Feedback on timber extension structure

[JackOfNoTrades]

Hi all, 

I'm just about to start doing a stick build kitchen extension with timber cladding and standing seam roof. 
I have had my first stab at a timber structure and wanted to run it by a few people before I send it off to the structural engineer. 

 

In the 3D mock ups, the blue beams are flitch beams - precise depth to be decided by the SE. I've also included the drawings that got me planning permission so you can see what it's building up to. As much as possible I'd love to keep it timber build and not introduce RSJs, which would be easier if I was just going for a box and not the roof shape we settled on in the planning stage! 

A few areas of trickiness:

- currently the flitch beam for the bifold pokes into the roof structure on the left hand side as viewed from the front. It will require a little notch on the end joist that is fully supported by that wall. 
- To the right hand side of the bifolds, I have support for the bifold beam AND the beam supporting the roof. This gives me about 32cm of studwork which is a pretty big thermal bridge. 

- might need to double up some of the roof joists as there will be a small rooflight installed. (smaller than in PP) 

- I have supported the flitch beam that goes into the original opening of the house with timber studs going onto old house foundation instead of putting the beam on a new RSJ across that opening. This is because I want to keep that original opening in the house as high as possible. 

 

Any thoughts would be greatly appreciated. 

 

Cheers everyone! 

Edited April 10 by JackOfNoTrades
reordering and updating photo

[stu w]

What is the opening span for the bi-fold doors 

[JackOfNoTrades]

9 minutes ago, stu w said:

What is the opening span for the bi-fold doors 

 3300mm 

Was originally going to be wider in the planning permission drawings, but it clashes with where I need the other support beam to be for the roof

Edited April 10 by JackOfNoTrades

[stu w]

4 hours ago, JackOfNoTrades said:

 3300mm 

Was originally going to be wider in the planning permission drawings, but it clashes with where I need the other support beam to be for the roof

I have recently designed a flitch beam and had it passed by an SE .

All depends on the total loadings , bolting pattern etc , the span shouldn't be a problem if designed correctly and the jack and King studs are adequate.

Cold bridging wise , can PIR cover the internal beam to help ? 

Edited April 10 by stu w

[JackOfNoTrades]

That's great to hear Stu. I had a go at designing the flitch beam but to be honest I'm not convinced I got the deadloads right. My estimate to put under the nose of the SE is 

 

Timber C24  47 × 195mm 

Steel S275  10 × 180mm 

Timber C24  47 × 195mm 

 

I'm hoping it wont need to be any deeper as it's already encroaching into that roof. 

 

Re thermal bridging - yes, I can have 50mm of PIR in front of all of those beams internally to help with that. I've got a continuous layer of PIR in my wall build up. (Included here for ref) 

 

 

[stu w]

17 minutes ago, JackOfNoTrades said:

That's great to hear Stu. I had a go at designing the flitch beam but to be honest I'm not convinced I got the deadloads right. My estimate to put under the nose of the SE is 

 

Timber C24  47 × 195mm 

Steel S275  10 × 180mm 

Timber C24  47 × 195mm 

 

I'm hoping it wont need to be any deeper as it's already encroaching into that roof. 

 

Re thermal bridging - yes, I can have 50mm of PIR in front of all of those beams internally to help with that. I've got a continuous layer of PIR in my wall build up. (Included here for ref) 

 

 

My extension is similar structure,  I am using 145 x 45 regularised , and the flitch beam is similar in size but the span is shorter at 2.5m

I personally would increase your upstand to 50mm, it will meet up with the internal PIR and you can tape them together. 

Plus I have opted to use kingspan but the nyrock frame slab as it is 32 not 35 with 50 PIR internally then my vcl then service batten.

What are you covering your sts with below damp , mosiac render or leaving plain ? 

 

Edited April 10 by stu w

[JackOfNoTrades]

Good point about the upstand matching at 50mm. Will look into that. Will also check out the switch form flexi to frame slab. Think I only defaulted to flexi because someone claimed it was easier to install. 

To be honest, I have made no decision about what to cover the STS with. I just know I don't want to be looking at breeze block and thermoblock. There's a strong chance I'll render it, but I was hoping to hold off on that decision until I'd seen the rest of the cladding on the building. 

