Cutting chases for gas pipes in concrete

A concealed gas pipe looks tidy and does not get in the way when you plan the kitchen. Concrete, however, is not a material you can cut into anywhere you like. Before you decide on a chase, ask yourself a few questions.

• Is the wall load-bearing? In cast-in-place concrete buildings and in blocks of flats built from precast panels, concrete walls are almost always part of the structure. A chase reduces the wall's cross-section and can hit the reinforcement.
• Is it a wall or a ceiling? Cutting into floor and ceiling slabs, especially prestressed planks, is generally not allowed. The steel in them is under tension, and damaging it is a serious structural problem.
• Does the route run horizontally? Horizontal chases weaken a load-bearing wall more than vertical ones. Long horizontal chases in concrete should only be cut after a structural engineer has assessed them.
• Would a surface run do the job? A gas pipe can often run on the wall surface, behind the kitchen units, inside a plasterboard service wall or in a cover trim. In concrete buildings this is often the more sensible choice.

In practice it often happens that the customer wants the chase exactly where the kitchen designer drew it, and only on site does it turn out to cross a bracing wall of the block. The route should therefore be decided by whoever designs the gas installation and, for load-bearing walls, by a structural engineer, not by the crew holding the wall chaser.

Marking out the route

The route is transferred from the gas installation drawing onto the wall with a pencil or chalk line. Gas chases normally run vertically and horizontally, never diagonally, so that years later it is still obvious where the pipe sits in the wall. The width is marked to suit the pipe diameter, with allowance for any protective wrapping or sleeve and for the filling.

Checking what is inside the wall

Before the first cut, the wall is scanned with a detector for metal and live cables. The crew looks for reinforcement, electrical cables, water and heating pipes and, above all, any existing gas pipe. If there is an old gas pipe in the wall, a qualified, registered gas engineer must safely disconnect it and confirm it is isolated before work starts. Nobody should ever cut or chisel into a wall that holds a pipe with gas in it.

Cutting the edges with a wall chaser

The edges are cut with a wall chaser fitted with two diamond blades, which make both parallel cuts in one pass. For concrete, blades designed for hard materials are used, not general-purpose blades meant for brick. The chaser is connected to an industrial vacuum with a filter class suitable for mineral dust. In hard concrete the cuts are often made in two or three passes, so the blade does not overheat and the machine is not overloaded.

Breaking out the core

The concrete between the cuts is removed with an SDS breaker or rotary hammer fitted with a flat chisel or a special channel chisel. The work moves along in short sections so the edges do not break off and cracks do not spread into the surrounding wall. Chiselling by hand without cutting the edges first is not used in concrete: the edges end up ragged and the chase far wider than needed.

Hitting reinforcement

If steel bars appear during cutting, work stops at that spot. The reinforcement is neither cut nor bent. Either the route is moved, or the chase is made shallower there and the pipe is run differently. The decision belongs to the structural engineer or the designer, not to someone with an angle grinder.

Trimming and cleaning

The bottom of the chase is levelled so the pipe lies straight and is never forced into a bend. The chase is blown or vacuumed out, and loose fragments and dust are removed. Finally the depth is checked along the whole length. The chase is then ready to be handed over to the gas engineer.

The price list has a separate item for cutting gas chases in brick. At first glance it is the same job, but apart from the final shape of the chase almost everything is different.

• Hardness and speed: work in concrete is noticeably slower, often several times slower than in fired brick or aerated blocks. Blades wear out faster and the breaker has to be more powerful.
• Reinforcement: brick masonry usually has none, concrete almost always does. Scanning is therefore a mandatory step in concrete, and the route is adjusted to what it shows.
• Structure: a chase in a brick partition is usually unproblematic, while in a load-bearing concrete wall or panel it is an intervention in the structure that may need a structural engineer's assessment.
• Edges: hollow clay blocks break out easily, and the chase grows wider. Concrete holds its edges better, but careless chiselling can crack it deep into the wall.
• Dust and noise: concrete dust contains more fine silica, and cutting is louder. Extraction and a proper respirator are a necessity with concrete, not just good advice.
• Equipment: brick needs a standard chaser and a lighter hammer. Concrete needs diamond blades for hard materials, a heavier breaker and sometimes wet cutting.

If your home combines both, for example a concrete load-bearing wall and a brick partition, the estimate is split according to the actual material of each section. It is worth finding this out early: if the guess is wrong, the concrete work drags on and the schedule of the other trades slips.

Cutting the chase itself is not a DIY job: concrete walls are often structural, the tools are heavy and the dust is harmful. What you can do yourself is prepare properly, because most delays on site are caused not by the concrete but by things that were not ready. Before the steps below, make sure the gas installation design gives the route, pipe diameter and material, otherwise the crew cannot know how wide and deep the chase should be. In a block of flats you need consent from the building management or owners' association, and for load-bearing walls usually a structural engineer's opinion as well. Check with your local building authority whether the work needs to be notified or approved under local building regulations, and do not simply assume it does not.

