Dormer construction detail advice, please

[Tony L]

I’m hoping you can help me steer my architectural technician in the right direction with these drawings he’s done for me.  It seems the arch tec wants to build the 1990s way, with concessions to the latest building regs.  I want to build the Build Hub way.  I don’t have an architect (although I have recently looked into bringing one on board). I drew the house myself to achieve PP & now I’m relying on an arch tec to get my design through full plans BR approval & then do the construction drawings & notes, so I can continue building (we’ve already built up to the B&B floor).

 

This Sketchup shows a very early draft design for my house (the chimney has since been deleted & other improvements have been made, including reducing glazing).  I’m seeking advice on the detail for the big dormer that hangs over the edges of the first floor.

 

[Tony L]

[Tony L]

[Tony L]

The main building construction will be block & block, 150 cavity with blown beads.  I’m aiming for high airtightness & heating on GF only (UFH).  The outer blocks will be rendered, so it makes sense to go right up to the flat roof, at the highest point, in blocks, rather than build the first floor gable end in timber.

 

Here’s the arch tec’s first attempt at the dormer. 

[Tony L]

[Tony L]

This design makes the bedroom 250mm wider than I’d envisaged in both directions, which would be great.

 

However, problems I see include:

 

1) outside wall becomes the inside wall, with no cold bridge mitigation (perhaps a few Marmox Thermoblocks could help here)

 

2) no VCL or airtight membranes shown

 

3) Posi joists butting up to outside wall – I prefer the idea of hanging the joists from a ledger board that’s bolted to the inner leaf block work (or structural outer external stud walls).

 

4) incredibly skinny flat roof section, with parapets, but let’s ignore the roof for now; I may come back to that in a separate post.

 

 

I should just add that these are supposed to be building reg. drawings & more detailed construction drawings will follow on, so perhaps I’m expecting too much from these drawings – am I? 

 

Q1) The first thing I need to understand is: would it be better to build most of the dormer in block & block, & if so, where does the block work end & how will the junctions from blocks to the sloping roof work?

 

Q2) If it’s going to be better to build the whole dormer from timber, how will the vertical junction to the block work on the side of the house work (where the chimney used to be)?  This junction could be hidden behind the cladding, so I’m not concerned about my cladding cracking, but I’m concerned about timber shrinking & pulling away from the masonry.  & is it OK to build the whole thing off the outer leaf of the blocks?

[Tony L]

Just in case it’s relevant, here’s a drawing of the GF corner immediately below the dormer bedroom.  The wall in the red ellipse is to be built from high density blocks laid flat, the idea being that if I have any money left as the end of the build is in sight, there’d be a fancy staircase hanging off this wall.

Edited Saturday at 21:55 by Nickfromwales

[Tony L]

[edit] there'd be a fancy staircase...

 

Why can't I edit my post?  I can see there's an argument for fixing historic posts, but this was less than an hour ago.

[Russell griffiths]

Just a quick idea, is it a big deal to have the bedroom inline with downstairs, but just space the cladding out to get that set out look. 
you loose your bigger bedroom. 
that lintel above the corner window is going to be grief, can you live with a corner post, could save you a good few grand. 

[Tony L]

26 minutes ago, Russell griffiths said:

Just a quick idea, is it a big deal to have the bedroom inline with downstairs, but just space the cladding out to get that set out look. 
 

I'd be happy with that, if that's what everybody recommends - that's the way I designed it, then the arch tec "improved" my design.

 

Yes, I accept my fantasy spec' glass meets glass cantilever lintel idea is out of the window, due to budgetary constraints. Maybe an internal post, so it still looks a bit special, is achievable, but more likely we'll end up with a big black post on the external corner where two windows meet.

Edited Saturday at 20:21 by Tony L

[Russell griffiths]

I have a 90mm square steel post with 25mm insulation on the outside then covered in an aluminium flashing to match the windows. 

[ETC]

The easiest thing to do is to build the walls as cavity and then build out in timber to create the look you want. Keep the window recessed.

[Tony L]

8 minutes ago, Russell griffiths said:

I have a 90mm square steel post with 25mm insulation on the outside then covered in an aluminium flashing to match the windows. 

That's good to know, thanks @Russell griffiths.  I will paste that sentence into my drawing notes.  & I must also compliment you on your wonderful view.

[ETC]

Also why are you building a cold roof?

For the corner use a Keystone post and lintel system - simple.

[Tony L]

17 minutes ago, ETC said:

Also why are you building a cold roof?

 

Thanks for pointing this out, but there's ample space in this dormer, so it will be drawn as a warm roof, once I have taught the arch tec what BH has taught me about roof systems. 

[Tony L]

41 minutes ago, ETC said:

The easiest thing to do is to build the walls as cavity and then build out in timber to create the look you want. Keep the window recessed.

Thanks, @ETC.

 

Are you suggesting I continue my block cavity wall up behind the areas marked in red, then I suppose, in timber in the parts marked yellow & orange (as I guess this will be cheaper/easier than strong lintels & blocks above the corner window) - please correct me if I'm wrong.  If timber, what happens at the interfaces marked with the blue lines?

 

I could continue the blocks behind the orange lines (2nd picture, below) -there’s a wall underneath that would support them, but then I have to deal with the problem of the block exterior wall becoming a block interior wall, as it passes through the sloped roof.

 

 

Edited Saturday at 21:29 by Tony L

[Tony L]

[Nickfromwales]

2 hours ago, Tony L said:

[edit] there'd be a fancy staircase...

 

Why can't I edit my post?  I can see there's an argument for fixing historic posts, but this was less than an hour ago.

