Afixing joists to wall for flat roof

[i_am_fubar]

I've been pretty set on using a 2x9 ledgerboard with joist hangers to join my replacement flat roof to the house wall. However, according to this document:

http://learning.covcollege.ac.uk/co...tion Sheets/CnJ/materials/CJ12_Roofs/M125.pdf (page 5 of PDF)

This is not an acceptable method. Joist hangers can be used if mounted into the masonry. But if a ledger board is used, then joists should be notched and a fillet used. to me, this seems to be a weaker aproach than timber to timber hangers :s

Anyone shed any light onto this?

Should also note that the roof is to be used as a terrace.

Cheers,
Fubar.

 

[tony1851]

The method shown in that document - fixing a fillet to a larger piece of timber - is indeed weak. The strength would ultimately depend on the shear reistance of the fixings of the fillet to the main timber.
Go with your own method of fixing the 2x9 to the wall and use timber-to-timber joist hangers.

 

[i_am_fubar]

Cheers buddy.

So point of failure in the fillet will either be sheer of the fillet/ledger fixing, or a splitting of the joist.

The POF in ledger with hangers is either the hanger fixings (more substantial than fillet) or the ledger board fixing (which will be huge).

Does anyone happen to have drawings of how many / what pattern of bolts to use? I was going to use 2 between each joist (diagonal to each other) on 400mm centres (per pair) M12 chemi bond studding into the bricks. Would use expanding, but worried about splitting bricks under the doorway.

Cheers,
Fubar.

 

[^woody^]

So point of failure in the fillet will either be sheer of the fillet/ledger fixing, or a splitting of the joist.

No, there will be no failure. Its a pretty standard method, the loads are not great

 

[i_am_fubar]

When I say POF, I mean that it would be the first component to go in a failure situation (such as massive overloading like a hot tub). Not that it would fail under normal conditions.

So, I would still say that of the two methods the absolute loading for hangers would be higher than the fillet. But I'm more than welcome to have someone explain further if I'm wrong.

Also, you say the loads are not great. This would be a paved deck flat roof, surely that is a reasonable load as far as loadings go? I'm not saying it would get close to the absolute load of either method, I'm just curious what the scale is? I would have put 'just a flat roof' as low loading and 'concrete screeded deck' at the high end, though I could be missing several scale factors of other uses (like a hot tub).

Cheers,
Fubar.

 

[tony1851]

So point of failure in the fillet will either be sheer of the fillet/ledger fixing, or a splitting of the joist.

No, there will be no failure. Its a pretty standard method, the loads are not great

OP said that the roof would be used as a terrace. Presumably normal domestic floor loading (1.5kN/m^2) will apply.

Putting in that fillet is adding an extra layer of complexity, and therefore increases the potential for failure. (May be OK for a small roof with no access, though)..

More straightforward and safer to use joist hangers.

 

[i_am_fubar]

There are specific tables for terraces (with access other than maintainance and repair) as, unlike domestic interior floors, there are snow loads to contend with on top of people and furnature.

I'm not so much worried about the increasing chance of failure. As Woody says, it won't fail, I just want to engineer this as strong as I can, as easily as I can. The original question was less towards if they would both work (of which i have no doubt), but are they both acceptable under building regs.

Cheers,
Fubar.

 

[tony1851]

The original question was less towards if they would both work (of which i have no doubt), but are they both acceptable under building regs.

There is no specific detail for this type of connection referred to in Building Regs.
You could make the connection with a few panel pins or some packing tape if you wanted.

But, if the building inspector wasn't happy with what he sees, he would probably ask you to submit structural calculations to prove - to the satisfaction of the council's checking engineer - that the connection was in fact OK.

So, to the question 'would it comply with Building Regs?', there is no straightforward answer, at least initially, unless you hired an SE to do you the calculations to prove that the connection was sound.


PS: as regards your general point that both would work, you could well be right. But remember that when fixing the fillet, you need to make sure that you dont split it with the fixings, so they are best staggered. This means that the fillet can't be too small. But the bigger the fillet, the deeper the notch on the joist, and you risk weakening that.

 

[i_am_fubar]

Cheers Tony,

So if a BCO came along and saw this:

Untitled

• i_am_fubar
• 12 Jul 2013
• 1

He shouldn't have a problem with it?

Couple of questions I do have, is 'Fixing point A' needed, or can it be excluded?

Can 'Dimension B' be such that the hanger is right up agianst the end of the joist?

Would it be possible to use another fixing method just for the end joists so the ledger doesn't stick out past the final joist at all?

I ask these two questions to work out how to hang the joists for the edge of the roof. Don't really want the ledger board poking out through the soffits.

Cheers,
Fubar.

 

[tony1851]

Unless you are using the terrace as a gym, I would say your bolting pattern is overkill.

Don't put any bolts nearer than 2" from the edge of the timber.

Fixing A could be omitted (the bearer will easily cantilever past the next bolt in);

Fixing the end joist flush with the end of the bearer could in theory be a problem because you won't have enough nails in the hanger to satisfy the mfr's recommendations. You could get the tin-snips out and try bending it round the end of the joist; in practice it would probably be OK. The end joist will probably not be loaded as much as the others.

 

[i_am_fubar]

If I'm going to use hangers, I'll be using them as per manf. instructions (all holes filled with twist galv nails).

May look at using an angle plate between them:

http://www.wickes.co.uk/reinforced-angle-bracket-e2257090/invt/214968/

With extra ladder noggins between final and penultimate joists. That'll also allow me to have a facia that actually protrudes past the wall. (ladder noggins resting on the walls)

Fubar.