the wall beams which would alleviate the problem of how to insert the tennon, but would I feel leave a point of weakness where the new and old beams are joined by a scarf joint. The un-rotten vertical timbers show some sign of bowing and general wear and tear so ideally I would like to add to the strength and was thinking of adding a doubler alongside.

I could of course cheat and fix the new uprights using stainless steel or galvanised brackets and coach bolts, but then I feel I should not visit this website again.

 

[Nemesis]

The option in your final para is a perfectly valid and honest SPAB type repair IMHO. If it's the most simple then consider it carefully! Although of course the more complex method might be more aesthetically pleasing.

However - whatever you do try to retain as much as possible of the original timber.

 

[Gervase]

Instead of scarfing you could try using threaded steel rod and epoxy for a virtually invisible fix, or a steel fitch in a slot down the middle of the two pieces, bolted through to external fishplates for a less pretty but equally effective join.
There are degrees of repair and renovation, and although your mortice and tenon approach is extremely purist, no-one will castigate you for steelwork on unseen timber (although the rod and epoxy system is virtually invisible).
For advice on the purist approach, try sending a PM or email to Middi, who hasn't posted here for a while, but who is a whizz at timber construction.
It's a bit like having to choose between knitting your own hair shirt or buying a good quality cotton one! :wink:

 

[Nemesis]

There is a large timber beam held together, very visibly, by metal plates in the roof structure of what is now the tea room (NT) at Acorn Bank, Cumbria.

Does the job!

 

[Gervase]

Aye; you can regard a visible but well-crafted repair the way I look at my grey hairs, scars and wrinkles - as evidence of a life well lived!

 

[Nemesis]

Oh - I think of mine as signs of a life very badly lived!

Or would I be far worse if I had avoided the alchohol? Is it acting as a preservative?

 

[Gervase]

I do wonder sometimes - if I'd done to a horse what I did to myself for a couple of decades, I would probably have been jailed by the RSPCA!

 

[Penners]

I, on the other hand, always treat my body as a temple. I just wish the temple had been better designed and maintained.

Philip - if you're still attracted to idea of a mortice and tenon, remember that the wedge of oak that you use to fill the wedge-shaped trench doesn't necessarily have to be the same bit of oak that came out of the trench.

So you could make your vertical cuts that will form the walls of the tapered trench, then simply chisel out the stock until you've achieved the desired trench shape. Once you've swung the tenon into position, cut a new wedge to fill in the trench and peg it into place.

 

[middi]

If you wish to have a traditional mortice and tenon joint to attach the existing wall studs to a new oak cill plate ( beam) with traditional oak pegs, you have no choise but to to lift the newly morticed plate into the existing tenons, with the " good" ends of the existing wall plate lapped to accept the new plate and these end joints also oak pegged.

This ideal repair will only be possible if the existing studs have a good existing tenon, a careful dismantling of the existing base wall to about 100 or more mm is exercised ( dependant on thickness of new cill plate ), and carefull measurement of distances of mortices and lap joints to be cut is carried out.

In the event that the old studs bases have rotted away where it is necessary for a few centimeters of the rotted material to be removed in order that a new tenon can be cut, therefore making the above approach not suitable, I would favour what I refer to the "double cill" approach. This would still require the section of rotted wall plate to be replaced by splicing a new oak cill of same dimension and pegged in position, but this new wall plate section should slide under an already repaired morticed and tenoned cill plate onto the " shorter" wall studs.

This second option does not involve any base wall dismantling, albeit this traditonal repair will show a slightly thinner cill pegged onto another. A lot of the original babric is maintained by what i would define an " honest" repair.

The other simple option is to use galvanised straps and angle plates, holding everything together by stailess steel bolts or screws and cover up the lot.

Whilst this latest option is favored by many as an acceptable repair, as it usually retains most or all the original fabric, it has to be appreciated that it does " stiffen" the building in those aereas where it needs to be most flexible, the joint.

Whilst metal straps cure one problem, they usually create another. Admittedly at times they are indispensable (and very quick).

The choise is yours.

PS. unless the existing wall studs are rotten and unserviceable, do not attempt to "straighten" the wall by adding extra support timber, you will loose the character.

 

[Philip]

Many thanks for all the comments and advice. At least I wont feel too guilty if I use metal brackets.

Middi I like the idea of the split cill, apart from the fact I have the cill already and I dont relish the thought of trying to cut it half. At 6inches square my circular saw isnt going to cope and im blowed if im going to saw it by hand. However if I only replace the minimum of the cill it may be possible. I will report back when I get around to doing the work. Ive got to wait until the blue tits and blackbird fledgelings have flown their nests.

Thanks also for the views regarding doubling and strengthening. Where some studs have bowed, I am concerned that they may eventually fail and they are supporting the roof. Whilst I can fully accept the point that I should not try to straighten the wall where this has occurred is it acceptable to add adjacent timber to stop it bowing further?

 

[Penners]

I like the idea of the split cill, apart from the fact I have the cill already and I dont relish the thought of trying to cut it half.

The other problem is that, if you already have the cill cut to size, then cutting it in half will reduce its thickness by the kerf of the saw.

I hope that salves your conscience! :wink:

 

[middi]

You can certainly strenghten the existing studs as you suggested if you are concerned, my guess is that it would not be necessary.

Usually oak wall studding was always well overengineered, normally the eaves plate ( or beam) supported at each corner, with perhaps a centre intermediate stud, would be adequate to hold the roof up.

It does all depend on thickness of the main structural beams and their condition.