Advice on joist support for lean-to structure

[hughcollier]

Hey everyone,

I am building a lean-to type structure a the side of my house. On the one side I have a retaining wall on top of which I have added a normal stud wall type frame using treated 4 x 2 CLS timber. The span required for the joists is 2m and so will probably go with 6 x 2 C16 for these.

I've build the frame that sits on the retaining wall but I'm a bit unsure about the best way to proceed with creating support for the joists.

Should I be using joist hangers (diagram B) or resting the joist on the timber attached to the garage wall (diagram A)?

I also need a slight pitch to the roof. Do I create this with furring strips or simply rest the joists at an angle (diagram C)?

Hopefully the below diagrams showing a cross section view demonstrate my questions a bit better than I have explained them above. Any thoughts or pointers on the best way for me to proceed here will be veery welcome.

 

[MARK.B.]

I would go with option B , if you are making your own furring pieces then it would be just as easy to cut the joists to the required angle and do away the strips

 

[Myfordman]

B with parallel joists tilted downwards to give >1:80 fall for drainage. a very small shaped cut end will get the joists resting properly on the hangers

 

[Inspector]

Just of B for me. I did a deck with a 2 1/2" slope in 12' with joist hangers and clips, no notches.

Pete

 

[Pedronicus]

What roofing are you using?

 

[Bingy man]

I would go with option b also I’d treat all end grain with a suitable end grain sealer even if your using treated timber. Did a similar job last year and used plywood for roof which was going to be felted ( torched on ) but customer then went with profiled roofing sheets . Due to the plywood i was surprised how quiet it was when the heavens opened up 20 mins after finishing the roof .

 

[hughcollier]

Thanks everyone for your input - I think I'm going to do option B but with 7 x 2 timber and cut the required angle in as per @MARK.B. suggestion to save me faffing around with making extra furring strips.

7 x 2 with a 1:60 fall still leaves me with a joist height of around 132mm at the thin end so should be fine.

This will also be cheaper too as opposed to doing 6 x 2 + furring.

@Bingy man I'll definitely use a treatment on the cut ends. I've even been splashing it into the counterbored holes where the bolts have been going in as well - just to be on the safe side.

What roofing are you using?

It's polycarbonate sheet type stuff - not the ribbed kind the flat sheets - so not going to be heavy. Don't know exactly what it is as it's leftover from a similar job a neighbor has done and he's been kind enough to let me have some to do this.

One other question - what's the best way to secure the joists to the top of the wall plate. Would some super long coach screws attached from underneath do the job?

 

[Jones]

B or C depending on whether you want to pay for hangers or not. I'd give the roof a good fall 1:60 or more, there's no benefit to going flatter with modern roof types. The right hand side seems a bit over engineered, why not just add the dwarf stud and strap it down ? You'll need ends or diagonals to prevent racking. Skew nail joists with 4" nails to wall plate, if you use hangers use square twist nails to fix.
Edit: with such a gentle fall you won't really need to birds mouth the bearings.

 

[Pedronicus]

It's polycarbonate sheet type stuff - not the ribbed kind the flat sheets - so not going to be heavy.

Exactly what we have. I'll try and take a couple of pics tomorrow and also post up some of our dimensions.

 

[baldkev]

How about cutting a 40mm slot out of the underside of the lower end of joists to go over the studwork? It'll help retain the studwork and save cutting. Obviously the slot will be out of square, but to get that accurate, pop a 40mm packer under the higher end and rest lower end on studwork, mark the plumblines to 40mm deep and away you go

 

[Molynoox]

Option C looks like best to me. It's the easiest and gives same result as the others for less effort, with the exception of the internal angles not being 90 degrees.
I've never understood firing strips and why people use them, unless you really need those 90s...
Martin

 

[Pedronicus]

I've never understood firing strips and why people use them, unless you really need those 90s...

Firing strips are used to introduce a fall on a flat roof construction to allow drainage of rainwater to a suitable gully/gutter.

 

[Molynoox]

Cheers but I understood that bit
Its so easy to simply angle the joists that I don't see the point introducing another component.

 

[John Hall]

Definitely B, without the strips, just cut the ends to the required angles.

 

[Jones]

Cheers but I understood that bit
Its so easy to simply angle the joists that I don't see the point introducing another component.

If you want a flat ceiling and fall to the roof you'll need firing strips but I'd agree it's best to avoid them if possible.

 

[Jonm]

I would put up guttering otherwise you will be dumping loads of water behind that retaining wall which presumably is holding up soil, structurally not a good idea.

 

[Spectric]

I would use C, use chemi fixings to hold the 6 by 2 to the wall as they are better than the old expansion bolts but notch the 6 by 2 so it has a birdmouth to sit on the top plate and same other end but make yourself a template first to get it bang on.

 

[Inspector]

One other question - what's the best way to secure the joists to the top of the wall plate. Would some super long coach screws attached from underneath do the job?

There are little metal clips sold where the joist hangers are. A 90º twisted piece of metal that takes a couple fasteners in the edge of the wall plate and another couple in the face of the joist. Keeps them solidly anchored in the wind. Sometimes referred to as hurricane clips.

I goofed in my reply in post 4 in that my deck was done as a combination of B and C. The joists in joist hangers on the ledger board and sloped to the wall plate, no furring strip, held by the clips. I used the specified screws as they were easier than trying to swing a hammer in the spaces.

Pete

 

[Pedronicus]

@hughcollier PM sent

 

[hughcollier]

I would put up guttering otherwise you will be dumping loads of water behind that retaining wall which presumably is holding up soil, structurally not a good idea.

That's a good point that I hadn't considered. I think I will take the opportunity to install a water butt as I don't really have anywhere else suitable around the garden for one.

There are little metal clips sold where the joist hangers are. A 90º twisted piece of metal that takes a couple fasteners in the edge of the wall plate and another couple in the face of the joist. Keeps them solidly anchored in the wind. Sometimes referred to as hurricane clips.

I goofed in my reply in post 4 in that my deck was done as a combination of B and C. The joists in joist hangers on the ledger board and sloped to the wall plate, no furring strip, held by the clips. I used the specified screws as they were easier than trying to swing a hammer in the spaces.

Pete

That's really helpful, thanks. I don't have a framing nailer and so skew nailing the joists to the wall plate isn't really an option so I'll definitely look into those clips.

@hughcollier PM sent

Super helpful. Thank you for taking the time to send me that.