green oak, timber framed houses and shrinkage

[RogerS]

Quite keen to have a timber framed extension to the cottage with some sort of infill material (probably brick). Most likely it would be from green oak.

However, my builder chum (and whose opinion I do generally respect) is saying that after about a year, the green oak will have shrunk sufficiently for there to be gaps between the infill panels and the oak timber frame.

I do see the logic in that but then got to thinking....how do the likes of Border Oak do it? There must be a way otherwise no-one would be making timber-framed buildings these days.

???

 

[mr]

Usual caveat applies - ie I know nothing. But then, ever one to pontificate, I might suggest that they use an infill material that moves as well?

Cheers Mike

 

[RogerS]

..maybe but an 8" green oak timber will shrink...what ?? 1/2" in a year or so? It would have to be pretty spectacular mastic

 

[mr]

Possibly a cob type infill and not brick ?

(you can tell I know nuffin can't ye )

Cheers Mike

 

[Paul Chapman]

They've featured a number of timber framed houses in Grand Designs. Possibly one of the books based on the series might provide you with a few ideas. W H Smiths seem to stock the books - a browse around there might be an idea.

Cheers :wink:

Paul

 

[gwaithcoed]

Sorry Roger can't help now but if the question had been asked earlier I could have asked Border oaks as they were at Woodfest at St. Asaph when I went last Friday.


Alan

 

[Wanlock Dod]

Might it have something to do with just quite how fresh your "green oak" is?

 

[engineer one]

surely if you design the b things you know how long it takes for them to shrink and move. i must confess when i saw the first one on an early grand designs, and the skill that went into it i wondered how it would work.

if you look at elizabethan buildings, and there are still a few, it seems that the lath and plaster will move with the construction timbers. of course it is said that many of the original elizabethan timber frame houses were built from timbers cut for the fleet planned by henry v111 and never built after the sinking of the Mary Rose, so the timber may not have been that green.

maybe that is the reason that someone can build a timber frame house with those wonderful pegged tenons. :?

paul :wink:

 

[RogerS]

Doing a quick Google shows some people advocating covering the timber-frame externally (which kind of defeats the object IMO) to eliminate the problem of shrinkage..ie you don't see it.

Others have a 'patented' technique...which seems worth following up

 

[paulm]

I'm wondering if you let it settle for a year or so after building and then seal it.

The other thought is whether they use a highly compressible foam insulation strip rather than mastic, so that it moves with the timber.

Don't know of course, just speculating as to what might work !

If you contact a specialist firm and tell them you may be interested I am sure they will tell you all you need to know.

Cheers, Paul.

 

[RogerS]

I'm wondering if you let it settle for a year or so after building and then seal it.

That's a possibility although having adopted that approach at our current house, it's a bit never-ending and the end result isn't too brilliant.

The other thought is whether they use a highly compressible foam insulation strip rather than mastic, so that it moves with the timber.

I think that that is what one company may use although exactly what type of foam they use will need to be discovered. I'm not sure how it would work in practice. For example, with a brick infill, do the bricks sit on the foam? I don't know.

Maybe a rebate cut all the way around each square hole (maybe on the inside face) and then something like Kingspan stuck in place with maybe mastic or similar. Then the brick infill can be put in place on the external face. When the oak shrinks, the Kingspan will prevent water getting to the internal surfaces but not stop the water getting into the frame...ideally the rebate would be on the external face but then you'd have nowhere to put the bricks!

If you contact a specialist firm and tell them you may be interested I am sure they will tell you all you need to know.

Ha..tried that. Tried to get a ballpark estimate to start with from three firms and had no response. Even spent £22 with one for a catalogue and that was paid for 18 May and still not arrived nor emails answered.

 

[DomValente]

I believe the framing will shrink, normally curved pieces are used to help hold the integrity of the structure.
It's simply a question of making the joints,M&T, and pegging.
The reason most are now clad is for heat retention.
But shrink and split it will.
My advice, if you are going to do this, get a professional in as angles etc. are important.

