Gap in block work

stef22

Hi all, we are at the plastering stage. Scratch coat done. Now we are face with a problem, there are 2 gaps in the block work on one of the gable. It is at the junction between the gable and ghe conservatory as you can see in the pics ( I hope they are clear enough). Any idea why the blocklayers left it like that and more importantly how best to seal them? It has not just a missing block , the gap goes all the way to the cavity of the conservatory of that makes sense.

Many thanks

stef22

One thing I should add, there is one gap on each side if the conservatory.

adgib

May have been a tie in for the conservatory wall, probably misjudged the roof height, just fill in with some concrete bricks

bfclancy2

https://www.boards.ie/discussion/comment/121690556#Comment_121690556

Looks like it, get blocklayer back to correct

mickdw

The gap is correct and is on the line of the cavity of the wall tying in to gable which must be retained up to underside of cavity tray at this location.

the problem is that the trays have not been installed and as such you now have a poor weathering detail and a gap in the wall that should not be visible or quite as high if trays were installed.

Did anyone mention cavity trays at all?

Stepped cavity trays with lead work should have been built into blockwork at that location. The holw would stop at underside of tray and the lead work cover off the hole.

You will have a wet wall in conservatory without trays.

Only real option now is to completely seal the section of external wall over the conservatory.

bfclancy2

https://www.boards.ie/discussion/comment/121693670#Comment_121693670

99% of block walls in this country are flashed this way with no lead trays installed, rightly or wrongly that's the way it is done

mickdw

https://www.boards.ie/discussion/comment/121693894#Comment_121693894

Wrongly and it will be wet internally if in any exposed location.

I'm signing off houses for 20 years and and using trays for 20 years. That detail is just madness at a position like that.

In this case I'd go so far as to say that if the blocklayer knew to leave the gap, that he was fully intending on using trays but someone refused to buy them as they are 35 to 40 quid each with lead attached.

10-10-20

https://www.boards.ie/discussion/comment/121693985#Comment_121693985

These lads?

https://buildstore.ie/block-catchment-left-hand-cavity-tray.html

How does the gap in the blockwork tie in with these, if you don't mind me asking?

bfclancy2

https://www.boards.ie/discussion/comment/121693985#Comment_121693985

well you are the exception, been on sites for many years, rarely see them used, no dampness internally either, not saying your way isn't the right way but its not the way that is used, tie in the blockwork doesn't relate to the flashing in any way thats a bizarre statement

listermint

I'm kinda confused how the lead tray with plastic back stop and open hole would provide in this scenario better protection than the undercut lead and render over?

Is the hole for cavity ventilation? And the tray is protecting that ?

Where is the moisture coming from in this scenario. I'm unclear.

bfclancy2

https://www.boards.ie/discussion/comment/121694200#Comment_121694200

well the lead is only nailed on to the wall in this image and rendered over, the other post is talking about dampness permeating the block, travelling down through the blockwork and into the building below

listermint

https://www.boards.ie/discussion/comment/121694236#Comment_121694236

Ah.i see it's to direct that moisture out through this detail from the roof line as there is no wall below that roof line, just ceiling. Makes sense.

mickdw

https://www.boards.ie/discussion/comment/121694127#Comment_121694127

Anyone not using them in exposed areas is asking for trouble.

I've seen multiple wet conservatories and alot of head strtching and blaming roofers etc when it's missing trays that are the problem.

The slot in blocks should continue up from cavity line of the front wall of the conservatory. It should continue up to hit the bottom of first plastic tray.

Any wall below tray level that continues through solidly from being an outer wall to an inner wall can become wet. Once above tray, all water to thrown out anyway.

It's common sense if you understand the detail.

I didn't mention anything about tieing in blockwork, the gap is a clear break in blockwork to separate outer wall and from what becomes a dry inner wall. The gap might be slightly too high in this case but you would go up to hit tray.

mickdw

https://www.boards.ie/discussion/comment/121694029#Comment_121694029

Yep. That a base tray for the bottom of the run to throw all water out. The others are the same except only one side on the plastic tray.