 

A little worried to hear your flitch for a smaller span was similar. I'm really hoping it doesn't need to be much larger as it's already a pain. 

 

Edited April 11 by JackOfNoTrades

[stu w]

2 hours ago, JackOfNoTrades said:

Good point about the upstand matching at 50mm. Will look into that. Will also check out the switch form flexi to frame slab. Think I only defaulted to flexi because someone claimed it was easier to install. 

To be honest, I have made no decision about what to cover the STS with. I just know I don't want to be looking at breeze block and thermoblock. There's a strong chance I'll render it, but I was hoping to hold off on that decision until I'd seen the rest of the cladding on the building. 

 

A little worried to hear your flitch for a smaller span was similar. I'm really hoping it doesn't need to be much larger as it's already a pain. 

 

I've used nyrock a few times and it's a good product.

Not always the depth what necessarily needs to be increase on a flitch , possibly just the thickness of steel , but ultimately the loading its supporting.  I have good for Hardie VL plank on mine , I've used alot of cedral in the past but Hardie is better in my opinion .

A king stud may have to go all the way up through in places also looking at your drawings 

[JackOfNoTrades]

13 minutes ago, stu w said:

A king stud may have to go all the way up through in places also looking at your drawings 

Good point. I assume you mean to the left of the bifolds. 
That part is a bit of a challenge - there is actually no fully binding top plate at the moment because of the shape of the ceiling. I've removed the joists in this image to show what that looks like. The current lintel is 75mm too high to sit under a top plate. I can't raise the top plate because a joist needs to sit on top of them ..  

unless I do have a higher wall and a thinner joist running along the top? 

[JackOfNoTrades]

21 minutes ago, stu w said:

I have good for Hardie VL plank on mine

This is what I'm using on the wall that faces the neighbour's extension. There's no access there so I need something that will need no maintenance and is nicely fire resistant. 

[Russell griffiths]

Have you had that wall design passed by building control. 
in my mind it doesn’t look good enough insulation wise. 

[JackOfNoTrades]

19 minutes ago, Russell griffiths said:

Have you had that wall design passed by building control. 
in my mind it doesn’t look good enough insulation wise. 

Not yet. I'm pre submission (just about to send it all off in next few weeks). 
It should be fine though. The required u value for an extension wall is 0.18 
This build up gives me 0.159 if I've done my sums correctly. 

 

Edited April 11 by JackOfNoTrades

[stu w]

What calculator did you use for that please ? 

4 minutes ago, JackOfNoTrades said:

Not yet. I'm pre submission (just about to send it all off in next few weeks). 
It should be fine though. The required u value for an extension wall is 0.18 
This build up gives me 0.159 

 

[JackOfNoTrades]

2 minutes ago, stu w said:

What calculator did you use for that please ? 

 

ubakus. It's pretty decent. 

[JackOfNoTrades]

If I raise the height of the front wall by reducing the depth of the fully supported joist, I can have an uninterrupted top plate that spans the distance (highlighted in green). Still not perfect but potentially much more structurally sound. In this render I've hidden the front joist so you can see everything. 

 

 

And a side view to show the full depth joist next to the wall, and the shorter joist resting on top of the (higher) front wall.

 

A little unconventional maybe, but could this have solved my problem of the lintel being too high?  

 

 

[Redbeard]

You are asking for comments on the TF, but may I ask about the floor? What U value do you get with 100mm PIR? Could you fit 150 in (or even greater thickness and use graphite EPS instead of PIR)? Will you have under-floor heating? If so others will be along to highlight the greater delta T (and therefore the justification for more insulation).

[JackOfNoTrades]

26 minutes ago, Redbeard said:

You are asking for comments on the TF, but may I ask about the floor? What U value do you get with 100mm PIR? Could you fit 150 in (or even greater thickness and use graphite EPS instead of PIR)? Will you have under-floor heating? If so others will be along to highlight the greater delta T (and therefore the justification for more insulation).

Hi Redbeard. Not going for underfloor heating here. Family member had a tradesman come and drill straight through underfloor heating after promising he wouldn't. I've stayed away from it ever since. Irrational I know ... but that's me. 

