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Bricklaying & Repointing

New cavity wall joined to existing rendered wall using furfix wall starter

Anonymous user 03/03/2024 - 3.04 PM

We have an extension being built and the brick layer used mortar to join the new wall to existing below floor level and furfix type wall starter secured to the external render of existing masonry. My understanding was that prior joining new to existing wall all render and debris needs removing first. Render is quite thick, and property is more than 100 years old. Should the brick layer have removed render first? Also is joining new to existing wall below ground level sufficient? And lastly, I noticed there is a mortar build up inside new cavity wall being built. Does this need removing before insulating foam is installed? Many thanks for your help and advice. Additional information (1) Thank you for clarifying the removal of render issue with cavity walls. With regards to insulation, this has not yet been placed in, it is currently stored outside with plastic covering. However the plastic does not cover the insulation boards fully and it has rained a log in the past 2 weeks. I also noticed build up of mortar snots at the bottom of cavity wall. Might have to mention this to the brick layer. Additional information (2) New wall is being built and the brick layer used mortar to join the new wall to existing below floor level and furfix type wall starter secured to the external render of existing masonry. My understanding was that prior joining new to existing wall all render and debris needs removing first? It's an older property.

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3 Answers

Anonymous user

When using board insulation in cavity walls the boards should be clipped against the inner leaf to prevent dampness occurring.The inner leaf, usually blockwork, is built first using cavity wall ties with plastic retaining clips to ensure the board is against the inner leaf allowing any moisture in the cavity to drop off the ties an harmlessly dissipate through the outer leaves. Some Clerk of Works like weep holes to be built into the first course brick cross joints to facilitate this. The method is referred to as partial fill method. The other method of full fill is an easier method but the material used must be rockwool, fibreglass batts or such like.Remember to meet wall heat loss values (U values) 100mm thick full fill must be used in a typical cavity wall. To prevent mortar build up in the cavity, a board should be placed across the wall ties to catch mortar droppings and raised each time a layer of wall ties is reached. Any mortar on the board can then be cleaned off. The wall connector should be fixed against the brickwork and not the rendering. It is important to ensure the fixing plugs are in firm masonry to make a proper connection. Rendering of this age is usually thick, quite rough, and made of weak lime mortar and it would not be right to merely fix the connector to it. However, one of the reasons for using a wall connector and not cutting into the fabric of the wall (toothing) is to allow any differential movement between the existing house which has settled and the new extension which needs around 6 months to settle to occur.

Answered5 January 2015

Anonymous user

If the work below d.p.c is less than 450mm then no reason to tie in. If over then yes it should be tied it without crossing the d.p.c . Building regulations in different areas say take render off /leave it on/cut cavity/cut in a d.p.c . If the new build is in water proof modern render I can't see a problem, there isn't a cavity as the property is 100 years old . The foam insulation bothers me more, it should be either 100mm mass fill dritherm ect or celetex type with cavity.

Answered28 December 2013

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Croydon

The PIR insulation should be placed against the inner wall like previously stated, wall ties go in every second block course (450mm) normally red plastic discs hold the insulation in place, once the outer course is built then there is little chance of getting insulation in.

Answered29 August 2016