Short joists, and rafters

[glangri2]

Linked to my other post about rafters and joists, but slightly different ................

A neighbour has just had an extension built, very similar to the one that I am doing, and I grabbed the chance to climb up the scaffolding and have a look at how the builder constructed the roof.

He has taken the joists across the short span (2.5m approx) and then from the bottom of the rafters has taken short joists that meet the cross joists at 90 degrees (See below)

View media item 37624
Then up at the ridge, he has placed short horizontal 'Ties' (see below).

There are also Purlins at about half height.

My question is - Is what he has done, sufficient to stop the rafters from spreading out at the wall plates? ..................

In picture 1, it looks like the short joists are just attached to the cross joists with nails, and not joist hangers. Without any other tie straps, any spreading load would surely just apply a tension load to the nails and possibly pull them out over time.

Now, I do not know if the builder fitted any extra ties after I took the pictures, so I cannot comment on the quality of his work, but how should it be done?

This method of construction would really work well on my job, and combined with the advice that the 'Oldun' has already given me, I could be close to a design that will work for me.

Cheers
Gary

 

[SKETCH3D]

From what I can see on the photos, your neighbor probably had to fix some binders across the ceiling joists to prevent the spread of the rafters. The tie at the top is too far from the base to have any real effect.

Your neighbors builder may have been relying on the floor boards (Assumed the loft space is going to be habitable), not something that I would be entirely comfortable with.

In my opinion, the better detail would have been deeper ceiling joists / floor joists forming the ties at ceiling level, however this may not have been specified due to the spans.

The other thing that may be happening is that the ridge has been designed as a ridge beam. That way the rafters are being hung from those and so they won't spread at wall plate level. This should be designed by a structural engineer, and in my experience normally comes as a steel or glue lam.

 

[garyo]

What process do these kinds of roof construction go through to get signed off? Do BCO's tend to eyeball the construction for the first time on-site and then sign it off if they like the look of it, or will the builder have to submit some calculations/drawings?

The reason I ask is that for my small front extension the former happened, but I'd assumed that for something at a larger scale there'd be a more formal process.

 

[SKETCH3D]

Speak to your local building control department. They are normally quite approachable.

If you have noticed then not inspecting certain parts of the construction then they probably know the builder and the builders work so don't see the need for unnecessary inspections.

Normally calculations are required or timbers determined from a pre-calculated span table like that produced by TRADA.

 

[glangri2]

Your neighbors builder may have been relying on the floor boards (Assumed the loft space is going to be habitable), not something that I would be entirely comfortable with.

In my opinion, the better detail would have been deeper ceiling joists / floor joists forming the ties at ceiling level, however this may not have been specified due to the spans.

I see what you are saying.
Would the BCO be likely to sign off floor boards being used as the binders?
What dimension would binders normally be?

On a similar note though, is there a metal strap or special hanger that could be used at the perpendicular joint where the short joists attach to the ones going across.

The long span of the room is in the region of 7m, and so that would have required very deep joists or a steel at half way. I know that below there is only one room, so there isn't a load bearing wall to support the joists.

The other thing that may be happening is that the ridge has been designed as a ridge beam.

No. It is just a conventional ridge board.

 

[jeds]

My specification here would be metal strapping across the joists from rafter to rafter. Floorboards and ceiling boards do act as a diaphragm but should be treated as supplementary rather than the primary structural member.

 

[Blagard]

My specification here would be metal strapping across the joists from rafter to rafter.

jeds,
If I correctly understand your comment, it would be a continous strap from rafter to rafter. So I wondered what material you would be expecting?

Exparmet do straps of standard duty to 4m long and heavy duty to 4.8m long. both are the same material normally used as hold down straps to wall plates etc. Both would be rigid metalwork, but not long enough to span the situation. Or am I thinking of too heavy a gauge material

EDIT: Cheers Jeds your post below noted. I was thinking that you might consider banding too lightweight for the job.

 

[jeds]

Builders band. 20mmx20 gauge. Comes in 10m and 20m rolls.

 

[^woody^]

The method as per photo is OK as long as there were some metal straps (5mm) across 3nr joists and alternate rafters, and noggins or bracing to the rest of the joists

As is, those small joists have just been end-grain nailed which is no good at all

But the purlins may be enough to prevent spread

 

[glangri2]

.............. metal straps (5mm) across 3nr joists ............

Builders band. 20mmx20 gauge...........

A quick look on the Sabrefix website suggests that Builders Band is 1mm thick !

Whereas 'Restraint straps' appear to be available in 2.5mm and 5mm.

For the straps to bridge the first 3 joists, they would have to be 1600mm.

The restraint straps idea seems more robust, but is it overkill, and are straps available that are long enough ?

I'll have to investigate this further.

 

[glangri2]

As is, those small joists have just been end-grain nailed which is no good at all

How should those joints have been made?
I agree that what is shown will not offer much tensile load resistance.

How about Jiffy Hangers?

I am going to go for a very similar design on my extension. I want it to be as robust as required, but don't want to go over the top.
In the grand scheme of things, I have only got 14 of the short joists to do, so 14 extra Jiffy Hangers won't exactly break the bank, but that is a lot of twist nails to hammer in !

 

[glangri2]

The method as per photo is OK as long as there were some metal straps (5mm) across 3nr joists and alternate rafters, and noggins or bracing to the rest of the joists

Woody, that is pretty much what the BCO said.
He has asked for twist straps from ALL rafters, across the 3 near joists, and then noggins on 2m spacing.

Simple enough.

Thanks everyone for all of the tips.