How to Lay, Cut & Fix into Aerated Concrete and Thermalite Blocks
How to Lay, Cut and Fix into Aerated Concrete and Thermalite Blocks
Aerated concrete is popular on site because it is lighter and easier to work than many dense masonry products, but good results still depend on careful setting-out, suitable mortar, accurate cuts and the correct fixings. Thermalite blocks can be sawn and chased relatively easily, which is useful around openings and services, yet that same lower density means fixing selection needs more thought.
This guide covers the practical basics of laying, cutting, drilling and fixing into the Forterra products available in our aerated blocks range, including 100mm and 140mm Thermalite Shield 3.6N and 215mm Thermalite Turbo 2.9N.
Before You Start: Check the Block Specification
Confirm the exact block type, width and strength before work begins. Do not assume every Thermalite pack can be used interchangeably. The wall drawing may specify a particular thickness and strength, and a change can affect structural or thermal performance.
Inspect packs for transport damage and store them on firm, level ground. Protect aircrete from prolonged saturation and keep the work area organised so blocks do not need to be moved repeatedly.
Tools for Working with Thermalite Blocks
Typical tools include a tape measure, level, string line, trowel, club hammer, masonry square and a suitable aircrete or block saw. Power saws may be used where appropriate, but dust controls and tool-manufacturer safety instructions are essential.
For drilling and fixing, use masonry drill bits and anchors rated for aerated concrete. A dense-concrete fixing should not be assumed suitable simply because both substrates are called concrete.
Setting Out the Wall
Good blockwork starts before mortar is mixed. Mark wall lines, openings and returns accurately. Check dimensions against the drawings and identify where cut blocks will be needed. Aim to avoid very small pieces at corners and openings where possible.
Dry-setting a few blocks can help confirm the bond and reveal dimensions. Remember to allow for specified mortar-joint thickness rather than measuring bare units only.
Laying the First Course
The first course establishes the level for the entire wall. Set it carefully on a suitable mortar bed, checking each block for line, level and plumb. Correct variations in the base at this stage rather than trying to compensate over later courses.
Ensure the supporting construction is ready for the masonry and that any DPC or specified interface is correctly positioned before laying.
Mortar for Aerated Blocks
Use mortar specified for the aircrete block and wall exposure. Lightweight blocks can have different suction characteristics from dense masonry, so generic assumptions about mix strength or pre-wetting should be avoided. Follow the project and manufacturer's guidance.
Spread bed joints consistently and fully fill perpendicular joints where the construction requires it. Scrape away excess mortar without smearing the block faces unnecessarily.
Maintaining Bond
Stagger vertical joints between courses according to the specified bond. Proper overlap helps the wall act as a coherent masonry element. Around corners and junctions, follow the wall detail for bonding, ties or proprietary connectors.
Do not create long vertical runs of aligned joints for convenience unless the design specifically allows them.
Keeping Blockwork Level and Plumb
Use a string line for each course and check with a level regularly. Aircrete is easy to adjust while mortar is fresh, but repeated tapping after the bed has started to stiffen can weaken the joint. Build steadily and correct errors early.
How to Cut Thermalite Blocks
Mark the cut on more than one face and use a square to keep it true. A dedicated aircrete saw can make clean manual cuts because the material is relatively soft compared with dense concrete. Support the block while cutting so the offcut does not break away and damage the required piece.
For repeated cuts, powered equipment can improve productivity, but silica-containing masonry dust must be controlled. Use extraction, suppression, respiratory protection and other site measures appropriate to the tool and risk assessment.
Cutting 100mm vs 140mm vs 215mm Blocks
Wider blocks require longer cuts and are more difficult to hold square. A 215mm block may need marking around several faces to maintain accuracy. Do not rush large cuts; an out-of-square face creates irregular joints and can affect finish and wall alignment.
Read our Thermalite size guide if you are planning different widths.
Chasing Aerated Blocks for Services
Aircrete is relatively easy to chase for cables and pipes, but ease of cutting is not permission to remove unlimited material. Chase depth, direction and location can affect wall strength. Follow the building design and applicable masonry guidance, especially near openings, bearings and slender walls.
Use appropriate chasing tools rather than repeated uncontrolled hammering. Keep service routes planned and consistent.
Drilling into Thermalite
Use the drill mode and bit recommended by the fixing manufacturer. Excessively aggressive hammer action can enlarge the hole in lightweight masonry and reduce anchor grip. Drill to the specified diameter and depth and clear dust as instructed.
