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New Build - gap under window sill

  • 04-01-2023 10:14am
    Registered Users Posts: 2,458 ✭✭✭

    I closed the purchase of a new build just before Christmas.

    Got it snagged twice and had my cousin who is a builder give it a going over.

    Over the Christmas I spotted that under the window sill of the upstairs main bathroom there was a gap between it and the roof of the extension at the back of the house. (Well it's part of the house rather than an extension, but it's a small part that juts out)

    Basically, it looks like they never put concrete under the window.

    Picture attached.

    Now, I said it to the builders yesterday and they said they are going to fill it in and paint over it.

    But, I am just wondering, will it be safe after that? They assured me it would. But, would any weather have gotten at the timber frame or other material inside the wall? I don't think so as all I could feel was some hard black like paper stuff under the window.



  • Registered Users Posts: 6,950 ✭✭✭10-10-20

    Is it a timer-frame house? Is there any sign of damp/leakage under that window or on the wall below it?

    The underside of the window cill should have a rebate cut into it which is called a "drip channel" and this should have stopped the majority of rain from tracking back into the construction.

    The DPM (which is probably what you felt) which is lifting over on the left side of the photo needs to be cleaned up and laid flat and the void inspected to ensure that it's solid before it's filled. The DPM in that position should have prevented any wind-blown rain from entering the building.

    Filling it should be a two step process - filling with sand-cement and then later rendering it with sand-cement. Painting should be done 4 to 6 weeks later.

    Others might be along with other approaches, but this is my opinion on it.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    Could you provide a few pictures so that I can see what you are talking about.

    Also, I hope that your cousin, who is a Builder, gave the external Joinery a going over, and checked every pane of glass of the house to ensure that there are no scratches on the panes of glass - internally and externally.

    To me this these are most important Snags and the House Builder is obliged to replace every scratched pane of glass.

  • Registered Users Posts: 2,458 ✭✭✭chops018

    Thank you both for the replies.

    @10-10-20 - yes, it is a timber frame house. There is no sign of leakage or damp inside or out, at least to me anyway. The way you suggested seems to be the way the builders are going to rectify it. They are working on it today filling it in. Presumably they will render it next and paint then so as that part looks the same as the rest of the back wall. Just wondering, once they have done that, is it all good and safe? I mean, extreme here, but that part won't affect the house overall or keep deteriorating now will it? It's a new build so I'm not happy about this at all.

    @C. Eastwood - the pic I attached is the best one. It was snagged and then re-snagged by an engineer who specialises in surveys and snags. I just got my cousin to give it a general go over/give me his opinion overall, he didn't go through it in full detail. That's why I'm very unhappy as it wasn't spotted at snag stage. But sure I suppose if it was they'd have just fixed it then the same way they are fixing it now. But at least that would have been before the purchase closed whereas the purchase is closed now so it's not like I can say give me my money back! I hope it doesn't cause further issues. I've been assured it won't but it's just really after annoying me!

  • Registered Users Posts: 6,950 ✭✭✭10-10-20

    Ask then whether the cill has a water drip channel and that the DPM in-situ was complete and undamaged and was in place with sufficient pitch to allow any water to runoff outwards, in writing/email.

    I'd be happy enough with that as that's not a very exposed location given that the cill is in place and should have thrown most of the water off the cill.

  • Registered Users Posts: 2,458 ✭✭✭chops018

    Ok thanks! I will email them and also text, and ask that, to keep a record of it. But I'm nearly sure they were going to fill it all in today! I will say when I saw it there did not appear to be any dampness and the sheet is covering the area. Also the window sills do have the groove for the water actually. Hopefully it is ok. As you say, the location is not exposed like other areas and with the extension part just below it that can catch water or wind. I'm just raging as the snagger didn't spot it!

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  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    Thanks chop018

    Do not worry about this. The Builders will patch this gap with Cement Mortar.

    it is difficult for me to see the details of the Cill in the photo above.

    It is important that rainwater landing on the cill will drop off and not get in to the cement mortar filling.

    All cills have a drip which is a grove on the underside of the cill. This groove is - an ‘anti surface tension groove’

    It is seen on the photo below and is described as an ‘integrated drip channel’

    When the filling below the cill is complete - the anti-surface tension groove should be away from the cement mortar filling. see the photo below-

  • Registered Users Posts: 1,231 ✭✭✭monseiur

    It seems that the mortar used to bed the cill did not adhere to the underside of cill or the DPM under, it just dried, cracked, crumbled and fell out. Fill in gap under cill with some type of packer and compress well leaving a gap of say 60mm for mortar at front. Scrape any loose residue off under side of cill & dpm and give it a coat of PVA Bonding or similar (Diluted 2:1) Fill with 1:1 sand & cement mortar, add a little PVA bonding to this mix as well it will make easier to work with and will bond easier, when dry paint to match.

    Option 2 - fill joint with epoxy resin & polyurea based filler but probably an overkill in this situation

  • Registered Users Posts: 2,458 ✭✭✭chops018

    Thanks all.

    Builders have since filled it in and assured me it is fine and won't affect the house (here's hoping).

    I just wanted to make sure any rain etc that got in would not be dangerous but the cill does have a groove for rain and also the DPM is there so hopefully it is all ok from here.

  • Registered Users Posts: 6,950 ✭✭✭10-10-20

    Glad that the collective could be of assistance. 👍️

  • Registered Users Posts: 93 ✭✭Dangee4050

    I have this in my house in certain windows. from my research they’re supposed to leave differential movement gaps under the cills because the frame shrinks. I don’t know how true that is though

    and it’s supposed to be filled with a compressible filler like compriband

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  • Registered Users Posts: 6,950 ✭✭✭10-10-20

    Compriband and the likes of expanding fillers are designed to be part of the air seal from block/cill and window and aren't an external finish. You would still need a durable finish under the cill where that junction is.

  • Registered Users Posts: 93 ✭✭Dangee4050

    for my own curiosity, is a differential movement gap not really needed?

    I’ve wondering about these gaps in my own house.

    i think I’ll just fill them with mortar.

  • Registered Users Posts: 6,950 ✭✭✭10-10-20

    If you specifically have "differential movement gaps" and not render gaps, we'd need to see photos of these rather than blanket comment out of sight. The gaps in the OP's post were clearly from sloppy workmanship as there is no expansion in a residential house which needs 3cm's of a gap. You could probably have hypothetically said that differential movement would probably have been supported by the DPC alone in that case.