What dimensions and lengths of underfloor heating to choose for an irregular room shape

Why an irregular room shape is the biggest challenge when choosing a heating mat

When a customer comes to the store or sends a query saying they have a bathroom or another room in an irregular shape, they usually assume the mat can simply be cut or sewn to size. This is the first and at the same time most dangerous misunderstanding I regularly encounter. An electric underfloor heating mat is not a textile or insulation foam – it is a precisely manufactured electric heating element with embedded resistive cables. If I cut the cable in the wrong place, the mat will not only stop working, but it may cause a short circuit or even a fire.

On the other hand, an irregular floor plan is more the rule than the exception in practice. Bathrooms with slanted walls due to a staircase above them, living rooms with niches, L-shaped hallways, bathrooms with built-in bathtubs or shower corners in the middle, kitchens with islands – all of these are situations I deal with every week. And one thing always holds true: a properly planned mat on paper saves time, money, and nerves on the construction site.

This article will guide you through the entire process – from measuring an irregular room, through selecting the right dimensions and lengths of the mat, to practical solutions for assembly and laying that work in real-life conditions. If you are unsure about basic terms such as power per m², type of floor, or the difference between a mat and a loose cable, I recommend reading the articles What power do I need for a bathroom or living room per square meter and Heating mat under tile vs. heating cable – which solution is better for your case, where these basics are explained in detail.

Basic principles: what can and cannot be done with a mat

Before we move on to actual measuring and calculations, I must emphasize several strict rules that apply without exception to all standard electric heating mats on the market, including HAKL products, which I work with most often.

• The mat must not be cut through the heating cable. The grid fabric (carrier fabric), in which the cable is sewn, can be cut – and this is exactly what makes laying in irregular spaces possible. I cut the fabric, the cable remains intact, and I change the direction of the cable.
• The mat must not overlap itself or another mat. Even if it would fit spatially, overlapping causes overheating, which damages the cable and may cause a fault in the entire circuit.
• The mat must not be installed under fixed furniture. We never place the mat under a bathtub, built-in shower, kitchen unit, or cabinets – not only because there is no need to heat there, but primarily because the heat would not be able to escape upwards and the cable would overheat.
• The minimum distance from the wall and fixtures is usually 5–10 cm. This value may vary according to the manufacturer, always check the specific product instructions.
• Power supply cables (cold end) are laid into a groove in the wall. Their position and length determine where the mat starts in the floor plan – this must be considered in the planning.

Once we know these rules, we open up space for creative solutions to adapt the mat to almost any floor plan.

How to accurately measure an irregular room

This is a step that most people underestimate, and then they ask why their mat does not work out. The measurement on paper must be accurate to the centimeter, not just "roughly enough". The procedure is simple, but it must be followed carefully.

Grid drawing – the foundation of successful planning

Take millimeter paper or a squared notebook and draw the floor plan of the room to scale 1:20 (1 cm on paper = 20 cm in reality). Mark all niches, columns, the position of the bathtub, toilet, shower, water heater, distribution box. Also mark the doors and their opening direction – there is usually no mat under the doors (it would hinder smooth opening and would wear out mechanically).

Then draw with a pencil where the thermostat with the temperature sensor will be placed (the sensor is laid into the floor between the cables). From the thermostat, draw a groove in the floor – the power supply (cold) lead of the mat will run through it. The location of the thermostat and the groove is important because it determines where the mat will start in the floor area.

On the drawing, then shade with a pink or other distinct pen the areas where the mat will not be – that is, under the bathtub, shower, under the furniture, 5–10 cm from the walls. What remains unshaded is your effective heated area. This is the area you must cover with the mat.

Measure twice – actual floor area vs. rough rectangle

Very common mistakes: the customer specifies room dimensions 3.0 × 2.5 m = 7.5 m². From this, they subtract the bathtub (1.7 × 0.75 m = 1.28 m²) and the toilet module (0.5 × 0.5 m = 0.25 m²). Result: 7.5 − 1.28 − 0.25 = 5.97 m². But watch out – the mat must physically go around these obstacles. It is not enough to subtract the area on paper if the mat cannot be laid there realistically due to the shape of the room. That is why the grid drawing is irreplaceable.