Dom

 

[devonwoody]

You wanna see some of the old oak timber frame houses down in Devon (2 to 300 years old) They have 3" blocks under their bedposts to make themselves comfortable in bed!

So its not only infill problems, its floor levels as well.

 

[inventor]

I think this issue of shrinkage, cracks and water ingress has to be handled with great care!

I've recently worked on a timber frame building built sometime before 1600. Inappropriate maintenance did a lot of damage! Keeping oak timbers exposed was fine. Start to hide them, especially behind a portland cement render, and the moisture content of the timber builds up, and, eventually, the rot sets in, even in oak. If you're going to use masonry infill, lime mortar is far better than portland cement, because it lets the water out once it's got in. Personally, I'd definitely ask an expert first - the owners of this building employ a structural engineer who only works on ancient buildings. He knows what will last.

Some of the structural timbers of this building were damaged by cement-retained water over a period of 50-100 years. They were replaced a few years back, meaning the surrounding infill was stripped out - masonry, lath and plaster, even plasterboard! The new infill is foam insulation board scribed to fit, held in place by lime mortar and lime putty. This is approved both by the structural engineer and by the listed building police (it's grade 2*). Yes, the timbers do move and shrink. Yes, you do get cracks. Yes, water does get in and yes, it's a problem. So the building owners have to go all over looking for cracks, every year, and filling them with lime putty. I guess this keeps going until all the new timber stabilises.

 

[houtslager]

I have built an extension to my humble house in East Friesland, it is a hybrid Straw bale construction.
Timer framed, with main load bearing posts staying outside the SB infill
the remaining purlins will be enclosed totally. From previous constructs, the timber frame was either infilled with SB, which inturn were forced in, ie a very tight fit, so when the timber shrank it was taken up by the bales. The timber and bales were rendered with a LIME based render, this allowed the bales and timbers to "breath" and any moisture was also allowed the escape.
Shrinkage of timber AND the render occured and this was treated by doing a lime wash on the smaller cracks, larger cracks was re filled with a thick slop.

I hope you do go the timber frame route, and can do it yourself, for there is a LOT of job satisfaction and pride in doing such a construction.

all the best in this, HS

 

[DomValente]

Another point is that, and this is important, when the timber shrinks the wood will also twist thus tightening the joints, this is where the construction gets its strength.

Dom

 

[Shultzy]

Roger, maybe a visit to the Avoncroft Museum would be beneficial, it only about 30 miles from you. They even have a Woodturner’s Workshop.

http://www.avoncroft.org.uk/sitemap2.asp

 

[jasonB]

Brickwork infill should be done with lime mortar which will remain flexible and allow the brickwork to settle as the oak dries out.

It is also possible to use a product like "Compriband" at the sides and top of each panel of brickwork, this is a compressed foam that comes on a roll, it will expand to its full thickness in 7-10 days but will be contained bty the new brickwork so will only expand as any gaps occur.

A lot of the oak frames are infilled with SIPs ( Structural insulated panels) which are covered in a wood strand board and then rendered with lime render which will again move with the oak.

Jason

 

[RogerS]

Thank you everyone for the advice, encouragement and suggestions.

We were struggling to see how we could realistically do it in timber and resigned ourselves to a brick extension (which is what most of them are made from in Herefordshire).

But, Jason, your input was especially timely as I think that we now have a fighting chance.

Down to the planners now

 

[jasonB]

Roger one thing you will have to decide is where you want to see the oak.

Unless you go for very large sections of timber the frame will likely have to be covered on one side as purely infilling will be unlikely to give the required "U" values.

So if you want brick infill you will in effect have to build an insulated stud wall inside the frame and face with plasterboard. Or if you want to see the oak on the inside walls then the oak frame will need infilling with a rigid insulation and the outside clad with something like ply and then weatherboarding. If you want exposed roof slopes thats easier as the insulation can be placed above the oak rafters and the roof counter battened and battened on top of that.

Jason