Building a Garage

You can build your garage in brick, concrete blockwork or assemble a prefabricated structure.

Concrete slab

It is generally best to erect a garage on or over a concrete slab. The garage may stand directly on the slab or may be built, in the case of brick or concrete block construction, on a shallow over-floor.

Since the average garage occupies a fairly large area, it may be worthwhile to hire a concrete mixer or arrange for delivery of ready-mixed concrete.

To construct the concrete slab:

1 Clear the site of any plants and roots.

2 Position four 2 in. (50 mm) timber pegs, one at each corner. Join these up with taut string or builder’s twine.

3 Lay a 3-4 in. (or 75-100 mm) base of hard core, “blinded” if necessary by sand or ash. Tramp this down well. For larger areas, the slab should be in two sections with a joint consisting of bituminous felt.

4 Position formwork of 1 in. (25 mm) timber, set on edge and nailed to stout pegs. Set out the edges accurately for, unless the perimeter is level, sections of any prefabricated structure may not fit and there will be particular difficulty in fitting the doors and roof.

6 Lay the concrete slab in widths of about 3 ft 4 in. (or 1 m), tamping down each section or bay before going on to the next. Separate the bays with screed rules. The gaps left by these when removed can be filled with concrete and tamped down. 6 Level off the formwork with a timber batten between rules. A wood float or soft-brush finish is best, for a roughened surface will be less slippery. Allow the slab four weeks to cure in normal weather. Note: If the area of the garage base is large, with the length exceeding twice the width, the concrete should be laid as two adjoining slabs with a joint consisting of bituminous felt.

Sump. To take away car-washing water, the slab may be sloped inwards slightly to a sump. This consists of a soak-away pit with a grating above. A piece of plastic rainwater or soil pipe can be inserted into the concrete lid on the soakaway sump below the level of the concrete slab.

Pit. For car maintenance a pit is useful. It can be dug out when preparing the site and should be as central as possible. The depth should be no deeper than to allow you to work comfortably beneath the car.

To make the pit:

1. On the base of the pit lay 4 in. (or 10 cm) of hardcore and on that 4 in of concrete (1: 2: 4).

2. Line the pit with concrete blocks.

3. In forming the edges of the pit, avoid ridges, which may make it difficult to move the car forward or backward. The edges should be recessed to allow a lid to fit on the pit.

4. The lid can be made from 1 in. (2.5 cm) timber and should have a hand-hold for easy removal.

5. Make a fillet in the corners of the pit to keep them free of dirt.

6. Place a sand-filled metal tray on the floor of the pit to catch grease and oil.

Access to the pit may be by manhole steps, set into its sides. Electricity can be taken into the pit but the cable should be well insulated and not likely to suffer from damage, damp or corrosion.

Brick construction

Brick garages may be joined to the house or built as a detached structure. A single-skin wall section will usually suffice. But if a room is to be built above where the garage is joined to the house, deeper foundations and cavity-wall construction may be needed.

The roof can be flat or pitched. Ready-made window and door joinery sections can be built into the structure. Piers should be twice the thickness of the wall structure.


Concrete blocks are a suitable material for garage construction. The blocks may be dense or lightweight. The standard sizes are 45 cm by 22 cm and 40 cm by 20 cm (roughly 1 ft 6 in. by 9 in. and 1 ft 4 in. by 8 in.). These standard sizes are available in a variety of thicknesses, with matching half blocks. Facing blocks are made with a wide range of finishes.

Cutting. A club hammer and bolster can be used to cut blocks but plan the work so that cutting is minimized. Do not add anything for mortar joints as these are allowed for in the block sizes.

b. Blocks may be laid “bond to bond- with staggered vertical joints. A stretcher bond — blocks in each course overlapped by half blocks — makes the strongest joint. Half blocks can be used as piers, wall ends and around windows and doors. Lay the blocks with a 1:5 (masonry cement) mix. Use the standard bricklaying technique, working to a string line and building up ends (quoins) first to some four courses. Check alignments with a spirit level and do not go higher than five courses at any one time.

Doors and windows should be tied into the blockwork joints with galvanized-steel ties. Seal the timber end grain with resin glue to prevent moisture from penetrating.

Ensure that doors, whether of the swing or up-and-over kind, are supported by piers and that there is a stout timber lintel (4 in. or 100 mm) above the door opening. If you fix a heavy pair of doors, reinforce the fixing blockwork with 2 in. by 2 in. (50 by 50 mm) reinforced-steel angle bars, bedded at least 10 in. (or 250 mm) into the foundation concrete. The bars should finish about 2 in. (50 mm) below the top of the blockwork.

Prefabricated Garages – Garage Kits

  • Timber, aluminum and concrete panels and posts are the principal structural choices.
  • Kits can be bought to meet average requirements for both double and single garages. Kits usually come complete in all respects — lintels, jambs, doors and mechanisms, in addition to the basic garage.
  • Some makers of prefabricated kits do no more than deliver the components. Here it is first necessary to put down a firm and level concrete base — always important with kits, for if the site is not properly prepared the prefabricated sections may not fit together because of uneveness at the base.

Mark out the position of the prefabricated garage in chalk on the base to serve as a guide when assembling it.


When ordering a precast-concrete panel garage, usually supplied with an asbestos-sheet roof, inspect the panels on arrival to ensure there are no breakages. Stack them carefully, close to the assembly point, to ensure that there are no hitches in work progress or likelihood of damage to the panels.

Assembly of a prefabricated precast-concrete garage begins at the rear corner.

Sections generally bolt together with galvanized bolts, fitting through holes pre-drilled in the ends of each section, which may have reinforcing ribs. The bolts clamp the panels firmly together but should not initially be tightened as some adjustment may be needed during assembly. Usually a mastic sealing strip has to be inserted between the mating edges of panels before the bolts are finally tightened but, in some cases, an exterior mastic is used, usually from a special “gun”. Corners may be built up before the sides, followed by construction of the rear wall.

Window panels may come as optional extras with some makes; these are interchangeable with standard panels and provide scope for positioning windows where wanted.

Roofs of kits may be supplied cut to size and sometimes with a tile pattern instead of the traditional corrugated one. Eaves fitting pieces, consisting usually of shaped foam rubber or polystyrene, may be sup- plied to blank off the holes at the edges of the corrugated sheet. The roof sheets may be supported by premade purlins and trusses and fixed with drive screws or galvanized screws and plastic washers. Start roofing assembly at the front. Holes normally have to be drilled through the sheets, which have an emple overlap to prevent direct rain penetration or capillary action. Shaped ridge pieces fit over the roof apex and are similarly drilled and fixed. Fit all overlaps away from the direction of prevailing weather.

Doors. Some kits provide the option of side-hung or up-and-over garage doors.

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