Timber Frame build up.

[JamesP]

I have enclosed a drawing of our proposed wall and roof build up, using ibeams to create a portal frame at 600 centres.

I would appreciate anyone's thoughts on this,

Thanks

[ProDave]

What provides the racking strength? Is that meant to be a layer of OSB on the inside of the frame for that purpose?
 

[JamesP]

Hi Dave

I hope the OSB will be sufficient, was going to have horizontal 50mmx50mm batten across the roof to then fix 25mm batten vertically for an air gap and to fix metal roof.

[ProDave]

You will almost certainly need to have it all detailed by a structural engineer. that is similar in some ways to my build, and the SE specified that on some walls, I had to have two layers of OSB with staggered joints to give adequate racking strength.
 

[ProDave]

2 minutes ago, Sensus said:

 

There's no 'almost' about it!

 

Apart from racking stiffness a portal frame works by having sufficient stiffness at the eaves and ridge connections; your sketch doesn't demonstrate how that will be achieved, but it will need calculations to prove it.

 

True.

 

I have seen this type of construction, and there are were gusset plates bolted at the eaves and ridge joints.
 

[Gone West]

I have a portal timber I-beam frame. It has 15mm OSB3 external racking on the walls and roof. The SE even details the spacing of the ring shank nails fixing the racking. The spacing of the I-beams varies all over the frame depending on the loading.

 

[ProDave]

^^ and that shows the substantial gusset plates either side of each beam at the joints.
 

[Gone West]

7 minutes ago, Sensus said:

 

That's standard practice for any timber frame, .......

 

Yes, I was trying to point out the importance of using a SE because they will be involved with all aspects even the seemingly less important.

[TerryE]

I went through the technical pack that the SE supplied and which I passed onto my BInsp.  It was quite an eye opener for me.  There was were as many calcs to do with stiffness / racking strength as there were to do with load.  Or in my summary they were as much concerned about the stiffness of frame and its ability to take hurricane force winds without folding like a deck of cards as the loading to carry its own weight and the contents.

[JamesP]

Hi Sensus

Thanks for your comments and drawings.

 

37 minutes ago, Sensus said:

1) It would be better to have the vertical battens on the roof directly on top of the membrane/sarking for the same reason (any water that finds its way in can run harmlessly off to eaves, instead of sitting against the horizontal battens, on top of the sarking) and;

 

Drawing is  a little bit light, my Father does them by hand...you are right, we have a vertical batten on top of the membrane / sarking to fix the metal roof.

We have switched the cladding to vertical and yes we now need counter battens for the continuous air gap.

 

[Crofter]

Agree re the detailing at the junction between top of cladding and roof.

Why not just fly the sarking over the top of the cladding, thus protecting the end grain, and providing a better lath for water runoff.

[DavidFrancis]

@Sensus: as you have access to TRADA's "External Timber Cladding"! Their document "Top ten tips for timber cladding" says "where the end of a board meets a corner post detail, a gap of at least 8mm should be left to reduce the chance of water sitting against the end grain and being held there through surface tension"

 

I assume this relates to horizontal cladding as I can't see how you can get a board end adjacent to a corner post with vertical cladding.  I don't think I've ever seen a gap of 8mm between board ends and a corner post.  Am I reading this right?  If I am, do most people just ignore this recommendation?

[DavidFrancis]

Ah - there's a four page extract available online and it includes fig 5.40 which does show a gap of 8-10mm.  The extract also includes fig 2.2 (junction of vertical and horizontal cladding) where the gap looks to be a lot less than 8mm!

 

I think in another recent thread some one talked about getting boards tight up against a corner post as if this was good.