Do not guess hole size. Aerated-concrete anchors often rely on a particular installation method to achieve their declared load.
Choosing Fixings for Aerated Concrete
Fixings for Thermalite blocks should be rated for aerated concrete. Options can include specialist plugs with deep external threads, frame fixings, proprietary screws or resin systems, depending on the load and substrate. The correct anchor is determined by tested performance, not appearance.
For light items, a suitable aircrete plug may be sufficient. For medium or heavy items, check allowable loads and consider the complete fixing arrangement.
Fixing Kitchen Units to Thermalite
Wall cupboards apply sustained load and can be subject to additional force when doors are used. Use a fixing system rated for the block and load, install the correct number of anchors and use a secure rail or bracket arrangement. Avoid relying on one or two generic plugs.
If the wall construction or loads are uncertain, seek fixing-manufacturer or competent technical advice.
Fixing Boilers and Heavy Equipment
Boilers, cylinders, large radiators and other heavy equipment require particular care. The appliance manufacturer may specify fixing requirements, and the anchor system needs to be suitable for aircrete. In some cases, fixing back to a stronger structural element or providing planned reinforcement may be preferable.
Fixing into Shield vs Turbo
Thermalite Shield has a higher published density than Turbo. Anchor performance can therefore differ. Do not transfer a load value from Shield to Turbo unless the fixing data specifically covers both substrates. Our Shield vs Turbo guide explains the material differences.
Wall Ties and Connectors
For cavity walls and masonry junctions, install wall ties and connectors at the positions required by the design and manufacturer. Embedment, orientation and drip position matter. Do not bend or reposition ties randomly to compensate for poor coursing.
Lintels, Bearings and Openings
Openings introduce concentrated loads and movement details. Use specified lintels, bearings and any padstones or higher-strength units required by the design. Do not cut away blockwork around a bearing simply to make services fit.
Weather Protection During Construction
Protect fresh masonry from frost, heavy rain and rapid drying. Cover the top of unfinished walls at the end of the day to limit saturation. Avoid letting water collect inside cavities or on floor edges around blockwork.
Plastering and Finishing Aircrete
Surface preparation depends on the intended plaster, render or drylining system. Aircrete's suction can influence finish behaviour, so follow the finish manufacturer's background preparation guidance. Remove loose dust and avoid contaminating the surface with unsuitable treatments.
Common Thermalite Installation Mistakes
- Using the wrong block strength or width.
- Starting on an uneven first course.
- Inconsistent mortar-joint thickness.
- Over-chasing walls for services.
- Using generic fixings not rated for aircrete.
- Drilling oversized holes with excessive hammer action.
- Failing to protect unfinished masonry from weather.
- Substituting Shield and Turbo without checking the design.
Thermalite Installation FAQs
Can you cut Thermalite blocks with a hand saw?
Use a saw designed for aircrete or masonry. The material is relatively easy to cut, but normal timber saws may wear quickly and are not the best tool.
Do I need a special drill bit for Thermalite?
Use a masonry bit and the drilling method specified by the anchor manufacturer. Some fixings advise against aggressive hammer action.
Can you screw directly into Thermalite?
Only with a fixing specifically designed and rated for direct installation into aerated concrete. Standard screws alone are not suitable for most loads.
What fixings are best for Thermalite?
There is no single best fixing. Choose from tested aerated-concrete anchors according to the load, fixture and block grade.
Can Thermalite blocks be chased?
Yes within the limits of the wall design and relevant guidance. Avoid excessive chase depth or locations that weaken critical masonry.
Should Thermalite blocks be soaked before laying?
Do not automatically soak them. Follow Forterra and mortar-system guidance for the actual site conditions.
How many Thermalite blocks are needed per m²?
About 10 blocks per m² is a common preliminary estimate for 440 x 215mm face blocks with typical joints.
Where can I buy Thermalite blocks?
Shop JJ Building Materials' Thermalite aerated blocks in current 100mm, 140mm and 215mm sizes.
Final Advice
Aircrete is forgiving to cut but deserves the same precision as any structural masonry. Get the first course level, maintain bond, use compatible mortar, cut accurately and select anchors specifically tested for aerated concrete. That combination gives a straighter wall and far more reliable fixings than treating lightweight blocks as a generic substrate.