Dimensions of HAKL mats and their suitability for different situations

The HAKL mat range we offer is available in a width of 0.5 m and various lengths. This width of 50 cm is not random – it is a standardized dimension that allows flexible laying in narrow spaces and at the same time makes sense for the snake-like cable routing. Strips are laid next to each other, with the cable turning at the end of each strip.

In terms of length, we have the following variants available:

• HAKL mat 0.5 × 2 m – area 1.0 m², power approx. 150 W; ideal for small areas, e.g. in front of the sink, entrance stairs, niche
• HAKL mat 0.5 × 5 m – area 2.5 m², power approx. 375 W; suitable for small bathrooms, narrow hallways
• HAKL mat 0.5 × 6 m – area 3.0 m², power approx. 450 W; most commonly used for standard bathrooms
• HAKL mat 0.5 × 10 m – area 5.0 m², power approx. 750 W; for larger bathrooms, living rooms, combined spaces

A width of 0.5 m means that one "strip" of mat laid straight will cover a strip of 50 cm. When I need to cover a width of 1.5 m, I lay three strips next to each other (3 × 50 cm). The total length of the mat is divided into these strips. For example, a mat of 0.5 × 6 m will cover an area: 3 strips × 50 cm × 2 m per strip (room length) = 1.5 m × 2 m = 3 m². This is the basis for understanding how the length of the mat correlates with the room dimensions.

Irregular shapes – specific practical scenarios and solutions

Scenario 1: L-shaped room

An L-shaped room is probably the most common situation. A typical entrance bathroom of 3.0 × 1.8 m with a niche of 1.2 × 1.0 m for a sink in the corner. Total area: 3.0 × 1.8 − (3.0 − 1.2) × 1.0 = 5.4 − 1.8 = 3.6 m². From this we subtract the bathtub (0.75 × 1.7 m = 1.28 m²), distances from the walls (estimated 0.3 m²) – effective mat area is about 2.0 m².

Solution: We lay the mat as two sections. The first section covers the long part of the L, the second section covers the niche. We use one 0.5 × 10 m mat, which we run along the long part in the first pass, cut the mesh at the end and turn into the niche. The cable always runs in one continuous piece – this is key. If the power of one 0.5 × 10 m mat is too high, I can choose two smaller mats (e.g. 0.5 × 6 m + 0.5 × 2 m connected in two circuits with one thermostat or two, depending on the situation). More about connecting the thermostat can be found in the article How to set up and connect a digital thermostat to underfloor heating.

Scenario 2: Bathroom with a slanted wall (under the stairs)

This is a classic in family houses. The bathroom is triangular or trapezoidal, one wall runs diagonally. The mat (and cable) always run in straight lines – it cannot be forced to follow the slant. What to do about it?

Procedure: We lay the entire mat parallel to the wall opposite the slanted one. Where the slant "loses" space, the mat simply does not reach there – and that is fine. At the end of each strip, we cut the mesh, turn and continue. The resulting mat area will be smaller than the actual room area, which also corresponds to the fact that the slanted part is the lowest, people do not move there and thermal comfort is not a priority there.

Example calculation: trapezoidal bathroom, shorter side 1.2 m, longer side 2.8 m, room length 2.5 m. On average 2.0 m width on 2.5 m length = 5.0 m². The mat will cover about 3.5–4.0 m² (after subtracting corners, slant and equipment). Suitable choice: HAKL 0.5 × 10 m (covers 5 m²), but actually laid out on an area of 3.5–4 m². The remaining length is "used up" on turns and gaps near the walls – this is normal and correct.

