vertical dpc detail at window opening with stone

Wavehopper · 30-10-2010 02:06PM #1

Hi all,
My windows went in on Wed and stone mason is back to finish off stone above window sill level next week. I'm wondering if anyone has advice as to how to correctly position vertical dpc when stonework is being returned into window.
I got the window supplier to make the windows wider where stonework is being used to allow stone to return to the frame without narrowing the overall window frame width.
There is a gap between edge of window frame and opening at external leaf, I thought the windows would be tight to the blockwork when measured but it didn't turn out that way.

Here are some sketches to explain the 3 options I was looking at. It would be great if I could get opinions as to which is the best option or if anyone has another suggestion other than these 3.

Thanks

Option 1

Option 2

Option 3

sinnerboy · 30-10-2010 05:16PM

2.

Simplify internally by carrying IPB around corner

Oversize the dpc to be trimmed back after . At least 50mm should stick its tongue out .

mickdw · 30-10-2010 05:58PM

Option 1 is totally wrong. It is allowing a clear water path to the internal. It would also be pretty much impossible fit the overhead damp tray to work with such a setup.
Ensure that the overhead tray comes down outside the vertical dpc.
I would personally prefer the window to sit behind the outside block as it would be that bit easier to install the dpc on site.

Wavehopper · 30-10-2010 06:28PM

Thanks Sinnerboy & Mickdw for your comments

I installed 300mm vertical dpc at the time so there is enough to come down inside of external leaf then wrap around edge of block and along outside face.....there is plenty left to trim back.

Regarding the Insulated Plasterboard inside, I was afraid I wouldn't have enough strength to fix plasterboard & skim given width of cavity and nothing to fix to apart from 100mm inner leaf. I thought that 3 or 4 battens fixed to inner leaf would give more strength when skimming.

The overhead dpc crossing the cavity and out over top of window is under the vertical dpc so is that likely to cause me problems?
What happens this overhead dpc once out over the top of the window? It's currently around 75mm out past top of window frame. I plan to install steel lintel to carry stonework so to I just bring over this and trim at the back of it?

mickdw · 30-10-2010 07:02PM

Wavehopper wrote: »

The overhead dpc crossing the cavity and out over top of window is under the vertical dpc so is that likely to cause me problems?
What happens this overhead dpc once out over the top of the window? It's currently around 75mm out past top of window frame. I plan to install steel lintel to carry stonework so to I just bring over this and trim at the back of it?

If your overhead tray is running inside the vertical dpc, it is in effect supplying water to the dry side of your vertical dpc and will show in patches of damp on inside of house.
How do you propose to finish the underside of the concrete lintel that will be exposed behind the steel angle?

Wavehopper · 30-10-2010 07:29PM

Mickdw, is there anything I can do to rectify the overhead dpc at this stage as both come out over the inner lintel crossing the cavity in the wrong order?

(it just seems to be like this for the 4 windows where I have stonework)

I was going to plaster the underside of the concrete lintel before the steel lintel goes in but I need to talk to my plasterer to see if this is possible via some timber shuttering.

mickdw · 30-10-2010 07:53PM

We need to be sure that we are not confusing what we are calling under/over etc.
If you understand the whole damp proofing system around the windows, it is all about downward movement of water. Then your outer blockwork leaf is a wet wall. The overhead tray is to catch all water falling sownwards that may get behind inside the vertical damps etc. So if you work on the idea of downward flow of water, you should be able to work out the correct possition for the overhead tray.
If they re wrong currently you should be able to change them easily still. If the vertical dpcs are built in at the top, you can cut them off as high as possible and reposition them to the other side of the tray. Again if you understand what exactly they are for, you will be able to see what will work and what wont.

Plastering the underside of the lintel should be fine. Is the lintel in the way of the cavity tray?

Wavehopper · 30-10-2010 08:54PM

Mickdw, I don't have cavity trays above window openings.....I have an overhead dpc coming from top of inner leaf lintel across the cavity and down under outer leaf lintel just above window. I then have vertical dpc at each side of window opening coming down from top of inner leaf lintel but sitting over the overhead dpc and across the cavity and down the inside of of the outer leaf.

If there happens to be any water falling soutwards then I presume the risk is that it will make its way down the room side of the vertical DPC and cause dampness across the insulation in the cavity to inside skim?

