Ledge Issue at Masonry-Timber Frame Junction

[R2021]

My timber frame company outsourced the masonry ground floor design to a structural engineer and didn't notice an alignment issue until now. I've been left of fix the issue so grateful for this communities help.

 

The timber frame aligns with the inner blockwork leaf as shown in the photo. The ground floor will be clad in slate slips adhered directly to the external blockwork leaf. The timber frame will have render board or timber cladding on 50mm counter battening creating a 100mm cavity as shown on the diagram below. The battening and render/wood cladding is yet to be installed but the diagram shows how it will align (or not) with the ground floor external leaf. I can’t increase the batten thickness because the window cills have been spec’ed and order for 100mm cavity by the timber frame company.

 

The first challenge is how do I stop any water that gets behind the battens going into the blockwork cavity and channelling it out over the slate slips. A cavity or lintel tray has been suggested but I've not found any with the right dimensions. Grouting out the inner leaf mortar in the course above the outer leaf block and inserting lead or DPC has also been suggested. Thoughts?

 

The second challenge, given the render board / wood cladding doesn’t sit flush or proud of the external block work leaf, is how do I get rain that runs down the cladding out and over the external leave & slate slips. A verge trim has been suggested and I believe that if I fitted a 140mm one between the horizontal counter batten and cladding it would reach over the slate slips. I've found aluminium ones online, which could work out expensive. My builders merchant recommended a fiberglass roofing fillet trim, which is cheaper, but flimsy, which might help with installation, but may not have a good lifespan. 

 

Any thought, alternative options or product recommendations would be very much welcomed.

 

 

[Roundtuit]

This really shouldn't be your problem to sort out if you've used a professional design service.  Gut feel is that your timber frame company need to use wider battens on the cladding and suck-up any additional costs on the windows.  Failing that, you can get aluminium pressings made in whatever size and colour you want that would sit behind the cladding, cover the cavity and overhang the slate slips (like a big aluminium window cill pressing).

[ProDave]

Does the blockwork need to be a cavity there?  I have only ever seen the blockwork on a TF house solid below DPC.  So I would say they need to knock down the outer leaf of blockwork to below ground level and rebuild with it stepped in and no cavity.

 

Agreed it should the the contractors problem (cost) not yours.

 

Another thought. How are you getting enough insulation just in the thickness of the timber frame?  I would look at external insulation over the frame before the battens and cladding which would widen the whole structure and improve it's performance, then you are back to changing the window cill depth.

[Gone West]

2 hours ago, R2021 said:

Any thought, alternative options or product recommendations would be very much welcomed.

Could you use a plinth brick on the outer blockwork. It's poor design and you shouldn't have to sort it out.

[ETC]

Some decent aluminium flashing should sort that out.

[Mr Punter]

Another option is the do the outer leaf in block or brick.  If you want to just clad the timber frame, do as @ProDave suggests and do away with the outer leaf altogether.

[R2021]

Thanks for all the advice so far. there are several similar comments so rather than reply to each, I will try to cover all points here...

 

The house is on a sloping site with a flood risk, so the masonry ground floor is doing two jobs: 1) acting as a retaining wall tied into the raft foundation; 2) providing tanking for rain water flowing down the hill - it's filled with water proof concrete between the two leafs to 1350mm above ground level, i.e. a semi basement. So there's no way that wall is coming down now.

 

The timber frame company were responsible for the detailed drawings for building regs, but they used a structural engineer to design the foundation and masonry ground floor to achieve a complete system for retaining walls and tanking. Unfortunately the two seemed not to collaborate during the design process. I was responsible for appointing a groundwork/building company to complete the foundation and ground floor, who done an excellent job following the structural engineers design to the letter. I've challenged the timber frame company, but their general attitude is that they weren't responsible for the ground floor design, which is rubbish as they are the company I paid for it, but what can I do. I don't want to destroy our relationship whilst their part of the build is in mid progress right now.

 

The timber frame company are providing the windows but they don't provide battens, so that's my responsibility to fit. However they have dictated 100mm of battening and ordered the windows based on that. If I increase the batten depth now, they will fine me for changing the order and for the delays it will cost, which will also cause me to delay the renders and cost me more scaffold rental. The timber frame company are never going to absorb the cost. They've already charge me an extra £11k because I was one month over their deadline for completing the foundations and ground floor. The ledge is not going to look pretty, but it is only down one side and 2.2.m at the back of the house, so won't really be seen.

 

@ProDave The DPC does stagger up the masonry wall to follow the slope. The timber frame insulation is already built into the frame (Ultima). It is supposed to be one of the best insulated frames on the market (before getting into SIP territory) at 0.13U. On the masonry ground floor, I need internal insulation because I can't further increase the perimeter with external insulation.

 

So I can't increase the batten depth, or build up in blockwork from the outer leaf as @Mr Punter says because of the windows. There are a few suggestions for Aluminium trims which was where my mind was at, but I'm also the plinth brick as suggested by @Gone West looks an interesting option I will explore further

 

[ProDave]

I would say the relationship is already soured.  I would have been livid at an £11K charge for being 1 month late. Is there a similar clause that allows you to charge similar if they are late?  no I did not think so.

 

I would be calling a serious meeting, telling them it is their issue as you contracted them to do the package, tell them the best technical solution is wider battens so the cladding lines up with the outside of the base blockwork and that will require deeper window cills which they will pay for.