Roof design question

[schiz]

I'm a bit curious about the way our builder's doing the roof on our extension. He told us the attic joists have to go front to back, to kind of 'hold the house together', which I presume is to stop the weight of the roof pushing the walls 'outward'. I assumed we'd have some joists running front to back, but there's a large beam going across in the other direction between the gables which breaks them in the middle.

The joists attach to the central beam with standard joist hangers that look like this: this, but that looks like something just used to resist downward pressure rather than side movement, so I don't understand how this works.

Do we need some kind of strap to join the joists across the beam and stop the roof moving them outwards, or are these hangers good enough for this? I'm sure the builder knows what he's doing, but it would be nice to set my mind to rest.

many thanks,
Sch.

 

[noseall]

Is it a pre-manufactured truss roof that he is building?

Sounds like he's using a main girder truss with all the other roofing elements built off the girder truss.

Post a piccie?

 

[schiz]

Is it a pre-manufactured truss roof that he is building?

Sounds like he's using a main girder truss with all the other roofing elements built off the girder truss.

Post a piccie?

No, it's not pre-manufactured, put together on site.

Hopefully you can see this:
//www.diynot.com/network/schiz/albums/4979/19855

It's 9-inch joists, rafters, and that's about it. No binders or purlins, but then it's a fairly small extension (garage with room above).

 

[noseall]

The image shows the floor lay-out and it looks ok. He has done it this way because of the spans involved. The larger central timber is simply splitting the span thus reducing the need for unfeasibly deep joists.

I am a little surprised as to the fact that he has been allowed to use timber. What is the span that this larger timber covers?

 

[schiz]

The image shows the floor lay-out and it looks ok. He has done it this way because of the spans involved. The larger central timber is simply splitting the span thus reducing the need for unfeasibly deep joists.

I am a little surprised as to the fact that he has been allowed to use timber. What is the span that this larger timber covers?

The span is 4m. It's the width of the garage.

It's not in the pic, but the rafters connect to the ends of those joists. So do you think those hangers are OK for this? I don't have much feel for the forces involved, weight of the wall, weight of the tiles, strength of the hangers when the resultant force of the roof weight is tending to pull them away from the central timber....

thanks,
S.

 

[noseall]

The hangers are fine, we have done a near identical set-up using a central steel spine, with timber bolted into the flange and jiffy hangers fixed to the flange timbers.

Some architects may specify an additional angle bracket, located at the junction where joist meets bearer, to offer belt and braces restraint.

But the jiffy's will work fine without the additional angle bracket.

 

[schiz]

Thanks noseall, nice to get a second opinion on it

 

[schiz]

Some architects may specify an additional angle bracket, located at the junction where joist meets bearer, to offer belt and braces restraint.

Just a follow-up to this, 6 months later some of the joists seem to be making their way away from the bearer, in one case I can now get a finger in the gap. I've seen a crack appear in the gable wall but I don't know if that's related. I'd expect cracks in the walls that the joists rest on instead if it was due to outward movement of the wall, but I don't see any there.

The odd thing is the gaps are only appearing on one side of the bearer and they don't affect the joists uniformly - only three out of about 10 appear to have moved.

 

[Deluks]

More pics?

Is there any internal strutting fitted since the last pic was taken? Has the underside been boarded out? what about the top? (are these supporting a floor?)

In the pics, there doesn't seem to be a decent connection between the hangers and sides of the joists (ie: some of the hangers are wider at the top than the bottom) Never mind jiffy hangers, these are well 'iffy' hangers, and something needs to be done to ensure a strong connection between main joists and the central one.

I can't help thinking that the central one should have been doubled (or flitched, or should've been a steel) and that the other 9x2's needn't have been that deep if they aren't supporting a structural floor.

6 months is too early to tell if more will move, my guess is that they will unless it's sorted.


If you want to move them back, I'd suggest bolting a block of timber to the side of opposing joists, and using a heavy duty T-bar sash clamp tighten it all up, just a couple of mil a day until the gaps are closed up. Then fix a long heavy duty strap across the top, or better, run sheets of 18mm sheathing ply the full length of the centre beam, screwed or nailed down every 150mm.

 

[schiz]

I figured it out. The central beam looks small to you because it started life as a binder. The builder asked if we wanted the floor to be strong enough for an attic room, and said they'd just increase the joist size to suit. We said yes. So they dropped the binder and hung the joists off it. That should never have happened, they should have upped the spec on the center beam, and, as you say connected adjacent joists to one another.

Maybe noseall got away with this because the the roof was supported elsewhere. Perhaps purlins would have done away with the need, but in our 'design' if you can call it that, we don't have any.

The builder nicely dodged this one by telling us all inspections had been done and then taking payment. On contacting the BCO he's told us the roof has yet to have the final inspection, so I guess that's when we'd get the fail. And, before you say it yeah, sure we should have checked, but everyone makes mistakes!

So if this is what you get from a member of the FMB I'm wondering how the f**k I find a decent builder these days. I give up .

Anyhow, the only option now seems to be to keep that level as just a ceiling, forget about the upstairs room and have done with it. Changing that beam probably means taking the roof off first, scaffolding and so on, extra expense and worse, trying to find another builder I can trust to actually do the job.

S