Rafters for lean to shed

[Jupiter01]

I have a 6 metre long lean to shed against the side of the house. It’s 70cm wide in some parts and then gradually narrows down to 50cm. The rafters will be built from 63mm CLS and go between the timber fixed to the wall and rest on the top timber of the studs. I will check the angle (against the wall timber) of the first rafter but wondering if all others will be the same too, given the reduction in width?

Can you advise on how I approach this please.

 

[noseall]

Can you advise on how I approach this please.

Where it reduces in width you have a few choices.
Either go with a flattie or..

Maintain the same rafter pitch and raise the height of the shed to accommodate the cut-back, or....
Maintain the same rafter pitch and length and have a large soffit over the narrower bit.

shed2.jpg

• noseall
• 18 Aug 2024

You'd still have to raise the shed wall, even for the larger soffit.

 

[Jupiter01]

Appreciate you sketching that out @noseall.
My situation is slightly different to that. The reduction in width is gradual. The wall timber will be level all the way through, against the wall.

Here are some pictures.

 

[freddiemercurystwin]

I guess I could work it out for you, it's actually dead easy with the software I use, in fact any cad software. You know your eaves will be higher at the narrow end - which begs the question why have you built the long stud wall at the front already when that will have a sloping top?

 

[Jupiter01]

Oh oh, excuse my simplicity but here is what I had in mind:


I would have a timber against the house wall running level. I’ve also worked out where my flashing will slot in above this.

My rafters will be cut out of 63mm CLS and without the bird mouth arrangement. I will screw them to the timber on the wall and rest them on the timber on my studs. I will then put some supporting braces across the rafters to make it all rigid. I thought I could just do this all the way through and when I put the OSB board on, it will be wider on the right side and slightly narrower on the left side. I can measure and cut the OSB once the rafters are in place. Similar to what I did for the base.

Also, unlike the above image, my rafters are almost flat. There a very slight slope away from the wall.

I am hoping and praying there’s a simple solution to this.

Thanks for your help.

 

[Jupiter01]

This may be a non-issue and I might be fretting over nothing….
Over to the experts

 

[Jupiter01]

Building on my simplistic understanding…
The book model seems to work. If you look inside, it’s wider at one end and narrower at the other. The book at the back is level and resembles the timber on the back wall.

I can then cut the front of the roof at the angle so the over hang is the same across the entire length.

 

[freddiemercurystwin]

So are you saying you want a consistent eaves height all along the roof? In which case it will overhang further at the narrow end.

 

[Jupiter01]

I would like to have an even overhang (for soffit and fascia) across the entire 6m length. This will be minimal but I’d like it to be uniform.

Again, the fall on this roof is very minimal and as indicated above, the floor is 70cm at one end and around 40cm at the other.

 

[Jupiter01]

To add, I am going to line the shed with plywood, membrane and then featheredge boards.

 

[freddiemercurystwin]

So if you want an evening overhang then the eaves will be higher at the narrow end. I'll let you know the few key dimensions I need and can tell you your wall plate height at the narrow end.

 

[Jupiter01]

So if you want an evening overhang then the eaves will be higher at the narrow end. I'll let you know the few key dimensions I need and can tell you your wall plate height at the narrow end.

That would be awesome.
Given the very small width and shallow pitch here, I think I am safe to fix the wall timber level? I can then raise the joists very slightly at the narrow end using packers? My stud is around 3cm lower than the wall timber which indicates how shallow this pitch is. If I’ve understood this correctly, the need to raise joists at the narrow end will be negligible.

My plywood and feather edge can go up to the top once the joists have been raised.

That’s what I had in mind. Please let me know as I was planning to put the wall timber in next.

Thanks for your help.

 

[noseall]

I guess I could work it out for you, it's actually dead easy with the software I use

...but dead hard for the builder to build, lol.

 

[Jupiter01]

Some more info on this.
Here are my plans until I learnt this may be an issue, due to the narrowing width.

As seen above, this is a very shallow pitch. Almost flat. Will it really pose issues if I carried this through the entire 6m length?
At worst, I could pack the rafters further down so they are slightly raised from the header stud. I can then cut all the rafters with an equal overhang for soffit etc.

I appreciate this may not be the textbook approach but is it fundamentally flawed? The attraction to this is that I can fix the wall plate and flahsing at the desired levels and they will be straight. It may also be easier for me - as a layman - to go along and adjust rafters if necessary.

That said, I am the layman and appreciate your advice on this. I thought it would be helpful to share the proposed pitch to put things into context.

I am keen to get the wall plate on as I can then temporarily secure this tarpaulin to it and protect the structure from rain, forecast for today!

 

[freddiemercurystwin]

If you tell me what dimensions 1 to 5 are I can tell you the elusive No.6 ....

This assumes your two side walls are perpendicular to the house wall?