Mortar Mixing
Mixing lime mortar correctly is one of the most important parts of any successful pointing, bedding, rendering, or masonry project. A well-made lime mortar is easier to use, more breathable, more compatible with traditional buildings, and far less likely to cause problems than an incorrect mix. Roundtower’s current guidance already stresses the fundamentals: measure materials by volume, dry mix first, add water slowly, and avoid overwatering the mortar.
At Roundtower Lime, we offer both Natural Hydraulic Lime binders for site mixing and premixed lime mortars for customers who want greater consistency and convenience. Our core site-mixed range includes Roundtower NHL 2, Roundtower NHL 3.5 Lt Grey, Roundtower NHL 3.5 White, and Roundtower NHL 5. We also supply premixed options including Roundtower NHL 2 Fine, Roundtower NHL 2 Coarse, Roundtower NHL 3.5 Fine, and Roundtower NHL 3.5 Coarse.
If you are looking for lime for on-site gauging, browse our Natural Hydraulic Limes collection. If you would prefer a ready-prepared option, explore our Lime Mortars range.
Why Getting the Mix Right Matters
Lime mortar is not mixed in the same way as cement mortar, and it should not be treated like it is. The amount of water, the mixing time, the type of sand, and the consistency of the gauging all have a direct effect on the finished result. Too much water increases shrinkage risk and can reduce final strength. Too little water can prevent the mortar from developing properly. Water should be added sparingly until a usable consistency is reached.
Correct mortar mixing helps to achieve:
- good workability
- better adhesion during pointing and bedding
- reduced shrinkage risk
- a more durable final mortar
- improved breathability and compatibility with older masonry
It is also important to remember that dry masonry can pull moisture from fresh mortar too quickly. Dampening the background before application is often essential, particularly in warm or dry conditions.
How to Mix Lime Mortar
A conventional cement mixer can be used for smaller jobs, although for larger projects a roller-pan or paddle mixer is preferable, as lime mortars mixed in drum mixers can be prone to balling. Materials should always be measured by volume, not by shovel, as shovel measurements are too inconsistent. A bucket is the right approach.
Mortar mixing method
Roundtower’s recommended sequence is:
- Start with an empty mixer
- Add 1 part sand
- Add 1 part lime
- Add 1.5 parts sand
- Dry mix for at least 5 minutes
- Slowly add water until the desired consistency is reached
- Do not drown the mix with too much water
- Once the correct consistency is reached, mix for a further 20 minutes
- This example is based on a 1:2.5 mix ratio
The mortar may seem dry at first, but as the mixing time increases it will become much fatter and more workable. That is normal with lime mortar and is one of the reasons patience during mixing matters.
Site-Mixed Lime Mortar with Wexford Sand
For customers who want to mix on site, Roundtower’s Natural Hydraulic Lime binders offer the flexibility to gauge mortar to suit the work in hand. These products are mixed on site with Wexford Sand, making them a strong option for contractors and craftspeople who want full control over the sand grading, colour, and final character of the mortar.
Roundtower NHL 2
Roundtower NHL 2 is a feebly hydraulic lime binder suitable for internal lime plastering, external renders on soft masonry, and fine pointing or bedding mortars. It is known for its workability, bright colour, and high vapour permeability, making it a very good choice for softer, more breathable work.
Roundtower NHL 3.5 Lt Grey
Roundtower NHL 3.5 Lt Grey is a versatile, moderate-strength binder for masonry, pointing, rendering, and external plasterwork. Its light grey tone gives a more natural appearance where a softer grey mortar is preferred, particularly when matching stone or traditional finishes.
Roundtower NHL 3.5 White
Roundtower NHL 3.5 White is a highly versatile natural hydraulic lime binder for external plasterwork, rendering, pointing, and general masonry applications. Its clean, bright appearance makes it especially useful where a lighter or colour-matched mortar is required.
Roundtower NHL 5
Roundtower NHL 5 is a durable, high-strength natural hydraulic lime intended for higher exposure applications such as limecrete floors, wall copings, foundations, and damp or marine environments.
Premixed Lime Mortars
For many projects, premixed lime mortars are the more convenient option. They remove much of the guesswork on site, improve consistency from bag to bag, and save time during preparation. Roundtower’s Lime Mortars collection is specifically positioned around consistency, quality, and long-term performance for traditional building, historic conservation, and breathable construction work.
Roundtower NHL 2 Fine
Roundtower NHL 2 Fine is a factory-prepared, dry ready-mixed lime mortar made with NHL 2 and 2mm kiln-dried sand in a 2:5 ratio. It is designed for pointing, bedding, and textured render topcoats where a softer breathable mortar is required.
Roundtower NHL 2 Coarse
Roundtower NHL 2 Coarse is a multi-purpose dry ready-mixed lime mortar made with NHL 2 and coarse kiln-dried sand in a controlled factory mix. It is suitable for bedding, pointing, and general work where a softer vapour-permeable mortar is preferred.
Roundtower NHL 3.5 Fine
Roundtower NHL 3.5 Fine is a dry pre-mixed lime mortar made with NHL 3.5 and 2mm kiln-dried sand for pointing, bedding, rendering, and general masonry work.
Roundtower NHL 3.5 Coarse
Roundtower NHL 3.5 Coarse is a factory-blended dry ready-mixed lime mortar made with NHL 3.5 and coarse kiln-dried sand. It is suitable for bedding, pointing, general masonry, and base coat applications.
Which Lime Mortar Should You Choose?
Choosing the right lime mortar depends on the strength required, the background, the degree of exposure, and the finish you are trying to achieve.
For softer, more breathable work, Roundtower NHL 2 or Roundtower NHL 2 Fine / Coarse are strong options. For general pointing, rendering, and masonry applications, Roundtower NHL 3.5 White, Roundtower NHL 3.5 Lt Grey, or the matching premixed NHL 3.5 mortars are often the most versatile choice. For floors, copings, foundations, and more exposed work, Roundtower NHL 5 is the natural fit. These product roles are reflected across the live Roundtower product pages and collections.
Common Mortar Mixing Mistakes to Avoid
Even good materials can perform badly if the mixing process is poor. Some of the most common problems include:
- measuring by shovel instead of by volume
- adding too much water
- under-mixing
- applying mortar to an overly dry background
- using additives or plasticisers
- poor storage of opened lime bags
- leaving aggregate uncovered so fines wash out or separate
Storage of Lime and Aggregate
Natural Hydraulic Limes should be kept dry and protected from moisture. Once a bag is opened, exposure to air begins to weaken the hydraulic set, so opened bags should be folded over carefully and stored in a dry place. In that condition, the material will remain usable for a short period only. Aggregate should also be stored properly and kept covered.
Final Thoughts
A good lime mortar starts with good materials, but it also depends on good mixing practice. Measuring by volume, using the correct sand, adding water carefully, and allowing enough mixing time all make a real difference to the finished result. Whether you prefer to mix on site with Roundtower NHL 2, Roundtower NHL 3.5 Lt Grey, Roundtower NHL 3.5 White, or Roundtower NHL 5, or you would rather use one of our premixed lime mortars, choosing the right product and mixing it properly will give you a better, more reliable result.
