8 " x 8" x 16" stretcher concre...

8 ” x 8″ x 16″ stretcher concrete blocks (Photo credit: Wikipedia)

Building a partition is the easiest way to rearrange the rooms of your house – to improve storage or simply to divide a large area into two smaller parts. Lightweight building blocks make a particularly solid structure, and they’re easy to work with.

E ven with the most ingenious space-saving schemes you may find that one room is simply not enough to cope with all the different activities and storage needs of your household. Often your problems would be solved if you had two rooms where there was one, and a partition is the obvious solution. You could, for example divide an open-plan living and dining room, or make a second WC or a utility room. Or it may simply be that you are dissatisfied with the way your rooms are laid out, and wish to rearrange them. CARPENTRY TECHNIQUES 8 describes how you can build a timber-framed partition wall, which you can clad with plasterboard and decorate. But there are advantages to using concrete blocks instead: they give a stronger, more sturdy structure than timber, help to reduce the transmission of sound through the wall and provide a solid fixing for items such as book shelves and picture frames. Blocks- also provide a greater degree of heat insulation. Because of their size you can build a fairly substantial floor-to-ceiling room-width wall relatively quickly which has the advantage that you can make it load-bearing.

Types of blocks

Building blocks are cast from concrete and come in a range of sizes with various densities. They’re laid in a similar bonding pattern to bricks for strength, and have mortar joints between.

Planning a partition

You must plan the shape, size and position of your partition wall if you’re not to end up with two dull rooms with unattractive proportions, but the most important factor to take into account is whether your floor can support a solid wall of blocks.

If you’re building onto a solid concrete floor you can position the wall where you want. Don’t worry if the surface isn’t perfectly smooth and flat: you can accommodate any – -». – unevenness in the thickness of the mortar screed on which you lay the blocks.

A suspended timber floor causes more problems, although you can build onto it if the partition doesn’t impose too great a strain on the structure. To ascertain this it’s wise to consult your local council’s Department of Building Inspectors.

Ground floor joists are usually spaced at 400mm (15in) intervals and supported on timber wall plates, which in turn are mounted on dwarf brick walls 1.3m (4ft 6in) apart over concrete foundations. Here you can build the partition on a stout timber floor plate, which will spread the load of the wall over the floor area. The floor plate should be of 100 x 50mm (4 x 2in) unplaned timber, nailed or screwed to the floor. It’s a good idea to fix a 100mm (4in) wide strip of carpet underfelt underneath the plate to help reduce any sound transmission. You can nail strips of expanded metal lath on top of the plate to improve the mortar bond with the blocks.

Upper floors of timber construction aren’t sturdy enough to support a blockwork partition and here you should build one with a timber frame. Similarly, upper floors of reinforced concrete construction aren’t designed to support the weight of a full-width, room-height block wall, but you can safely build short-length dividers and half-height partitions, the top section of which can be glazed.

Structural requirements

The type of wall you wish to build is entirely up to you. It can be floor-to-ceiling and full-room-width; it can incorporate a window or door ; or it may be a half-partition. But if you choose a full-width wall, there are some important structural rules to keep in mind.

On very long walls you’ll have to install a ‘vertical movement joint’ of flexible mastic every 6 to 9 square metres (20 to 30ft) -usually against a door or window frame – to allow for flexing of the structure. A wall of this size should also be ‘toothed’ into the existing walls. Alternatively you can make a room-height frame long plank with a nail driven into the top.


You’ll have to cut some blocks to size at the corners and the ends to maintain the stretcher bond throughout the rest of the wall.

– form the corner at ground level with full blocks then cut three-quarter-size blocks (A) for the second and then for the alternate courses.

– use bricks or cut 100mm (4in) pieces of blocks (B) to use at each course on alternate sides of the corner.

Marking out the partition

Before you start to build the wall on a solid floor you must remove any loose floorcovering such as linoleum, sheet vinyl, carpet or tiles, so that you have a firm, flat base for the blockwork. You can lay the blocks directly onto thermoplastic, vinyl or ceramic tiles as long as they’re firmly stuck down. You should also remove any wallcovering from the existing wall where the partition is to be fixed.

Mark on the floor in pencil or chalk parallel lines that indicate the exact position of the partition, and carry them up the adjacent wall or walls to ceiling height. Use a spirit level and a long timber straight edge to ensure these guidelines are evenly spaced and straight. Next, chisel out a section of the skirting board and ceiling moulding, if any, between the guidelines so that the blocks can be laid flush with the wall; this makes for a sturdier structure and means that you don’t have to cut any blocks to shape. When the partition’s built you can fit new skirting and moulding to match that on the adjacent wall. You also need to chop out ‘bonding pockets’ in adjacent walls so that the new structure is properly tied in.

Space out the blocks dry for the first two courses, without using any mortar, so that you can plan the best bonding pattern with as few cut blocks as possible. You’ll probably find that stretcher bond, as used in brickwork, is the best and simplest arrangement. When placing the blocks, leave finger-width joints between them. You’ll have to cut some blocks to size for corners and ends to maintain the bond throughout the 1 a rest of the wall. Alternatively, you could use bricks to fill in some spaces. When you’ve dry-laid the first two courses of blocks you can fix the door frame, if any, in place. Hold it erect with a temporary timber strut wide screed of mortar about 9mm (3/ain) thick onto the floor over your guidelines and trowel it smooth. Then scribe a guideline marking the outside face of the wall onto the screed using your trowel and a timber straight edge. Bed the first course of blocks in mortar up to the line, starting at the wall profile. Make a mortar joint with the existing wall.

