Demising Wall

All about Demising Wall

The Importance of a Demising wall well built:

First we will explain how we arrived to use the term Demising Wall and the root source of the term. 

So you will learn the etymology of the word, then we will discuss what a demising wall really is. 

Please note that the term is “Demising wall”, not “demise wall”, like many people have erroneously mentioned on the comments before.

You can skip to “definition” if you need to know what it is right now.

If you prefer, you can also Skip to “Who builds a demising wall”, if you want to know how we can help you.

Finally, you can skip to the “Demising Wall Special Report” below if you are interested in the content related to the example report written by our team.

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In this page you will find these things:

Watch a demising wall video instead of reading:

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demising wall basic answers:

What is a demising wall?

What is a demising wall in a building?

What is a demising wall in construction in general?

Demising wall - Simple Explanation:

A demising wall is an element of a construction that divides two spaces within a property that is under only one ownership but is intended to be leased separately. 

Its a term used mostly in construction and turns out to be mentioned many times in legal procedures, specially because when its not built correctly, the issue becomes a legal battle.

Demising wall - Etymology:

The term demising derives from the Latin word dimittere, and dīmissum (past participle of dīmittere), that means to send away, to dismiss. 

With that usage of the word, later the Old French started using it differently and it became demis (past participle of desmetre), which created the use on our later version demising, as we know it today.

Demising wall - Definition:

The term demising, refers to the division in a lease situations. There is where the property has to be divided in two or more partitions, to be sub-leased to different tenants that in turn, depending of the use, tenants have to have certain privacy and safety elements.

Therefore, these walls have to conform with a minimum amount of resistance to impacts, special constructive elements, and most importantly, features that are mainly physical devices implemented as a critical noise barrier.

For an external link for a website that specializes in definitions, Click here.

Who Builds a Demising wall or a partition Wall?

The Ultimate crew can Build demising walls and partition walls all over Los Angeles. 

In addition, we can help you with your demising wall and partitions even if compliance requires only a piece of paper to get it approved. 

Sometimes an assessment of what has been built and how it has been built will answer all questions about the demising wall in question.

In short, we can write a report of how the wall needs to be built and you can handle the construction yourself.

But we have to point out that we can help you with Reports, Plans and Designs that will address noise issues and safety issues.

Given these points, our professionals can also help with considerations and assessments, that you or your landlord needs to implement. in order for you or your tenants to have complete enjoyment of the premises, without disturbances of neighbors or danger of a break in though an improperly built wall.

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Preface of a long story:

The following text is a report that we wrote, paid by a tenant, living on a Penthouse that was divided in two smaller units by the Landlord.

Since the wall was built, the tenant always wondered if wall was built up-to-code, specially because she could not sleep with the noise from the adjacent unit.

When the tenant contacted us, she said that she had done all she could do, to have this problem remedied. But the landlord refused to help.


She contacted the department of building and safety, and Housing Department requesting help to resolve this issue.

The housing Department came in and noted issues with smoke detectors and hand rails that were promptly addressed. But the noise was still unbearable to the resident.

The Department of building and safety had said that the only reason they would send someone to investigate, was if she could verify that the wall in question, was not built according to the current local building codes. So she reached out to us and we wrote the following report:


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Demising wall inspection Report:

1. Aim and intent of this report:

This report was requested by Ms. XXXXXXXXX XXXX in an attempt to determine if a wall in between her residence and the neighboring, which is also the subject of this report, is in fact a demise wall, following all basic standards for such use.

1.1 Demising wall, accepted definition related to the intent of this report:

A demising wall is a wall that separates spaces belonging to different tenants, as well as separating private tenant areas from common areas. 

Such walls may be treated separately in the building code to address concerns about energy efficiency, noise pollution, and tenant safety.

A simple example of a demising wall can be found in any apartment complex, where the shared wall between two apartments represents a separation between areas that belong to separate tenants.

The subject of this report, as it was indicated to us, is a wall that was built after the building had received it’s Certificate of Occupancy, to divide the original penthouse unit into two smaller apartments.

In the subject area the building code requires insulation in a demising wall for a number of reasons. 

Insulation can keep down noise and minimize complaints about noise from shared walls. In addition, it can increase energy efficiency. 