Could I take out the 4 windows, cut the existing overhead dpc and mastic new overhead DPC to the top of the existing vertical DPC.....or are there other options here?

Thanks.

mickdw · 30-10-2010 11:39PM

By cavity tray, i meant a DPC so that is fine.
If the dpc is lapped the wrong way at the top, only by looking at it would it be possible to see how to fix it but sticking it to the bottom of the overhead one is not a good idea unless you can get it lapped behind it.
Also by the sounds of it, you overhead dpc is sloping only from top of lintel which would give it a very shallow slope. there are 2 issues with this - firstly, I would prefer that it was built in at a higher level at the inner leaf to eliminate any possibility of parts of the dpc settling in a fashion that would allow water to travel sideways - you dont want water running off the edge of the top dpc and travelling onto the underside of it for example. Secondly (and this might cause some debate), the dpc being built into a wall between lintel and blockwork is not the best idea as the lintel must be part of a composite structure and has little strength if not in a bonded structure with blockwork above. Thats a complete seperate discussion though

My final point - Where your overhead damp comes down and out over the window, Does the lintel position mean that this comes down inside the outer lintel but then has to be brought out a bit (2 inches or so) before it can get down over the front of the window? If so, this is very messy and again is creating a flat area in this damp which will possibly allow water to travel off the sides of the dpc tray (unless the ends have be turned up)

To be honest, you should get someone to have a very good look at these details for you. Some who is prepared to standover their opinion as the various trades will all say it looks fine until you then get wet spots and suddenly nobody wants anything to do with it. If you are in an exposed area, all this is super important but Ive seen all sorts of wrong details in housing estates and the extra shelter meant there was no problems but in the west of Ireland where I am, if anything is even a little wrong, the driving rain will always find it.

Wavehopper · 31-10-2010 09:15AM

Thanks once again for your comments Mickdw.

Yes, the DPC is quite shallow as it travels from inside to outside and I did raise this with my engineer at the time especially given comments from boards about dpc between lintel & blockwork but he was ok with the way it was done.
The overhead dpc does flatten out once again when it comes over the window frame by about 50mm as you say.

This detail is around 4 windows on a single storey sun-room and there is only 150mm of blockwork above lintel level as far as fascia/soffit so maybe the risk of water falling down onto overhead dpc and travelling inward is reduced due to this.
Anyway I'll go through this with my engineer to see if he's happy.

mickdw · 31-10-2010 05:29PM

Yes the little amount of wall above the DPC in this case is a major help to you. If that detail was below an exposed gable, I would be very worried.
Did your Engineer comment/notice the reversed lap on the dpc?

Wavehopper · 31-10-2010 08:07PM

No, wrong order wasn't noticed during site inspections.

I went around all my other windows today and half of them have the incorrect order so it's worrying now that this may cause problems on the main part of the house.
I used Xtratherm insulated cavity closers on all window openings which I believe also act as a dpc themselves so hopefully these along with the fact that pumped bead is being used in 150mm cavity will reduce risk of dampness internally.

mickdw · 31-10-2010 10:11PM

I would not proceed until you get those DPCs sorted given that its a bit hit and miss around the entire house. If there is any access at all to the DPCs, it might be possible to cut the vertical ones at the top, and push them in behind the overhead one.
I design and certify houses and this kind of stuff stands out immediately on site. Sort it now before you have your house plastered as it can turn into a nightmare very quickly when you get to a later stage of construction.

Given that your blocklayer knows nothing about DPCs, are there any other parts of your house that are likely to cause trouble? A prime lcation would be if you happen to have a conservatory or any roof coming in against a higher wall. Do you have anything like this?

Wavehopper · 01-11-2010 08:33AM

Thanks for the advice. I'll disuss with my engineer and call off my plasterer who is due to batten for patent window reveals today.
I have a ground level sun-room that meets a higher wall at the main part of the house and same with ground level porch. Here I used lead cavity trays rather than DPC so I should be ok here.

braftery · 01-11-2010 09:26AM

Looking at your drawings I am a little confused.

Normally, in the case of stone work the windows are handled in one of two ways.

1. Window postioned in line with the rear face of the outer black (as per standard plaster positioning. A wider DPC can be placed in the normal position, giving you a good straight edge.

