Thin Joint Constuction

What is Thin Joint Construction?

Thin Joint mortar construction is an accepted Modern Method of Construction (MMC) that utilizes 2-3mm thick mortar joints. This method of construction is very well established throughout Europe and with the ever increasing tightening of Building Regulations and Codes of Practice, Thin Joint mortar construction will assume greater significance in the UK due to the numerous benefits offered by this method.
 
Applications for Thin Joint construction include: Internal and External leaf of cavity walls, Party walls and Flanking walls, Internal Partition walls, Solid walls and Foundation walls.
 
 

Are Airtec products Thin Joint compatible?

Yes. Because very thin 2mm mortar joints are used, the dimensional accuracy of the block is absolutely critical since there is less leeway in adjusting coursing heights by varying the thickness of the mortar bed as with traditional mortar construction.
 
All Airtec products comply with the most rigorous dimensional tolerance category as defined by the latest European Standards, ‘Thin Layer Mortar category B’ (TLMB). Unlike other dimensional tolerance categories (General Purpose or TLMA) very tight constraints on the flatness of the bed faces and block parallelism to a tolerance of 0.1mm are imposed.
 
Due to the completely unique Airtec manufacturing process, this accuracy is consistently achieved on all Airtec blocks. 
 
 

The Benefits of Thin Joint

Thermal insulation:

Thinner mortar joints means significantly less thermal bridging through mortar joints. U-Value improvements of up to 10% can be achieved over conventional mortar walls made with the same block. This could reduce the thickness of expensive secondary insulation in the wall, whilst still achieving the required U-Value.

Quicker builds:

Quick setting proprietary mortar essentially ‘glues’ the block together and the next course is ready to lay within a very short space of time. Purpose made serrated Thin Joint mortar trowels allow for quick, easy and accurate application of mortar to the correct thickness.

Easier to build with:

The accuracy of Airtec blocks and the use of tools available to ensure correct mortar bed thickness and width mean that less skill is required by the bricklayer to form correct mortar joints. By ensuring that the first course is laid completely level using conventional mortar, blocks are effectively glued together from thereon.
In addition, the rapid setting of the mortar allows for more stable walls during construction and no chance of blocks settling down out of true as is possible with conventional mortar joints.

Improved airtightness:

As with the improvements in thermal insulation, the significant reduction in the proportion of mortar in a wall greatly improves airtightness of the construction.

Less waste, better quality construction:

A bucket and stirrer is all that is required to make the mortar. Therefore, no need for bags of sand, cement and lime or a cement mixer on site.
 
The inner leaf of the cavity wall can be built before the outer leaf because the wall ties are the helical twist-type and are driven into the Airtec leaf at a point where it will course with the outer leaf as the build proceeds. This eliminates the problem of mortar snots forming on the wall ties and in the cavity.
 
 

Thin Joint Mortar

Mortar for Thin Joint construction is widely available through most builders’ merchants in the form of 25kg bags of dry, pre-mixed powder. Mortar mixing is simply done by adding water to the bagged powder according to the mortar manufacturers instructions and if made correctly will ensure that the correct mortar strength is achieved for use with aerated blocks.
 
Typically, a 25kg bag of dry mortar mix should yield enough mortar for 10 – 15m˛ of blockwork, although the face size and block thickness will of course influence the yield.
 

Tools and Ancillary Components

At present Thomas Armstrong (Concrete Blocks) Ltd do not supply tools and ancillary components for Thin Joint construction as they are widely available through builders’ merchants. We are able to recommend suitable suppliers on request.
 
These include a variety of mortar applicators of various widths to match the most commonly used block thicknesses along with aerated block hand-saws, whisk drill mixing attachments, sanding boards and specialist wall ties.
 

Laying

  • It is absolutely essential that the first course is laid accurately level to +/-1mm using conventional mortar and left to cure overnight.
  • The Thin Layer mortar should be prepared strictly in accordance with the manufacturer’s recommendations. A hand-held slow speed drill with whisk attachment may be used. Once mixed, the mortar should remain workable for up to 4 hours typically.
  • Remove any dust, dirt or debris on the block bed face to be mortared using a stiff brush.
  • Avoid using overly wet blocks as this can delay the setting of the Thin Joint mortar.
  • Using the specialist serrated trowel, the mortar is spread evenly across the bed face to a thickness of 2-3mm, ensuring all joints are fully filled.
  • The blocks should be pressed firmly against the mortared vertical face of the preceding block whilst being lowered onto the mortar bed below. A rubber mallet can be used to tap the block into position and make level. Remove any excess mortar as soon as possible.
  • After application, the mortar will begin to set within 10 minutes typically.
  • Normally in Thin Joint construction the inner leaf can be built well ahead of the outer leaf. However, if there is a delay due to weather conditions for example, the inner leaf wall should be supported and propped suitably until such time it can be tied to the outer leaf. Similarly in bad weather, the exposed inner leaf should be protected as far as possible.
  • The same guidelines on the accommodation of movement applies to Thin Joint blockwork as for conventional walls (refer to the Movement section for further guidance).
  • Specially designed Bed Joint reinforcement is produced specifically for Thin Joint masonry and we recommend that this is used if possible.
  • Helical ties can be hammer driven into the Airtec inner leaf at the appropriate mortar joint level of the outer leaf as construction progresses.
  • Alternatively, ties that are to be face-fixed to the Airtec leaf should be anchored through the hole provided in the tie using an expanding nail or similar fixing (refer to the section on Fixings for further guidance). Frame-fix ties must be installed correct side up and placed horizontally or with a slight fall to the outer leaf.
  • Other detailing such as cavity closing, lintels, finishes and fitting out is the same as for conventional blockwork.
  • At window openings, use full block heights and then use cut pieces of block to build up to sill level and lintel height. Alternatively, whole blocks can be cut to accommodate the lintel.
 
 
Please refer to our BBA Certificate 06/4309 for further details.
 
 

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