Each tenant can control the temperature and other conditions in his own space, without worrying about paying for conditioning in the neighboring space.


1.1.1 important intent of the demising wall in question:

This can also be important when a demising wall separates a usually hot area from another region, like a shared wall between a kitchen and a private bedroom, which is the case in the subject wall.

Such walls also need a fire rating, to prevent fires from spreading between tenant areas. Walls and doors with fire resistance are an important part of construction to control fires in the event they break out.

A demising wall with a high rating between two spaces can limit liability issues; if one tenant’s carelessness causes a fire, for example, the neighboring tenant’s space will be safe behind a fire wall; therefore, the concerns of the resident requesting this report is well founded.

2. location of the subject wall:

The wall in question is located in between the units 501 and 502 at the penthouse of the building located at XXX North XXXX Drive, in the City of Beverly Hills, CA.

Samples were removed from the bathroom wall adjacent, and between, the toilet and the shower. See attachment “A” for a Floor plan.

3. Sample Collection

3.1 Methodology

With authorization of the resident, and with a signed agreement stating that the resident is responsible for restoring the wall to it’s original condition, we removed a square of approximately 1.5” by 1.5” from a location at the subject wall.

Before we removed any samples, we also used a secure method of determining the identity of the person requesting this report. 

In order to positively determine the identity and to validate the signed paperwork and the responsibility of restoring to its original condition any damage caused by the samples to be taken, we collected a payment on Ms. XXXX XXXXXs’ valid credit card.

Only after payment was authorized by the credit card company, is that we proceeded with the collection of a sample.

The location of sample removal was chosen by the resident, for the purpose of determining the make up of the materials used to build the wall.

We removed a sample layer from the subject wall, from only one of the sides, the side for which we had physical access and authorization to remove a sample.

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3.2 Materials Collected

The sample removed included the following layers:

(1) Paint Layers ,

(2) Plaster Layer,

(3) Drywall Layer and,

(4) Insulation Layer

3.2.1 Materials Physical Properties

The samples removed have the following properties:

1) Paint Layers:  Less than 1mm of brittle, thin, solid layer with multiple coats of paint.

2) Plaster Layer: Typical, plaster application of a mix of sand and Plaster of Paris, with thickness of 3/8” of materials at the sampled area.

3) Drywall Layer: Typical 3/8” gypsum board

4) Insulation Layer: Brown Fiberglass

4.Fire Rating

Fire rating is the term used to specify and compare the resistance of certain materials against fire. Fire rating is used to determine what materials should be used to resist and/or block, and/or deter the spreading of fire for a determined amount of time.

For the purposes of this report, fire rating is used to determine the appropriate fire rating of materials used to build a demise wall. For this purpose, the ideal material, which is also required by current local code, is type X Gypsum wallboard, 5/8” in thickness.

4.1 Fire rating video from X-Plan-Nation YouTube Channel:

4.2 Fire rating data

Type X gypsum wallboard, 5/8″ in thickness (“5/8″Type X wallboard”), is manufactured for use as one component of an assembly/system (such as a wall) where a fire resistance rating is required in a residential, commercial, or other structure by an applicable building code.

5/8″Type X wallboard must be manufactured in accordance with established ASTM standards defining Type X wallboard as that which provides not less than one-hour fire resistance when tested in specified building assemblies/systems in a laboratory setting under certain controlled conditions and pursuant to certain ASTM procedures.

Because ASTM procedures require that fire tests be conducted on complete building assemblies/systems and not just on the wallboard by itself, the ability of a particular 5/8″Type X wallboard product to pass a specific ASTM fire test may well depend on factors other than the fire resistance of the wallboard being tested. These factors include the other components used to construct the building system being tested, the manner in which the system is constructed, and the inherent variability of ASTM fire tests.

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4.3 Fire rating conditions:

Independent third-party organizations, such as Underwriters Laboratories Inc. (UL), may authorize manufacturers to certify or label their 5/8″Type X wallboard as acceptable for use in one-hour fire-rated systems based on criteria established by the third-party organizations. The third-party organizations may approve changes to certified formulations using criteria they believe appropriate, such as new full-scale ASTM fire tests, small-scale fire tests, or engineering studies and evaluations, and without requiring a full-scale ASTM one-hour fire test. Accordingly, the fact that a particular 5/8″Type X wallboard product has been certified as acceptable for use in one-hour fire-rated systems by a third-party organization does not necessarily mean that wallboard made according to that specific formulation has been subjected to a full-scale ASTM one hour fire test.