1. Collect the gas design and any old drawings or photos of earlier renovations showing where cables and pipes run.
2. Ask the gas engineer to isolate and make safe any existing gas pipe in the room before the cutting day.
3. Move the furniture out completely and cover the floor with protective sheeting or boards.
4. Seal doors to other rooms with plastic sheeting and tape, including the gaps at the bottom, because fine concrete dust gets through very small openings.
5. Check that a working socket is available on a circuit that does not trip when a large machine starts up.
6. Agree who will carry out and dispose of the rubble, and where a skip or bags can stand.
7. Read the house rules of the building and note the permitted hours for noisy work.
8. Tell your neighbours the date in advance, since cutting concrete is one of the loudest jobs in any renovation.

There is one rate per linear metre, but individual walls vary a lot. These factors decide whether the work goes smoothly or slowly.

Things that make it easier

• Vertical chases on an open, easily accessible wall
• Concrete without dense reinforcement in the chase zone
• An empty room with no furniture and no finished surfaces
• A clear design and a route marked before the crew arrives
• Longer continuous runs instead of many short pieces

Things that make it harder

• Very strong, old or high-strength concrete that wears blades down quickly
• Dense reinforcement that forces route changes and piecemeal cutting
• Chases high up near the ceiling, where the crew works overhead
• Many corners, branches and short sections
• An occupied home where every room must be thoroughly protected from dust
• Restricted working hours in a block of flats
• Existing tiles that have to be removed first

If the wall has old tiles or thick plaster, removing them is usually a separate job. Ask whether it is included or priced as another item.

A gas chase is not a finished job on its own. It is an intermediate step that has to match what comes before and after it.

Before cutting

The gas designer sets the route, the structural engineer assesses any work on a load-bearing wall, and the electrician should know where their cables will go. A gas pipe must not share a chase with electrical cables, and a minimum separation is kept between them according to the relevant EN standards and gas installation rules. The designer specifies the actual distances.

Laying the pipe

Only a qualified, registered gas engineer may lay the pipe in the chase. Pipes in walls are run without demountable joints, and valves and fittings stay accessible. Depending on the pipe material, corrosion protection or a sleeve is used, and the pipe must be able to move slightly as temperatures change.

Pressure test before filling

The chase must not be filled before the gas engineer has carried out a tightness test and the pipe is confirmed sound. This is where site mistakes happen most often: the plasterer has a deadline, the gas engineer has not come yet, and the chase gets filled. Later it has to be opened again for the test and inspection.

Filling and finishes

The chase is filled with mortar or another material recommended by the gas engineer for the pipe type. Gypsum-based products are generally avoided around gas pipes because they can encourage corrosion of metal pipes. Only after filling do plastering, tiling and kitchen fitting follow. Take photos of the chase with the pipe in place and a tape measure held against the wall. Years later, when you drill, you will be glad you did.

• Chase too shallow: the pipe sits almost at the surface, the plaster over it is thin and cracks. Later a hairline crack appears exactly along the pipe.
• Cut reinforcement: nothing is visible at first, but the load-bearing wall is weakened. In panel blocks this can be a problem for the whole building, not just your flat.
• Chase in a prestressed slab: a serious structural intervention that cannot easily be put right.
• Diagonal chases: after a few years nobody knows where the pipe runs, and there is a real risk of drilling into it.
• Ragged edges: a chase broken out without pre-cut edges is wide and uneven, needs a lot of filler and the filling often sinks.
• Filling before the test: the chase has to be reopened, and you pay for labour and materials twice.
• Gas and electrics too close: insufficient separation can cause problems at inspection and is a safety risk.
• Poor dust protection: concrete dust throughout the flat, in neighbouring flats and in shared areas. It is not only about cleaning: fine silica dust irritates the lungs.

If you smell gas during the work or at any time later, do not operate switches or use naked flames, open the windows, leave the building and call the local gas emergency number or 112.

The pace in concrete depends on its strength, the reinforcement, access and the number of corners. As a rough guide, a crew can usually cut a few metres of chase in a kitchen within one working day, while hard reinforced concrete or overhead work can take longer. Cutting itself needs no drying time, but allow time for cleaning up, for the gas engineer's visit and the tightness test, and only then for filling. Mortar in a chase usually needs a few days before it can be plastered over; the manufacturer states the exact time.

What to look at when checking

• Route: matches the design, chases are vertical and horizontal, with no unnecessary curves.
• Depth: even along the whole length, the pipe fits with its protection and there is room left for filling.
• Bottom: flat, with no lumps the pipe would rest on at single points.
• Edges: clean, without cracks running into the wall.
• Reinforcement: nowhere cut or bent. If bars are exposed, the designer must know about it.
• Cleanliness: the chase is vacuumed, free of dust and loose pieces.

A simple straight batten or spirit level held against the wall helps a lot. You will see straight away whether the depth changes anywhere.

• The chase route matches the gas installation design
• Chases run vertically and horizontally, with no diagonal sections
• Depth and width are sufficient for the pipe and its filling
• Edges are straight and show no cracks into the surrounding wall
• Reinforcement is not cut, and any exposed bars are recorded and discussed with the designer
• There are no electrical cables or other services in the chase
• The chase is clean and ready for the pipe
• Rubble has been removed and the room tidied
• Adjacent rooms and shared areas are free of dust
• The date of the gas engineer's visit and tightness test is agreed
• The chase will not be filled until the pipe has been tested and approved