Then the stone or a hidden plaster reveal can be finished back to the window.
If you PM I can send you a picture of this method.

2. Window is position in line with the front face of the outer block and a wider DPC is attached to this face, again giving you a good straight edge.

The stone is then built out to cover the face of the window.

In your case it seems to have ended up in between these two places.

Is this because of the cill head positioning ?

Was there a site meeting with the stone mason, window supplier and who ever fitted the window cills to work out this detail ?

If so what was agreed ?

Did the window installer pump the gap between the window and the wall with a slow release expanding foam ?

If not, this should have been done and needs to be done.

Using blown material here is probably not the best suggestion.

Also make sure you get a decent quality slow release foam designed for window installation .. and at this time of the year make sure the cans are not cold .. if they are the foam will expand far to slowly and there is a strong possibility that too much foam will be put into the gaps ... possibly leading to problems later as it expands out and presses on the window frames.

On the internal reveals as you have the available space why not use a warm board reveal ... with 25mm of insulation backing the plaster board.

I would consider connecting the vertical DPC to the front face of the block and wrapping it back along the block and then turning it out where the front face of the window is.

The head dpc then needs to be in front of this.

A full fill of expanding foam behind.

How will you finish the stone to the window ?

A cement plaster joint ? Silicone ? Depending on the smoothness of the edge of the stone you need to make sure this is worked out.

There are very good water proof expanding tapes that are specially designed for use with stone, your window supplier should have talked to you about them.

I hope some of that helps.

Wavehopper · 01-11-2010 12:15PM

Hi Braftery,
Thanks a lot for you comments & advice.
See answers below

1. Window postioned in line with the rear face of the outer black (as per standard plaster positioning. A wider DPC can be placed in the normal position, giving you a good straight edge.

Then the stone or a hidden plaster reveal can be finished back to the window.
If you PM I can send you a picture of this method.

In hindsight, this is what I should have done but I had trouble getting 17" wide brown granite sills to allow window to sit this far back. I also wanted a deep reveal on the inside so they are the two reasons how I ended up in this half-way-house. Maybe not the best reasons given the potential problems I now have!
I suppose I could take out these 4 windows and get the wide sills I need and re-fit. Problem here is that my outside window opening will need to be narrowed to get the tight fit required but no idea how I'd now rectify this other than waterproof ply or some other treated timber.

2. Window is position in line with the front face of the outer block and a wider DPC is attached to this face, again giving you a good straight edge.

The stone is then built out to cover the face of the window.

I read here on boards the you shouldn't bring the back of the window frame out past the back of the outside leaf block to avoid major cold bridge so with a 70mm uPVC frame I didn't want to go with this option.

Was there a site meeting with the stone mason, window supplier and who ever fitted the window cills to work out this detail ?
Yes, went through this with my stonemason and engineer on site and then explained to my window sales rep what I was doing.

If so what was agreed ?
Nobody raised any concerns with what was proposed so this is where I ended up.

Did the window installer pump the gap between the window and the wall with a slow release expanding foam ?
Yes, this gap is pumped with expanding foam (will look at make of foam later). If I fold back the vertical dpc on outside I'll get window suppliers to fill any gaps I see tomorrow as they are back on site.

On the internal reveals as you have the available space why not use a warm board reveal ... with 25mm of insulation backing the plaster board.
Yes, I think I'll now go with insulated slab on the inside of the opening

I would consider connecting the vertical DPC to the front face of the block and wrapping it back along the block and then turning it out where the front face of the window is.

The head dpc then needs to be in front of this.
I might PM you to get more details on how I'd do this if you don't mind?

How will you finish the stone to the window ?

A cement plaster joint ? Silicone ? Depending on the smoothness of the edge of the stone you need to make sure this is worked out.

I need to talk to my stone mason but I'd have assumed cement plaster but maybe I can have a bead of silicon between stone and frame along with plaster joint.

There are very good water proof expanding tapes that are specially designed for use with stone, your window supplier should have talked to you about them.

Again I might PM you how these tapes can be used externally between stone and window.

Thanks again.

Robert Allen · 16-11-2016 11:47AM

do you mind me asking how this worked out for you? Im in the same boat at the moment. Thanks.

vertical dpc detail at window opening with stone

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