Given the very different circumstances that may exist from one fire to another, the differences between conditions in an actual fire and the laboratory conditions in which a test is conducted, and the inherent variability of ASTM fire tests, passing an ASTM fire test in a controlled laboratory setting or certifying or labeling of 5/8″Type X wallboard as acceptable for use in one-hour fire-rated assemblies/systems under third-party certification or labeling procedures does not mean that either a particular assembly/system incorporating 5/8″Type X wallboard or any given piece of 5/8″Type X wallboard will necessarily provide “one-hour fire protection” in an actual fire or last for an hour in a new laboratory fire test.

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4.4 Fire rating disclaimers:

Even if 5/8″Type X wallboard is referred to using terms like “one-hour board” or “has a one-hour fire rating,” this does not mean that either a particular assembly/system incorporating 5/8″Type X wallboard or any given piece of 5/8″Type X wallboard will necessarily last for an hour in a new laboratory fire test or provide “one hour fire protection” in an actual fire.

Once a 5/8″Type X wallboard formula has been certified by a third-party organization, the company using that formula to manufacture 5/8″Type X wallboard does not have to conduct periodic fire tests on that wallboard as long as the company follows the procedures established by that third-party organization to ensure that the wallboard is manufactured in compliance with the certified formula.

4.5 Fire rating Background and discussion:

5/8″ Type X gypsum wallboard, 5/8″ in thickness, is currently manufactured in accordance with ASTM Standard C 1396. One section of ASTM C 1396 requires Type X gypsum wallboard to meet specific requirements when tested using the test method defined in an ASTM Standard, the E 119 test, Standard Test Method for Fire Tests of Building Construction and Materials.

Additionally, all gypsum wallboard manufacturers in the United States subscribe to an independent third-party certification and labeling service such as UL to ensure product consistency and quality.

ASTM E 119 was initially published in 1918 (as ASTM C 19) and has been used continuously since then as the principal method to test the fire-resistance of construction assemblies/systems. (Similar protocols from other standards groups or laboratories were developed later and are used in some cases.)

Refinements have been made in the standard since 1918; however, several of its test criteria, including the conditions required to “pass” an individual fire test, have remained essentially unchanged.

fire rate drywall

4.6 Fire rating furnace testing:

The test method described in the ASTM E 119 standard evaluates (in terms of endurance time) “the ability of an assembly/system to contain a fire or to retain its structural integrity, or both, during the test conditions imposed by the standard.”

By the standard’s own definition, ASTM E 119 does not assess individual materials or products for their fire-resistance characteristics. ASTM E 119 does, however, generate results that can be used to evaluate the general fire resistance of assemblies/systems made up of multiple components, such as walls, columns, slabs, and floor- and roof-ceiling systems under laboratory conditions.

The ASTM E 119 standard does not contain specific details for construction of the test furnace.

Since test furnaces are subject to variation due to individual characteristics of construction and design, including ventilation, atmospheric conditions, and general thermal tendencies, test results are typically not fully repeatable or reproducible from one laboratory to another.

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4.7 Fire rating furnace results disclaimers:

Test results attained in an E 119 test are not precise predictors of future performance.

Additionally, differences in assembly/system components and construction methods, the design and control features of individual furnaces, and other variables regarding the testing regimen, can cause wide fluctuations in ASTM E 119 test results.

A fire test, therefore, is a snapshot of a single assembly/system at a given time that includes the measurement of the performance of a specific assembly/system, composed of specific materials, constructed in a specific test furnace, on a specific day.

This simply means that for a “one-hour fire rating” of a gypsum board assembly/system, all requirements of an ASTM E 119 test were successfully met in a testing laboratory furnace for at least 59 minutes and 30 seconds for that specific assembly/system and with those specific components of the assembly/system.

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4.8 Fire rating comparative standard:

The ASTM E 119 test method does not incorporate all dynamics essential for fire hazard analysis or fire risk assessment of the assemblies/systems under conditions in an actual fire situation. The results of an ASTM E 119 test, therefore, should be regarded as one component among a variety of factors used to assess the potential of a system to perform as part of a structure.

