grading sweet chestnut tie beams...

[guston]

Hey everyone - first post here on the forum. I've been lurking for a while, what a great forum with great people too! I have already learned so much.

I'll get to it. We're based in Portugal and are restoring a 100+ year old random sandstone building (7x8 metre footprint). It has two existing stone gables. We will be making two king post trusses to go in between the gables (span between the walls is 7m). These will hold up the ridge beam and purlins, with the common rafters laying on top. Traditional joinery all around, hand riven oak pegs, draw bored joints etc The trusses will sit on both the walls and posts (4 posts in total). Roof will be tiled, with insulation between the rafters, plasterboard on the underside. No snow here. Basic plan below:





We just received our chestnut beams and posts for the roof a few days ago - exciting. Stacking it all today and tomorrow (flat, stickered, raised up, out of sun, borax treatment etc) ready to be worked on when the time comes. All looks fine apart from a few timbers we are concerned with. We understand what we can about grading timbers for their individual roles in the frame, positioning defects, crowning upwards etc but are lacking experience when it comes down to the line where defects become unacceptable in the primary beams and any beams under tension. 2x timber framers and 2x engineers have given our plans (dimensions, joints, materials etc) the thumbs up, but where we are located it is not an option to have someone with experience check over our beams. They come from a saw mill in the north or Portugal close to where the sweet chestnut grows. 

One tie beam (235mmx175mmx7300mm or 7x9x22ft) has some rot at one end, starting in the heart, with some signs of it showing on the outer edges about a third of the way along. The other tie beam has several clusters of knots (not dead) towards one end close to where the principal rafter joins. Im worried/unexperienced to know whether these are really an issue. Both beams have fairly straight grain running throughout with some run out around the knots. 

Should we be concerned about these kind of defects (structurally speaking)? Or are we worrying too much and should just crack on?  We have seen plenty of 100 year old+ timber frame buildings with all sorts of defects in the timbers that are still going strong - but on the other hand, reading through all the modern day grading guidelines has made my head spin. If anyone has any practical advice then we would be interested to hear.

I can provide more photos/vids if needs be! 

Angus 



 



 



 

Knots:



 

[Jim_Rogers]

If you specified a grade to the sawmill, and if these do not meet the grade, I'd ask to replace them.

I'm not sure what grade rules your country has for timbers but that rotten one may not meet a grade #2 in our area. 

As for the one with the knots, it depends on the grade needed for that tie beam. And where the knots fall in relation to the joints being cut into the tie.

Personally, I would not use the one showing the rot.

Jim Rogers

[guston]

I should of added - we specified to the sawmill that beams should be free of defects where possible, particularly tie beams, principle rafters and king posts.

No grading system here for this kind of timber, not that I'm aware of anyway. 

Thanks for your input Jim 

[Don P]

I'm not seeing a truss in the drawing. The parts are all there but with the joinery as drawn that is a post sitting midspan on a tie that is acting as a beam. The post supports the ridge and half the roof width. I'm hoping that is a simplified sketch.

The beam with the rot and shake is an easy call, come get your trash. The one with knots is hard to tell and I don't know your design assumptions or the species. If there is nothing else in that area on the other faces it is probably a #2. This means absolutely nothing if we don't know the species design values. If it is a tie beam and if indeed you are using it as a heavily center point loaded beam, I probably wouldn't want that near a point of high bending moment.

[guston]

I'm not seeing a truss in the drawing. The parts are all there but with the joinery as drawn that is a post sitting midspan on a tie that is acting as a beam. The post supports the ridge and half the roof width. I'm hoping that is a simplified sketch.

Hey Don, thanks for your reply. It is indeed a simplified sketch. The final design is a full king post truss - if you follow a line down the centre of the king-post, all parts on the left side of this drawing are mimicked to the right, they are just not drawn on this particular drawing. Does that make sense? Each side of the truss is symmetrical to the other, so only one half is shown.

Species is Castanea Sativa (European Chestnut or Sweet chestnut). Like I said, all dimensions, joints and the design have been checked over by 2 engineers and 2 framers. Both of the framers have experience working with this species. 

Im going to map out where the joints will land on the non-rot beam today and see what I can work with. Maybe I will add some photos later