Repair cracking roof strut

Section62

WIAWSNB said:

stuart45 said:

It looks vertical because of how the photo was taken. Looks like one of the web members of a Fink trussed rafter. It will be either in tension or compression, so these parts can crack. Repairs to trussed rafters are above my pay grade.

Such a 'split' would be of more concern if under a bending force. In comp or tension, timber is immensely strong.

How have you ruled out bending forces in this case?

Timber is good in compression and tension, but in compression the load bearing capacity will be affected by the slenderness ratio and loading eccentricity.  Excessive compressive load on a slender member causes bending/buckling, more so if the load is eccentric.  It isn't possible to rule out bending forces in this member without doing some further investigation.

WIAWSNB said:

I think it is clearly over-spec'd in any case, at least partly to take into account potential 'flaws' like natural shakes and large knots, as one look at the relatively small size of the fixing plates show.

As stuart45 says, this really isn't the case.  Trusses are designed to minimise the timber required.  For cost reasons, but also to reduce weight... if the truss itself is lighter then it has less of it's own weight to support which means smaller sections can be used.  To allow the smaller sections the timber has to be graded to eliminate pieces with excessive defects... like knots, splits and shakes.  Timber is natural and rarely perfect, but the timber used for manufactured roof trusses is supposed to be much better than average.

In this case, there is a knot (under the printed word 'graded') which is about a third of the breadth of the member, plus what appears to be a lengthwise split through the remaining 'sound' wood.  That piece will likely be less than "immensely strong" as a result. I'd doubt that split was visible when the truss was manufactured, or else the grader was having an off day.

The small size of the fixing plates is because they are an incredibly effective method of joining timber.  Nothing to do with whether the timber is overspecced or not.

WIAWSNB said:

I personally wouldn't give that 'crack' a moment's concern, but if in any doubt would check it at regular intervals to see if it develops.
Which it won't do.

You can't know that.  In the first post in the thread you said "But others will advise better."...  that was correct.  In this case it would be worthwhile asking a SE to check the roof and advise.

Section62

WIAWSNB said:

stuart45 said:

By 'over-spec'd' I just mean that the size is chosen to take into account expected irregularities. Yes I know it's graded, but it ain't a homogenous material, so chosen size covers these variations. If it were homo, then it would almost certainly be a smaller profile.

...

That isn't what overspecced means.

The size is specified taking a certain level of defects into account.  That is the specified size and grade.

Overspecced would be using a section larger than the specified size because you want a bit more factor of safety, or to allow for future modifications, or you've got excessive stock of a larger section.  Although with manufactured roof trusses the process is iterative, and calculations would be needed to confirm the increased weight of the truss isn't going to be a problem.

stuart45

If you look closely at the timber it's stamped TR26, which is a high grade timber, higher than C24. A truss shouldn't be delivered in that condition, although they do occasionally get damaged on site and the chippies still pitch them.

WIAWSNB

Section62 said:

How have you ruled out bending forces in this case? I haven't. I never said I did. 

Timber is good in compression and tension, but in compression the load bearing capacity will be affected by the slenderness ratio and loading eccentricity.  Excessive compressive load on a slender member causes bending/buckling, more so if the load is eccentric.  It isn't possible to rule out bending forces in this member without doing some further investigation. And in tension the capacity would be limited by the nailing plate. In fact, both forces are pretty much limted by these plates, as in compresion you'd need to ensure no contact area movement.  

WIAWSNB said:

I think it is clearly over-spec'd in any case, at least partly to take into account potential 'flaws' like natural shakes and large knots, as one look at the relatively small size of the fixing plates show.

As stuart45 says, this really isn't the case. And I explained to him what I meant by this. Trusses are designed to minimise the timber required.   For cost reasons, but also to reduce weight... if the truss itself is lighter then it has less of it's own weight to support which means smaller sections can be used.  To allow the smaller sections the timber has to be graded to eliminate pieces with excessive defects... like knots, splits and shakes.  Timber is natural and rarely perfect, but the timber used for manufactured roof trusses is supposed to be much better than average. Whilst ensuring adequate spec'ing to include inherent variations. So, size will be based on 'worst case' timber example, so effectively over-spec'd. 

In this case, there is a knot (under the printed word 'graded') which is about a third of the breadth of the member, plus what appears to be a lengthwise split through the remaining 'sound' wood.  That piece will likely be less than "immensely strong" as a result. I'd doubt that split was visible when the truss was manufactured, or else the grader was having an off day. Very possibly. But shrinkage shakes occur. 

The small size of the fixing plates is because they are an incredibly effective method of joining timber.  Nothing to do with whether the timber is overspecced or not. But it has a literal bearing if it's in tension or bending. So, that strut is almost certainly a strut. 

WIAWSNB said:

I personally wouldn't give that 'crack' a moment's concern, but if in any doubt would check it at regular intervals to see if it develops.
Which it won't do.

You can't know that.  In the first post in the thread you said "But others will advise better."...  that was correct.  In this case it would be worthwhile asking a SE to check the roof and advise.

Yeah, I know it. I'm willing to bet my life. Or a £enner. Come on - or are you chicken? Baaaawk baaawk baaaaawk!

(I'm ashamed)

diveunderthebonnet

I would get a large jubilee clip and fit it round the timber to pull the crack back in ,might even need two or three ,cheap repair and piece of mind.

stuart45

If the webs or chords of a trussed rafter are splitting, it's worth finding the reason. Wind racking is a common cause on the older ones. A couple of jubilee clips isn't going to change that.

revengep8nt14

stuart45 said:

It looks vertical because of how the photo was taken. Looks like one of the web members of a Fink trussed rafter. It will be either in tension or compression, so these parts can crack. Repairs to trussed rafters are above my pay grade.

Apologies for the late reply. Thanks for all the comments.

That's correct, it is a web section of the rafter. I'll post a better pic in a moment. I came to the conclusion it would just need sistering to stop it from cracking further.

revengep8nt14

Cracked web section is the first bit of timber on the left.

stuart45

That ones in compression. You can always call one of the truss manufacturers, who might give you some advice on a repair. It's possible the web timber had a small shake which has split more under compression over the years.
You've got diagonal braces so you shouldn't have a wind racking issue.
I've seen the chippies repair them with strips of 18mm plywood, glued and screwed each side, although done under the direction of an SE.

Section62

revengep8nt14 said:

Cracked web section is the first bit of timber on the left.

It looks like there 6x2 rafters sistered onto the original rafters of the truss - I've never seen anything like that as an original build (the truss should have been specified/designed with a larger section rafter if necessary) which possibly means there was either a problem with the trusses, or else the roof has been modified in some way at a later date.

Can you post a picture of the outside of this section of roof, for context.  Has the house been extended or had solar panels added?  And is the roof original to the house, or has it been re-roofed? (the trusses and membrane look much newer than the brickwork)