Scenario 3: Entrance in the shape of a U or with a column

Entrances are often long and narrow, but sometimes they have a column in the middle or a technical shaft that protrudes. The mat cannot smoothly bypass the column – we must plan in advance. Solution: Divide the entrance into two "zones" in front of the column and behind the column. Each zone gets its own mat, or we use one mat, but plan its route so that it does not reach the column at all. A distance of 5 cm from the column (as from the wall) is mandatory.

Scenario 4: Mat in the kitchen near the kitchen line with a recess

Modern kitchens have recesses for the refrigerator, dishwasher or stove. The mat must never be there – not even in front of the refrigerator, nor under it. We lay it only in the clear passage. Typically it is a strip in front of the table or work surface, where people stand. Narrow strips (0.5 m wide) are an advantage here – they fit precisely into the corridor-like space in front of the line.

Calculation of the required mat length for an irregular floor plan

Now comes the practical math. I have a drawing of the effective area, I know where the strips will be, and I know how many strips (how many lengths of 0.5 m width) I need. How do I calculate the total mat length?

The procedure is as follows:

1. Measure the length of each strip from the drawing (in the direction of laying). Strips are not always the same length in irregular floor plans.
2. Add up the lengths of all the strips: L_total = L1 + L2 + L3 + ...
3. Add "losses" for turns. At each turn of the mat, a space equal to the width of the mat (50 cm) is consumed without generating any output – this is the area where the cable follows an arc. In practice, this loss in area is minimal, but you must include it in the total cable length if you are buying loose cable. With mats, this arc is already included in the delivered length, so it is enough to add up the lengths of the strips.
4. Compare the result with the available mat lengths (2, 5, 6, 10 m). Choose the next larger size.

Practical example: An L-shaped bathroom. Long part: 3 strips, each 2.2 m long = 6.6 m. Short part of the niche: 2 strips, each 1.0 m = 2.0 m. Total: 8.6 m. Available lengths: 10 m. I choose a 0.5 × 10 m mat. The remaining 1.4 m of mat length will naturally be "lost" by laying the strips with small predefined gaps or by cutting the mat a few centimeters shorter from the wall – this is not a problem.

Important: I must never shorten the mat to an arbitrary length. I can use a shorter dimension than available (e.g., buy a 10 m mat and unroll only as much as needed, without hiding the remaining part under the floor!). Therefore, the calculation must result in one of the available dimensions. If it does not come out "cleanly," I always choose the larger dimension and slightly increase the effective area, or use a combination of two smaller mats.

When it is better to use loose cable instead of a mat

We are talking about mats, but in truly complex floor plans – many corners, several niches, irregular polygonal shapes – loose heating cable may be a better option. I can lay the cable with any step and route shape, which gives me maximum flexibility. Its disadvantage is that installation is more laborious and requires more precise spacing to avoid overheating. A detailed comparison of both solutions can be found in the article Mat under tile vs. heating cable – which solution is better for your case.

General rule from practice: If the room shape is such that the mat covers at least 70–80% of the effective area without complicated cuts and turns, the mat is a faster and cheaper option. If I am forced to make more than 6–8 cuts in one mat, I start considering loose cable.

Practical tips during installation in an irregular space

Along with theoretical planning, there are a few practical things I want to highlight for every customer planning a DIY installation. The entire procedure is described in detail in the article Installation of electric floor heating mat step by step, but here I focus on the specifics of irregular spaces.

Draw the mat area on the floor before laying

Before unrolling the mat, draw the entire route of the mat on the clean concrete floor with a marker or chalk, according to your drawing. Indicate each strip with a line showing where it starts and ends, where the cut in the mesh is, and where it turns. You will save a lot of improvisation in real time. Believe me – when you have an unrolled mat in your hands, you are kneeling on a hard concrete floor, and trying to align it, all your nice plans go out the window.

Photodocumentation before tiling

This is an absolute must. Before the mat is glued with adhesive or fixed with glue, photograph every centimeter of the cable route from several angles. Save the photos to the cloud, to your phone, and to your email. If a fault occurs in a year or in five years and it is necessary to diagnose where the cable is exactly located, these photos are irreplaceable. More about faults and their diagnosis can be found in the article Common faults in electric floor heating and how to diagnose them.

