External Wall Masonry - Carcass Construction up to 30 cm Thickness (Brick, Porotherm, Ytong)

External walls carry the floors and the roof. They also protect the house from cold, damp and noise. That is why they are built strictly to the drawings, never by eye. The designer sets the material, the wall thickness, the position of openings, the lintels and the ring beam on top of the wall. A structural engineer checks that the wall can carry the loads placed on it.

One thing to know about walls up to 30 cm thick: on their own, they rarely meet today's insulation requirements. In practice they are almost always combined with external wall insulation, an insulation layer fixed to the outside and finished with render. Single-layer walls without added insulation are built from much thicker blocks.

New houses, extensions and any work on load-bearing walls are covered by local building regulations. Do not assume that a simple notification is enough. Check with your local building authority what approval you need before work starts. If you live in a block of flats, discuss any work on the external walls with the building management or the owners' association first.

The method is similar for all three. They differ in the material, the mortar or adhesive, and how quickly the wall goes up.

Traditional solid clay brick

Small units laid on ordinary mortar in both horizontal and vertical joints. Progress is slow, because each square metre takes many bricks. Today solid brick is used mostly in renovations, for patching or extending old walls, for chimneys and small structures, and less often for the whole external walls of a new house.

Clay blocks such as Porotherm

Large fired clay blocks with internal cavities and tongue-and-groove sides, so the vertical joints get no mortar. There are two main types:

• Unground blocks are laid on ordinary or insulating mortar, with a bed joint of roughly one centimetre
• Ground (precision) blocks are machined to an exact height and laid on thin-bed mortar or on the manufacturer's masonry foam, following its instructions

Fired clay stores heat well, is strong and holds wall fixings well.

Aerated concrete blocks such as Ytong

Light, very accurately sized blocks bonded with thin-bed adhesive. They are easy to cut and plane, so corners and openings come together quickly. External walls use blocks of the strength set in the design, not thin partition blocks. Aerated concrete soaks up water, so the wall needs protection from prolonged rain, and heavy items need fixings made for this material.

The design decides the material: loads, required insulation, sound protection and how the wall connects to other parts of the building. Mixing different systems in one wall without the designer's approval is a bad idea.

1. Checking the base. The slab or floor must be solid, clean and roughly level. The bricklayer measures the height differences and sets out the wall lines according to the drawings.
2. Damp-proof course. A horizontal damp-proof membrane goes on the slab under the external walls, so moisture cannot rise into the masonry.
3. Setting the first course. The first course sits in a bed of cement mortar that evens out the base. Precision systems use a levelling mortar and a levelling kit. Accuracy matters most here, because nobody can correct a mistake in the first course later.
4. Laying the courses above. Blocks are laid in a bond, with each vertical joint offset from the one below. Corners and wall junctions follow the bond specified for the system. The bricklayer keeps checking that the wall is plumb, flat and that the courses are at the right height.
5. Openings and lintels. Window and door openings are left exactly as drawn, and lintels go above them with enough bearing on the masonry at each end. Keep in mind that cutting a new opening into a load-bearing wall later is a structural change.
6. Cut pieces. Blocks are cut to size with a saw, not broken with a hammer, around openings and under the floor above. Small gaps and joints are filled straight away during laying.
7. Ring beam. A reinforced concrete ring beam goes on top of the finished wall. It ties the walls together and spreads the load from the floor. Its outer face is usually insulated to avoid a cold bridge.
8. Curing and protection. The wall is protected from rain, frost and strong sun until the mortar or adhesive has gained strength. Curing time depends on the product and the weather, so follow the manufacturer's guidance.

The effort, and therefore the cost, varies a lot from one site to another. The job goes faster with:

• A flat, accurately cast slab
• A simple floor plan with long straight walls
• Large-format blocks and thin-bed adhesive
• Good access for a crane or forklift and space to store materials

It takes longer with:

• Many corners, curves, angled walls and small openings
• An uneven slab that has to be corrected in the first course
• Taller walls that need scaffolding
• Small bricks, which mean more units and more mortar
• Tying new work into old walls during a renovation
• Bad weather, when work stops and the walls have to be covered

Building load-bearing external walls yourself is not recommended. It is a structural element, and mistakes are hard to fix once the floor and roof are on.

• A careless first course. Unevenness then gets taken up with thicker mortar higher up, which wastes the advantage of precision blocks in thin-bed systems.
• A missing or damaged damp-proof course. Damp at the base of the wall is very difficult to repair later.
• Poor bonding. Vertical joints lined up above each other weaken the wall and can lead to cracks.
• The wrong mortar or adhesive. Ordinary mortar on ground blocks, or adhesive applied only in strips, reduces both strength and insulation.
• Lintels with too little bearing. Cracks then appear above the windows.
• Blocks broken with a hammer. This leaves large gaps filled with mortar, which act as cold bridges.
• Walls left unprotected over winter. Wet blocks and frozen mortar lower the quality of the wall.

Before work moves on to the ring beam and the floor, walk round the walls with the bricklayer or the site supervisor:

• Wall lines, thicknesses and openings match the drawings
• The damp-proof course under the external walls is intact
• Walls are plumb and flat, checked with a spirit level and a long straightedge
• Courses are level and vertical joints are properly offset
• Corners and wall junctions are bonded as the system requires
• Lintels sit correctly with enough bearing on the masonry
• There are no large gaps, broken blocks or unfilled areas
• The mortar or adhesive matches the manufacturer's system and joints are full
• The masonry is protected from rain and frost
• Product and system documents are handed over for the site records