SodiumCooled

https://www.boards.ie/discussion/comment/121695773#Comment_121695773

Slightly tangential we have a somewhat similar roof joining a larger gable, we have cavity trays with the bottom tray having the weep vent through the external leaf. What I was wondering is how is the performance of the trays going to be impacted when they undoubtedly get packed full of insulation when the cavity is pumped or the weep vent possibly gets blocked from the inside etc?

mickdw

Even without the weep hole, they tend to still be successful as its not a volume of flowing water more seeping water which does make its way out via seepage again.

The tray should fit outside a partial fill board.

Pumped bead imo should be carefully managed around those areas. I prefer not to have bead there as it will deform the trays possibly and generally not be a good idea at that location.

stef22

I managed to access the gaps from the inside of the house and bit from the outside. Boxed it off and filled the gaps with concrete. Will tell the plasterer to be extra careful when plastering over the lead and bead. Hopefully that will do the trick. If not then don't know what else to do.

mickdw

no trays.jpg

trays with no gap.jpg

Trays with block gap.jpg

Some sketches that might clarify for anyone not clear on the purpose of the block gap

mickdw

https://www.boards.ie/discussion/comment/121694029#Comment_121694029

See above sketch that might explain. You need a gap where wall turns from outside wall to inside wall. Everything under the tray and inside the gap is a dry wall. Everything over the tray and outside the gap is wet wall.

The whole idea of a cavity wall is that the outer block may be damp all through. If you look at the situation with that in mind, it is clear enough why the trays are a requirement.

Single leaf cavity block walls work in city builds. They would be wet wet wet all through if built on the west coast. This is similar.

We even build all our sheds here with double wall and step in foundation too which would be overkill in a back garden in Dublin For example.

10-10-20

I get it now. Thanks for the explainer! I had not seen those style of trays previously.

sydthebeat

excellent work mick!

SodiumCooled

https://www.boards.ie/discussion/comment/121697268#Comment_121697268

Maybe I'm just missing it but I don't see the need for a gap? Won't the tray catch any water that gets through the wall and stop it getting to the inside wall below? In mine the cavity of the wall on the lower roof runs into the larger gable and joins the larger gable cavity - the cavity tray on the gable comes comes past this lower wall cavity so where can the water get in? Maybe I missed it but I dont recall any gap standing out to me during the cavity tray installation where exactly would be?

10-10-20

https://www.boards.ie/discussion/comment/121705069#Comment_121705069

The way I understood it was that the trays would be fitted into the blockwork with the lead flashing lapping outside. The gap at the wall joint was there there to facilitate the water cascading down the inner trays and then turning 90-degrees right out through the blockwork (probably through a weep hole) and trickling outwards and into the guttering and away.

But the question that I now have is... if the blocklayer knew that the cavity trays and lead flashing wasn't being fitted (as he had laid the gable blocks at that location) why did he persist in leaving a gap given that there was no need for it? Not getting that part of the logic...

mickdw

https://www.boards.ie/discussion/comment/121704459#Comment_121704459

Thank you.

mickdw

https://www.boards.ie/discussion/comment/121705069#Comment_121705069

Without the gap, there is a moisture path from wet outer wall (assume outer leaf wet all through) to inside of the building. The outer leaf of main gable of the house becomes an inner wall in conservatory.

The gap is just a continuation of the cavity that exists where front conservatory wall meets main gable. That cavity is continuous from floor until it hits tray.

Not leaving the gap means you are crossing the cavity with solid block from outside house to inside house.

SodiumCooled

https://www.boards.ie/discussion/comment/121709147#Comment_121709147

Is this assuming the conservatory just built into meet the outer gable wall? If so I think I understand. In my case it’s not a conservatory but rather an two story L part of the house and at wall plate level is the same height as the rest of the house but the roof is lower than the main roof hence the cavity trays. My cavity continues from the L part into meet the main cavity in the gable so is this the gap being created that you refer to (or equivalent anyway)?

mickdw

https://www.boards.ie/discussion/comment/121710133#Comment_121710133

The gap is the continuation of the cavity that would exist at the point where extension meets main house.