Place the temperature sensor in the axis between two cable lines

In an irregular shape, it is sometimes difficult to find the right place for the sensor. Rule: the sensor must be in the middle between two adjacent cable lines (i.e., 25 cm from each line at a standard spacing of 50 cm), at least 50–80 cm from the wall, and never directly on a joint or a bend of the cable. In irregular rooms, it often happens that the ideal place for the sensor is in a corner or near a device – this is not suitable. In such a case, move the thermostat to another wall and extend its wiring.

Combination of multiple mats in one room

If the room area exceeds 5 m² (which is the maximum area of a 0.5 × 10 m mat), or if the shape is so irregular that one mat cannot cover the entire effective area, the solution is to combine two or more mats. Each mat must have its own power outlet and its own temperature sensor, or they can be connected to a common thermostat, provided that the total power does not exceed the capacity of the thermostat (usually 3,000–3,600 W).

When combining two mats in one room, the following applies: the distance between the edges of the individual mats must be at least 5 cm (to prevent one mat's cable from overheating near the cable of the other mat). Mats never overlap.

Example: A large bathroom 4.5 × 3.5 m with a bath, a shower area, and two sinks. Effective area: approx. 7–8 m². Solution: two 0.5 × 10 m mats (each 5 m² = 750 W), total power 1,500 W. Each mat covers one "zone" (e.g., near the bath + central area / area near the sinks + bathroom entrance). Both mats are connected to one dual-zone thermostat, which allows different settings depending on the need.

Most common mistakes when choosing the size of a heating mat – what I've seen in practice

Over the years of practice, I have encountered these recurring mistakes that have cost customers time and money:

• Purchasing a heating mat based on the total room area, not the effective area. A customer has a 6 m² bathroom and buys a 5 m² mat, only to later discover that 1 m of length remains, which must be rolled up – and that is not allowed.
• Forgetting about fixed built-in equipment. A masonry shower enclosure, a fixed cabinet, or furniture – these are areas under which the mat must not go, yet customers often include them in the calculation.
• Ignoring the direction of laying the long side of the mat. The mat has a fixed width of 0.5 m and a variable length. Mats should be laid along the long side of the room, not across. If the customer gets this wrong, the mats are too short and the mat won't fit at all.
• Too many cuts in the mesh without planning the route. If the mat is cut too often and at random places, the shape becomes unstable and unintentional cable shifting may occur.
• Purchasing a mat without verifying the availability of 230V power nearby. The power supply must be connected to an electrical outlet or directly to the distribution board via a circuit breaker – if this is not pre-arranged with the electrician, complications may arise. In bathrooms, the strict requirements of the ČSN/STN 33 2000-7-701 standard on bathroom zones apply.

Related accessories: What else you need besides the mat

The mat alone is not enough. During installation, you will need a thermostat with a temperature sensor, mounting tape to fix the mat (some types have a self-adhesive side), suitable adhesive or tile adhesive for the floor compatible with electric heating (flexible and with good thermal conductivity), gaskets, and possibly connectors for other plumbing in the bathroom.

If you are also addressing other heating systems during the bathroom renovation, for example, connecting circuits or connections, you will appreciate that the same category also includes brass threaded connectors and other heating accessories – gaskets, reducers, elbows. More on choosing the right gaskets and connectors can be found in the article Threaded connectors and gaskets in heating – how to choose the right type and size.

When choosing a thermostat, remember that a mat with a power of up to 3,000 W can be connected to a standard single-phase circuit with a thermostat. If you are combining multiple mats, consult an electrician about the electrical installation. The entire issue of underfloor heating in the bathroom – including zones, insulation, and regulatory requirements – is discussed in the article Underfloor heating in the bathroom – what to watch out for when installing under tiles.