Using Celotex 100mm I'm getting 0.18 for the floor. (the required u-value for the extension) 
Using the same thickness of kooltherm k103 I get a pretty decent 0.15 ... although at a whopping 3 times the cost. 
EDIT - and upgrading to 150mm of Celotex would give 0.13 


While I'm all for improving u values where necessary - particularly in a new build - I'm a little conflicted about spending much extra money on incremental improvements .... when the extension is attached to a building with solid brick walls with a u-value of 2.0. 

 

Edited April 11 by JackOfNoTrades

[Redbeard]

12 minutes ago, JackOfNoTrades said:

While I'm all for improving u values where necessary - particularly in a new build - I'm a little conflicted about spending much extra money on incremental improvements .... when the extension is attached to a building with solid brick walls with a u-value of 2.0. 

I take your point, but if you look at it in the context of a whole-house plan enacted over a very long period (possibly finished by the person(s) after you), there's still logic in getting the best you can from each element that you do 'enact'. I fully expect my refurbishment to be finished sometime after I am dead! Depending on how much work you are doing yourself the actual 'spend' could be as little as the cost of an extra 50mm. Agree also with 50, not 25mm upstand at perimeter.

[stu w]

3 hours ago, JackOfNoTrades said:

If I raise the height of the front wall by reducing the depth of the fully supported joist, I can have an uninterrupted top plate that spans the distance (highlighted in green). Still not perfect but potentially much more structurally sound. In this render I've hidden the front joist so you can see everything. 

 

 

And a side view to show the full depth joist next to the wall, and the shorter joist resting on top of the (higher) front wall.

 

A little unconventional maybe, but could this have solved my problem of the lintel being too high?  

 

 

Yes that could be an option, looking at it on my phone but can't see why it won't work 

[stu w]

4 hours ago, JackOfNoTrades said:

Not yet. I'm pre submission (just about to send it all off in next few weeks). 
It should be fine though. The required u value for an extension wall is 0.18 
This build up gives me 0.159 if I've done my sums correctly. 

 

Only minor I know and I'm sure you haven't,  but don't forget the breather membrane.

I use TF200 thermo , a superb product in my eyes .

[Iceverge]

51 minutes ago, stu w said:

Only minor I know and I'm sure you haven't,  but don't forget the breather membrane.

I use TF200 thermo , a superb product in my eyes .

 

I've used TF 100, cheap and plasticky, and TF400, top notch in my opinion. Better than Tyvek, 

 

is TF200 closer to 100 or 400?

[JackOfNoTrades]

1 hour ago, stu w said:

Only minor I know and I'm sure you haven't,  but don't forget the breather membrane.

I use TF200 thermo , a superb product in my eyes .

 

Well spotted that I didn't have that in the u calc I shared. 

I'm using open joint cladding so I'll need a UV resistant breather membrane. So I'm looking at TRASPIR EVO UV. Used tyvek on my other timber build just because it was the standard and was my first build. 

Edited April 11 by JackOfNoTrades

[stu w]

7 hours ago, JackOfNoTrades said:

 

Well spotted that I didn't have that in the u calc I shared. 

I'm using open joint cladding so I'll need a UV resistant breather membrane. So I'm looking at TRASPIR EVO UV. Used tyvek on my other timber build just because it was the standard and was my first build. 

Looking at you drawings you are doing 600 centres ? If so BC may require noggins on load bearing walls also doubles at the existing walls.

Hardie Plank rather than VL ? 

Are you going to fix through your dpc for sole plate fixings , or try and fix it down externally/internally ? 

Edited April 12 by stu w
Added

[stu w]

7 hours ago, Iceverge said:

 

I've used TF 100, cheap and plasticky, and TF400, top notch in my opinion. Better than Tyvek, 

 

is TF200 closer to 100 or 400?

Is 400 not predominately a roof membrane or have I got mixed up without looking ? 

[Russell griffiths]

With your gable ladder type arrangement 

would you not be better taking the smaller, flying rafters internally an extra rafter gap, so you get a counterbalance effect on them. 
depends if you need extra support if your soffit is large.