Help with lean to roof spec please

[HobyTom2012]

I have a single story extension project. It is 6m wide and the outer wall is 4m from house wall. I have 4.8m wide bifold doors specified in the outer wall. I want a vaulted roof i.e ceiling on bottom of rafters. The pitch I can achieve is 17.5 degrees. I want 3 no velux and I am using concrete tiles. What size timbers do I need for the rafters please? I assume 400mm spacing? Thanks if someone can help. Have searched everywhere!

 

[catlad]

I would say 9x2 would do, but one of the others with tables will be able to give you reg's, you do not say which tiles ? 50kg 2m ?

 

[big-all]

are you allowing for a flashing lip 'tile thickness 'batons and rafters when calculating you angle ??

 

[tony1851]

8x2 C16 @ 400c/s would be OK. Remember to double up at Velux.

 

[^woody^]

And how will the wall be retrained from the roof pushing it outwards - especially with such a wide opening in it?

 

[tony1851]

And how will the wall be retrained from the roof pushing it outwards - especially with such a wide opening in it?

Not again!

 

[HobyTom2012]

Thank you all. Very useful. I have a 5.4m catnic lintle (300mm wall with 100mm cavity) to go over the doors. I presume that if the rafters are properly attached to the house wall there will be no danger from excessive outward forces? I can just get the pitch angle. Thanks again

 

[^woody^]

I presume that if the rafters are properly attached to the house wall there will be no danger

Does the council inspector make the same presumption?

 

[HobyTom2012]

I presume that if the rafters are properly attached to the house wall there will be no danger

Does the council inspector make the same presumption?

I will be chatting to building control this week. I want to put a design proposal to him hence the reason I am asking for help here. He has been very helpful so far.

Surely the risk is the wall coming inwards if anything? Can anybody advise from a position of knowledge? Answers please not questions. Thats why I am here!

Thanks

 

[joe-90]

I'd ask for a refund if I were you.

 

[tony1851]

so far.

Surely the risk is the wall coming inwards if anything?

No; the tendency is for the lean-to roof to push the new wall outwards.

You need something to effectively tie the rafters at the top (where they meet the house wall). Inspectors vary on how they see this detail, but I've used a timber plate firmly bolted to the wall, and notched/nailed the rafters firmly to it, and then fixed a 6" Strongtie bracket between the pole plate and each rafter.

 

[HobyTom2012]

225 x 50 rafters doubled each side of Velux
Cover with 12mm ply
Bolt 150 x 50 to house with 12mm raw bolts at 600 centres
Lintel over bifold doors to be calculated by Catnic or Engineer to be able to take load.
Tiles to be appropriate design for pitch.

THANKS FOR CONSTRUCTIVE COMMENTS. NOT SURE WHAT THE OTHER REPLIES ARE TRYING TO PROVE?

 

[chickenlips]

If you are concerned about BI querying the lean to roof pushing out the new walls, then you could consider using 50mm glide shoes which would help to reduce this.

They are more commonly used in Trussed Rafter roofs and not in traditional cut roofs (well certainly not in my area anyways). But they are designed to reduce the horizontal deflection onto walls.

 

[^woody^]

Glide shoes won't do a thing to alter the moment from the rafters.

Those rafters will be trying to push the wall out. Tying one end of them won't alter this basic principle

 

[HobyTom2012]

Glide shoes won't do a thing to alter the moment from the rafters.

Those rafters will be trying to push the wall out. Tying one end of them won't alter this basic principle

Plenty of people have extensions this size. What is the answer please 'Woody' Surely it is down to the lintle being man enough?