BH policy sorry. You can ask for an amendment any time you wish, just shout  I have edited the post for you. 👍

 

I think the timeout for editing is 20 mins. @MikeSharp01?

[Tony L]

2 minutes ago, Nickfromwales said:

BH policy sorry. You can ask for an amendment any time you wish, just shout  I have edited the post for you. 👍

 

I think the timeout for editing is 20 mins. @MikeSharp01?

Thanks for explaining.  I wouldn't want to trouble the mods with something so trivial, when it might divert your attention from the important business of house construction. 

[Nickfromwales]

1 minute ago, Tony L said:

Thanks for explaining.  I wouldn't want to trouble the mods with something so trivial, when it might divert your attention from the important business of house construction. 

Always a pleasure never a chore, lol. Plus my OCD often sees me stealth-editing.

 

My turn to construct something is WAY off lol, I just to work for others who are doing it a lot (and now don't make the mistake of showing the wife!!).

[ETC]

No - I’m suggesting you take up all cavity walls to the flat roof. The only wall that’s not a cavity will be the wall between the staircase and the bedroom which you can either build as a traditional timber stud wall (insulation between and inside with a vented and drained cavity or take the 215mm wall up and insulate the inside and clad the outside.

 

Once you’ve the cavity walls built up clad the outside with vertical and horizontal battens to create the look you want.

[Tony L]

7 minutes ago, ETC said:

The only wall that’s not a cavity will be the wall between the staircase and the bedroom which you can either build as a traditional timber stud wall (insulation between and inside with a vented and drained cavity or take the 215mm wall up and insulate the inside and clad the outside.

 

Thanks for that.  Building almost everything in block cavity walls sounds nice & straight forward.  It would be good to get some opinions from others on which of these options (blocks vs timber) might be best for the wall with the orange lines drawn on, where "best" means a compromise between expense & a solution that will make it difficult for the builders to get wrong.

 

My concerns are:

 

If it's blocks: the outside wall becomes the inside wall & there's a significant cold bridge (perhaps I have stepped Marmox Thermoblocks along the line of the sloped roof). 

 

If it's timber, the junction with the pitched roof is easy because it's like any other dormer, but I don't know how the timber joins the block cavity wall across the front (where the longest blue line is in the Sketchup image above).  Also, I was imagining wet plaster on the interior wall next to the stairs, going all the way from the ground floor to the top of the interior wall where it meets the high point of the vaulted ceiling, & if the top part is timber, there’s another problem to solve.

 

Thinking about this has made me realise things don’t quite line up in the way I imagined.  I’ve drawn the bedroom dormer & the edges of the pitched roof on top of the SE’s foundation plan.  I’ve drawn the inner wall of the dormer in orange & the squiggles represent insulation against the dormer block wall that sits on top of the 215 wide blocks laid flat wall that the stairs might hang on.  Have I got this right?

 

Ideally, I’d draw all this up in 3D so I could understand it better, but I don’t have time to improve my very basic Sketch up skills (I paid someone to produce the images at the top of the thread). I can see I’m going to have to reposition the windows in the pitched roof, so they’re centred between the dormers, rather than the walls either side of the hall – I wonder if it will look odd from the inside.  Alternatively, I could build out the cladding on the small dormer (ie more substantial battens), but then it won’t match the approved planning drawings, & it may make the pitched roof section between the dormers too narrow.

 

 

[Tony L]

11 hours ago, Tony L said:

It would be good to get some opinions from others on which of these options (blocks vs timber) might be best for the wall with the orange lines drawn on

 

Sorry: on re-reading I see this is ambiguous because I used orange again on the next drawing.  It's the dormer cheek marked with diagonal pale orange lines on this drawing, below, I'm struggling with.

[Russell griffiths]

Can you use an EWI on the section of cavity wall coming up against the sloping roof. 
it would be behind your cladding. 
if you use one of the aluminium bracket and rail systems for the cladding the Ewi can fit behind the rails. 

[Tony L]

Thanks @Russell griffiths.  That sounds like a good idea, but it’s going to need some thought to make it work. 

 

Although you say “cavity wall coming up against the roof”, my current understanding is it won’t be a cavity, it will just be a continuation of the inner leaf from the cavity at the front of the house, & this single skin block wall will sit on the edge of the 215 wide block wall, so the wall from the hall up to the landing area above will be a nice flat surface, ready for wet plaster.

 

My current front runner for the cladding (Dura Cladding Flush Resist) is designed to fit to 25mm Dura Aluminium bearers (will be 50mm by the time I’ve got them going both ways for vertical cladding).  They’re expensive, of course, so I’ll be using wood for the cross battening for the walls other than this awkward cheek.

 

I didn’t know anything about EWI until I started Googling 30 min.s ago.  I realise now, I can’t just stick 135mm+ EWI against the single skin block wall then 50mm crossed battens, due to the lateral loads (the cladding needs to be 185mm+ off the wall we’re talking about).  I found this diagram in a cladding handbook.  There’s some cold bridging, but it’s only a small section of wall, so perhaps I shouldn’t worry about that. I’m assuming wooden battens with EWI in between is not going to work, so perhaps I could source some bigger aluminium brackets/battens to get to 160 off the wall, with 150mm EWI embedded in these then the 25mm Al horizontal battens on here (not sure if the air gap is desirable/necessary behind composite cladding) - I could pack the whole 185mm out with EWI.

 

The heaviest part of the triangle (at the front of the dormer cheek) will be going onto the small section of cavity wall, so the battens won't have to reach out very far here, which is good.

 

Any thoughts on this anyone?  Do I need to think about using VCL with EWI in this scenario?