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Rendering Victorian fireplace: builder added scratch coat, already cracking.

Discussion in 'Plastering and Rendering' started by Beezle, 23 Sep 2018.

  1. Beezle


    23 Sep 2018
    Thanks Received:
    United Kingdom

    Currently renovating my Victorian house and on the newbie learning curve, I love it.

    Two differing builder perspectives: old school and new school and I need another viewpoint. Eeek! Lots of info here:

    After we opened the large fireplace (exciting), Builder One filled in a vertical line of bricks (probably part of the old style victorian range) with sand and cement and asked me to apply three coats of PVA later that day to everything (note: no raking, wire brushing or washing of bricks/joints). The next day, we installed a concrete lintel and he then used a mix of 4:2:1 soft sand: concrete: lime. This mix was applied to the dried PVA as a scratch coat. It was applied quite thickly (more than 15mm in some places) and after 24 hours is full of cracks. It was also applied around the lintel and is cracked there too. Note the lintel has not yet been back-filled and we used some bathroom tiles bits as the key stone between the top of the lintel and the middle of the shallow arch of bricks.

    A second builder, let's call him Builder Two, told me that the scratch coat shouldn't crack and that it should all be taken down and started again and that way too much sand was used. He reckoned it wouldn't have bonded and will likely chip off in chunks and cause problems later. (BTW - The feel of the render is dusty and sandy). Builder Two also recommended using a ready-made lime mortar mix and just adding water rather than making up our own batches. Also on Builder Two's advice, I have added a wood board coated in a plastic bag horizontally across the inside of the chimney to prevent rain water from coming down and soaking the bricks to be rendered.

    Builder One has agreed that he probably applied the coat too thickly and that sharp sand would be better for the second coat but doesn't think this first attempt needs to be removed. He also said as there are too many points of contact in the bricks behind the scratch coat, the moisture from the mix has been drawn out too quickly and caused the cracks. He also said that there was no need to fill the gaps between the bricks before adding the render as the gaps act as a good key for the scratch coat. He said he normally finds the first scratch coat will crack and then just adds the second render over the top. He also doesn't like to use ready made mixes and said they take ages to go off and need a cement mixer machine and doesn't rate them.

    My gut feeling is to remove the current render, rake, wire brush, soft brush and wash the bricks. Where there are huge deep gauges between bricks, point with normal sand and cement. 24 hours later, PVA everything and then apply a lime cement render at ratio 5:1:1 sharp sand: cement: lime at no more than 10mm thick. Then scratch the surface in wavy lines and leave for minimum 72 hours and then apply a second scratch coat (as we have to make up for some wonky lines in the original bricklaying). Leave for another 72 hours and then apply a final top coat (what ratio is best and how thickly should each layer be applied?)

    Any advice would be greatly appreciated for this first timer, what am I missing??

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  3. darrington


    28 Dec 2010
    Thanks Received:
    United Kingdom
    No way would I leave that on. and also, it's been put on far too thick for a scratch coat, causing the render to "slump" and crack,, 15mm thick,, wow.
    Another thing, you should "never" apply plaster, render or any other coating to a "dry" pva. That's probably another one of the reasons for the "slump". The "dried out" pva would have made the brickwork "slippery" due to the lack of suction, causing the "over thick" scratchcoat, to slide/slump down the wall and crack.
    I would have also scratched the area "across" the way with a scratcher, rather than the diagonal way the builder did it. Scratching it the way he has, leaves "far to many smooth areas" between the diagonal marks.
    All in all, i'd go back to square one and start again, especially where heat is involved.
    Last edited: 24 Sep 2018
    • Thanks Thanks x 1
  4. lostinthelight


    11 Feb 2016
    Thanks Received:
    United Kingdom
    Builder?? looks more like a DIYer to me and a poor one at that!!
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