Fixing joists to walls

[bobbybob]

Hi Guys

I have some questions about joists fixings, if someone would be willing to address them, that would be most appreciated.

1. What is the best way to fix 8x3 joists to 100mm blockwork (flat roof) ?
2. how deep should the fittings go into the blockwork ?
3. Is it okay to fix the joist to a lintel ?

 

[John D v2.0]

1 either built in a under the roof covering or you can rest on a wall plate bedded on the wall. Hangers wouldn't work right at the top of the wall.
2 usually want 100mm bearing but for a roof it's less critical
3 yes as long as the load is properly transferred as above

 

[^woody^]

With new build the joists would go straight on top of the block wall. Fixing to an existing wall would need either individual joist hangers in the wall,
or a 3x2 timber fixed to the wall face as a bearer plate with the joists notched or resting on top of this, or a 8x2 timber fixed to the wall face with timber joist hangers on the front for the joists.

m10 expansive bolts/sleeve anchors at 600mm centres would be fine, with at 70mm into the wall (120mm bolts).

You can fix to the face of a concrete lintel, but wouldn't that be too low for a roof?

 

[mangosRus]

Thanks guys. I have some more questions if you don't mind.

1. How far should the joists extend out of the wall?
2. How would I go about attaching a pvc fascia board to the joists? I was under the impression you just nail it on, but I read you should put some ply on the joists and nail the fascia to that?

 

[John D v2.0]

You were up late!
1 out in which direction? From a structural point of view not all at, but if you want to create an overhang to protect the wall, that's fine.
2 fixing into ends grain isn't very strong especially if it gets wet, but fascia is not heavy. Thinking about it, I'd probably put blocking at the end of the joist and screw to that. But the pros probably have a better answer

 

[^woody^]

You need to allow such overhang to accomodate any soffit, vents, or just the return leg of plastic fascia plus 5mm to stop water running down the wall.

Plastic fascia capping needs a base board. Normally 25mm treated timber nailed to the ends of the joists.

You also need to form a drip around the top of the fascia for the roof material to hang over - don't stick the roof covering to the fascia. This drip is a piece of tanalised timber 50mm deep and minimum 25mm wide.

 

[mangosRus]

You were up late!

Hah, I'm a night owl and relish the stillness of the night

Thank you very much for the replies guys, I have a much better idea of where I'm heading with the roof, and thanks woody for going out of your way to make a picture! That is very kind of you.

1. So the length of the joists will be around 5.6m from wall to wall. I am a little confused on what dimension joists I should get since different tables give me different recommendations. Currently I have 11 joists spaced at around 475mm. I was considering 75x220 C24 but not sure if that is overkill. The only added weight will be of the garage door.
2. At what distances should I put in the noggins? I was initially going to put them at the centre mark, but not entirely sure if that is proper.
3. Is OSB3 preferred in all/most cases ? I've read it provides 5% reduction in water swelling, which does't sound like much.

Thanks once again for your informative replies.

 

[John D v2.0]

You'll struggle to span that far, can you run the joists the other way? At that point the self weight of the joists ends up using all your deflection. However if the calculator says it's fine then go for that. Check whether you gave enough labour and tools to lift them up there as they'll be heavy.
Noggins/blocking should also be at the ends, so the joists stay parallel and spaced correctly.
I'd say use board that's rated for humid conditions, as you'll get condensation in certain weather conditions. Either class 2 ply or osb3 I think are the standards there. The problem wouldn't really be swelling it would be losing the fall on your roof because the board's no longer flat.

 

[^woody^]

75 x 220 C16 @ 450 centres will do.

Noggins or strutting at 1/3 span. The ends will already be restrained, but you'll need end noggins or smaller section timber for any plasterboard ceiling.

I've always preferred ply instead of OSB, as it tends to resist water damage better when the roof eventually leaks. But in service, there is not much, if any difference.

 

[mangosRus]

Once again, thank you guys for the very helpful repiles..

I have gone for the 75x220 spaced @ 450. I will be receiving the timber tomorrow and will be bolting the joists into the top of the walls.
Woody, you recommended 120mm anchor sleeves bolts, with 70mm in the masonary. That means I will have to drill 170mm into the joist with an auger bit and then a 12mm hole the rest of the way for the bolt to get through so I can nut it up. Does this sound about right? I am a little concerned about weakening the joists by doing this.

 

[^woody^]

120mm anchor sleeves bolts, with 70mm in the masonary.

That means I will have to drill 170mm into the joist with an auger bit and then a 12mm hole the rest of the way

I dont understand.

You are fixing a bearing plate to a wall. The timber is 50mm (or 47) and you will go into the wall 70mm, so that's 120mm.

But thinking about it, it will depend on what type of bolt you get. If you are using a masonry bolt or a hammer fix type, then yes a straight 120mm will do. If using a sleeve anchor or a expansive type bolt then you will need perhaps 130 or 140mm or so.

 

[mangosRus]

I dont understand.

You are fixing a bearing plate to a wall. The timber is 50mm (or 47) and you will go into the wall 70mm, so that's 120mm.

But thinking about it, it will depend on what type of bolt you get. If you are using a masonry bolt or a hammer fix type, then yes a straight 120mm will do. If using a sleeve anchor or a expansive type bolt then you will need perhaps 130 or 140mm or so.

Thanks for the reply. I'm not entirely sure what you mean by bearing plate here.
I've attached a picture of what I had in mind based on your replies. The hole was only there because the sleeve has a nut which needs to be tightened,

I do have some 120mm x 7.5 concrete screws, which would be much easier than using bolts.

 

[^woody^]

I thought you were doing this

or a 8x2 timber fixed to the wall face with timber joist hangers on the front for the joists.

which would need the wall bolts

At the other end, you don't bolt joists down on top of a wall. You fit vertical straps every 2m.

 

[mangosRus]

Aha, cool. I was a little worried about having to drill through 220mm of timber, hah.
Those vertical strips look like a much quicker solution, I may just go with them.