Table: Approximate choice of mat according to effective area

Effective area	Recommended mat	Power (150 W/m²)	Typical use
0.5 – 1.2 m²	0.5 × 2 m	150 W	In front of the sink, small niche, WC
1.5 – 2.8 m²	0.5 × 5 m	375 W	Small bathroom, narrow entrance
2.5 – 3.5 m²	0.5 × 6 m	450 W	Standard bathroom with a bath, entrance
4.0 – 5.5 m²	0.5 × 10 m	750 W	Larger bathroom, living room, kitchen
6.0 – 10 m²	2× 0.5 × 10 m or 0.5 × 10 m + 0.5 × 6 m	1,200 – 1,500 W	Large bathroom, combined room

Most frequently asked questions (FAQ)

Can I cut the mesh to adapt the mat to any room shape?

Almost yes, but with limitations. Cutting the mesh changes the direction of the cable laying, so you can create an L, U, or S shape. What is not possible: copying diagonal edges, circular or curved shapes. The mat always goes in straight lines. For truly complicated floor plans (many curves, irregular polygons), a free heating cable is a better option, as you can lay it with any step and direction without the limitation of the mesh.

What happens if I accidentally cut the heating cable while cutting the mesh?

Cutting the cable means the mat will stop working – the electrical circuit is broken. A broken cable in a standard mat construction cannot be repaired by yourself and usually means replacing the entire mat. Therefore, always cut slowly with sharp scissors at a precisely marked location, and move the cable away from the cutting line before cutting. Test the mat with an ohmmeter before installation – the resistance must match the value in the instructions.

Can I use one mat for two separate areas (e.g., an area in front of the bath and an area in front of the sink separated by a shower enclosure)?

Yes, if the cable route is physically possible – i.e., if the mat can be bent to bypass the shower enclosure without the cable having to pass through a forbidden area (under the shower enclosure). In practice, this usually works if the shower enclosure is against a wall and the mat can go around it along the wall. If the shower enclosure is in the middle of the room or if the route would be too long and complicated, a cleaner solution is to use two separate mats, each for its own zone.

What thickness should I account for with the mat when planning the floor height?

The mat itself has a thickness of about 3–4 mm. Add a layer of flexible adhesive (minimum 3–5 mm over the mat) and the thickness of the tiles (usually 8–12 mm). Overall, plan for an increase in floor height of 15–22 mm compared to the original concrete layer. This must be considered when planning door thresholds, the height of the WC, bidets, and other built-in equipment. In bathrooms with old wooden floors (not concrete), an additional preparatory layer is needed – this is a topic for a separate consultation.

Is a power of 150 W/m² always correct for irregular areas?

Not always. When the mat covers only part of the floor (e.g., 40–50% of the room area due to fixed equipment), you must calculate a higher specific power per effective area, as the entire room's heat loss must be covered by this smaller area. A typical recommendation for bathrooms with a mat as supplementary heating is 100–150 W/m² of effective area. If the mat is to serve as the primary heat source, calculate 150–200 W/m² of effective area. More in the article What power of mat do I need per square meter of bathroom or living room.

Can I secure cut areas of the mesh with adhesive or tape?

Yes, and it is even recommended. Along the cut edges of the mesh, you can use glass mounting tape or special fixing tape, which holds the mesh in the correct position during the application of the adhesive. Some meshes have a self-adhesive side that partially fulfills this function. It is important that the cable is not compressed anywhere or folded over itself – each such spot is a potential hot spot.

Conclusion

Planning a mesh layout for an irregular room shape is not rocket science, but it does require a systematic approach, paper and pencil (or a good drawing program), and enough patience before purchasing anything. The basic rules are fixed: cut the mesh, not the cable; meshes do not overlap; devices are excluded areas. Everything else is a matter of thoughtful routing and choosing the correct length.

If you are unsure about the calculations, the drawn plan, or the selection of a specific product – write to us or call before placing an order. It is much cheaper to correct the plan on paper than to change the mesh after the tiles have been laid. The HAKL mesh range in widths of 0.5 m and lengths of 2, 5, 6 and 10 m covers the vast majority of common situations – the only important thing is to calculate correctly which one to choose.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.

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