Without continuing the cavity right up to join underside of tray, you are basically leaving a block running straight across from outer to inner. You wouldn't do that anywhere else on the house, so why do here.

Mellor

https://www.boards.ie/discussion/comment/121715397#Comment_121715397

This 100%

Imagine a plan detail through the wall junction inside the room. Then imagine the same walls above the roof. One wall goes from inside to outside. That need a weather line.

another issue is the nailed on stepped flashing. Smh

SodiumCooled

https://www.boards.ie/discussion/comment/121715397#Comment_121715397

Thanks I understand now. The reason I was struggling to visualise is I was thinking of my own house where this gap is created by the cavity of the L part being joined to the cavity of the main house so there is that break up to the trays.

standardg60

I imagine the gap is purely to prevent cold bridging horizontally rather than stopping moisture transfer, they're expecting the render will seal the outer leaf.

Could see trays being a requirement with a brick outer leaf but not with rendered block.

mickdw

https://www.boards.ie/discussion/comment/121730821#Comment_121730821

We have been building this detail long before cold bridging was even a concern but it clearly does help in that regard also.

This idea of render keeping the outer leaf dry is completely incorrect. You need to consider the outer leaf entirely wet through and then detail accordingly.

window head dpc, vertical dpc aroound windows, trays etc all make absolute sense then. Why even use a dpc tray over a window or door if the outer render is waterproof?

standardg60

https://www.boards.ie/discussion/comment/121731021#Comment_121731021

That makes no sense to me, if the outer leaf is considered to be wet why put a dpc in at ground level?

What is to stop a cavity block built house being wet through if the render is not to waterproof it?

mickdw

A cavity block house will be wet through if build in an exposed location. That's why they are not built that way for the last 50 or so years other than in city estates.

standardg60

https://www.boards.ie/discussion/comment/121731213#Comment_121731213

Sorry but i don't buy that at all, exposure should make no difference.

In the OP's case the gap extended above the roof line, if they'd installed trays there they'd have been pointless, a wet outer leaf clearly wasn't a concern. Neither was nailing the flashing to the blockwork as it was all going to be rendered over.

sydthebeat

https://www.boards.ie/discussion/comment/121731181#Comment_121731181

Why does dpc step down from inner leaf to outer leaf?

Furze99

https://www.boards.ie/discussion/comment/121731181#Comment_121731181

I built two sheds - one with rendered cavity block and the other with a rendered cavity wall. Both get driving rain against one side, the cavity block construction has dampness on inside after a gale, the other is dry.

mickdw

https://www.boards.ie/discussion/comment/121731338#Comment_121731338

This has been thoroughly explained in this thread. The gap will extend to the underside of the tray. The wall to the left of the gap in the OPs original picture would be 100 percent dry if tray was used and would be 100 percent weathered by the lead work.

mickdw

https://www.boards.ie/discussion/comment/121731479#Comment_121731479

Exactly. These designs work in sheltered positions only.

You can get into rendering systems that are weatherproof but sand cement externally is not. It does harden and become slightly more waterproof with age imo but design is based on outer leaf being wet. It's the whole idea of the cavity and the inner wall.

I don't profess to know alot about ICF etc but I'm along time looking at blockwork.

I would bet good money that if the ops gable is on the windy side of the house that it will have wet area immediately inside the conservatory roof within 1 year.

standardg60

https://www.boards.ie/discussion/comment/121732113#Comment_121732113

How has it been explained?

The gap extends above the conservatory roof. How would installing trays here work, the flashing would have to extend from the roof up to them, which must be 18 inches, who would design that?

mickdw

I give up. Read the thread. I even did some sketches.

standardg60

https://www.boards.ie/discussion/comment/121736575#Comment_121736575

I did read it and found it very enlightening, especially your input re the trays.

I just don't think there was ever any intention, rightly or wrongly, to install them and the gap was just to prevent cold bridging.

Apologies if you're frustrated, it's not intentional.

listermint

https://www.boards.ie/discussion/comment/121736688#Comment_121736688

The gap was to prevent cold bridging?..

bfclancy2

https://www.boards.ie/discussion/comment/121736856#Comment_121736856

gap was left to maintain cavity nothin else