The most accurate way of working is to build up the corners or ends of the wall first, to about four courses, so that a string line can be stretched between them at each course as a guide to laying the intermediate blocks. Check constantly with a long spirit level as you work that the partition is level and upright.

Unlike brickwork, laying blocks that are going to be plastered or boarded doesn’t call for perfectly regular size joints between the blocks. So long as there’s sufficient mortar between each one, and they’re laid so that the face is vertical, minor irregularities in thickness don’t affect the strength of the structure. If you’re building a large, room-width wall it’s wise to carry out the work over two days to give the mortar in each part time to set. To tie the partition to the existing wall of the house you can either recess alternate courses of blocks into bonding pockets cut into the wall or insert galvanised metal frame cramps at these positions.

Finishing the partition

If your partition is to be in a garage or utility room you might not feel a perfectly smooth, plastered finish is necessary, in which case you can simply form ‘struck’ or bevelled mortar joints with a bricklayer’s trowel as you work and then give the wall a coat of emulsion paint when the mortar has set.

On the other hand, if the partition is to be in a habitable room within the house you won’t want to see the outlines of the individual blocks. Here you should spread on two coats of plaster. You can then either paint or waiipaper the partition to match adjacent walls.

Alternatively, you can screw or nail horizontal timber battens directly to the blockwork and clad the surface.


If you’re building a very long wall it’s advisable to include a vertical ‘movement’ joint every 6 to 9m (20 to 30ft). Using a flexible mastic compound (available from builders’ merchants) instead of mortar, the joint allows for a little movement in the solid structure and prevents cracks appearing in the mortar joints. If you’re including a door or window, alongside the frame is an ideal place for the joint.


To form a strong bond with an existing brick or block wall and to prevent sideways movement of the partition, ‘tie-in’ the blocks at alternate courses.

– cut bonding pockets 50mm (2in) deep x 130mm (5in) wide x 245mm (91/2in) high (to allow for mortar joints) in the existing wall with a club hammer and cold chisel.

– slot the last whole block of alternate courses into the bonding pockets, bedding them in mortar.


You’ll need to install a lintel over the doorway to support the blocks above. Because of the lintel’s size you’ll have to fill the gap above with bricks or cut blocks.

Building a bedroom partition

Along narrow room can be a problem to furnish. No matter where you place groups of furniture they’ll appear to cut off other parts of the room; single items such as tables, chairs – and even the bed – will tend to look ‘lost’. Doors that open into a room will severely limit where you put things, and if you set your furniture round the walls you’ll only succeed in emphasising the tunnel-like appearance of the room.

Here a partition wall comes into its own, splitting the area into two – or more -separate parts of more adaptable and attractive proportions. You could build a timber-stud partition, clad it with plasterboard and decorate it, but one made of lightweight concrete blocks is far more sturdy and can be plastered so it seems an integral part of the room. However, you should check with your local building inspector before erecting such a wall, upstairs the weight may be too great for the existing structure to bear safely.

Although you’ll create two smaller areas where there was a single large one, you’l find that you’re able to position furniture with much more flexibility, and you will have the benefit of extra wall space, too.

You needn’t build your partition so that it divides the room exactly in half: if you’re making a shower-room or dressing-room, for example, you’ll probably want only a fairly small annexe to the bedroom. In practice the shape of your two new rooms might be limited by existing door and window positions.

In this long and narrow room a spacious double bedroom and a smaller, adjoining study have been created by building a block partition wall. The two rooms are linked by open doorways, which admit plenty of light to the bedroom and provide ample access. Spotlights recessed in timber channels fixed to the inside edges of the openings cast a mellow light on the walls at both sides of the partition. Each can be angled to change the lighting scheme.

Wall construction

The two courses of blocks above each doorway were supported by lintels. The end blocks of the lower course were mortared into the wall; they were cut to length to maintain the stretcher bond throughout the rest of the wall. Aerated concrete blocks measuring 440 x 215 x 100mm (17 x 8V2 x 4in) were used to build the partition wall because they’re lightweight and give a degree of sound and heat insulation. A course of bricks was laid at the level of the lintels to bridge the gap left by its thickness; this avoided the need to cut blocks to awkward shapes. A backing coat of Carlite Browning plaster followed by two coats of Finishing plaster were applied to the blockwork to give a smooth, matt finish; then the surface was decorated.

Fixing the floor plate

If the partition wall is to be built onto floorboards, you’ll need a timber floor plate to spread the load of the blockwork over the entire surface. (Check with your local building inspector that your floor’s strong enough to support the wall). The plate of sawn timber, measuring 100 x 50mm (4 x 2in), can be screwed to the floor joists over a length of 100mm wide underfelt, which will help absorb any noise caused by movement in the floor. A 100mm wide strip of expanded metal lath fixed to the top of the plate with U-shaped staples will give a key for the mortar screed on which the first course of blocks is laid. When the wall is complete, the floor plate can be concealed by the skirting board. If you’re building on concrete bed the blocks on a mortar screed.

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