Model building codes reference the ASTM E 119 standard test method (or one of its closely allied counterparts). Gypsum board systems are tested based on the requirements of the ASTM E 119 standard. Referencing the same test method(s) by the building codes facilitates the descriptions and comparisons of fire resistance ratings of assemblies/systems that have been objectively evaluated.

Numerical fire resistance ratings created by ASTM E 119 tests may be considered as benchmarks for comparison purposes. The higher the numerical rating (i.e., one, two, three, or four hour(s)), the longer the assembly’s/system’s comparative endurance.

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4.9 Fire rating further disclaimers of methods:

The hourly fire resistance ratings found in commercial and residential building codes that refer to an ASTM E 119 test do not imply that a specific assembly/system will remain intact for the prescribed time of the hourly rating in an actual fire situation.

Fire resistance classifications are based on results of tests conducted on assemblies/systems created with specific materials and built in a specified manner; therefore, variations from the test conditions or the construction specifications (including, but not limited to, the type and size of materials and the method of construction) will affect the results of fire tests.

Because fire exposure conditions vary with changes in a wide variety of factors, including the amount, nature, and distribution of available fuel; ventilation; and the size, configuration, and other characteristics of the compartment, the test method contained in the ASTM E 119 standard should not be considered to be representative of all fire conditions.

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4.10 Fire rating more info directly from Underwriters Laboratories:

Fire resistance ratings created through use of the ASTM E 119 test method reflect a relative measure of comparative assembly’s/system’s performance under specific fire test conditions. ASTM E 119 test results should not be construed as having determined performance of an assembly/system under different conditions.

To maintain industry-wide quality assurance standards for 5/8″Type X gypsum wallboard, the Gypsum Association requires that all member companies relying on the generic assemblies/systems contained in the Gypsum Association’s Fire Resistance Design Manual subscribe to an on-going, third-party, in-plant product inspection and labeling service.

This objective certification and labeling process ensures that manufacturers continue to manufacture the same quality of product as that originally tested. For more information on Underwriters Laboratories testing, certification, follow-up, and labeling procedures, visit its Web site at


Please note:

There are systems that have third-party-approved ratings for fire resistance which may utilize gypsum wallboard other than Type X or which may use more than one layer of Type X along with other materials.

These ratings can vary from 30 minutes to 4 hours, and like the one-hour rating utilizing 5/8” Type X gypsum wallboard are based upon standard ASTM test methods for determining system fire resistance.

The third party approvals for such other systems, like the one-hour systems using Type X gypsum wallboard, may be based upon full-scale ASTM fire tests, small-scale fire tests, or engineering studies and evaluations.

While this article on the subject of Fire and Safety Information deals expressly with 5/8” Type X gypsum wallboard, the principles discussed may also be relied upon to help understand the meaning of the fire resistance ratings of these other systems.

For further information, see the current edition of the National Gypsum Construction Guide or call 1-800-NATIONAL®


4.11 fire rating determinations of samples- for demising walls:

1) Paint Layers: The fire rating for the paint layers is negligible for the aim and intent of this report.

2) Plaster Layer: Typical, plaster application, a mix or sand and Plaster of Paris, with thickness of 3/8” of materials at the sampled area. Plaster does not have the certified properties of type X 5/8” sheet-rock as described in item 4 above and application practices have been long discontinued for its fire rating is dependent of materials used during its application and this cannot be determined by an inspection as the one performed here.

3) Drywall Layer: Typical 3/8” gypsum board, single layered. Does not comply to min of 1 Hour fire rating necessary for the intended use.

4) Insulation Layer: We collected a small sample of brown fiberglass and we could notice empty pockets of missing materials on the area probed for sampling. For the intent of this report, we noticed that fiberglass insulation is present.

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5. Inspection Recommendations:

The sample removed, proved that the wall in question does not have a Type X 5/8” sheet-rock as recommended by current local building code for demising walls. The 5/8-inch, “fire-code” drywall (called Type X) increases a wall’s fire rating to a minimum of 1 hour, from the 30-minute rating from the standard ½-inch drywall, and the subject wall has only a 3/8” drywall and 3/8” plaster, but it’s not just thickness that makes the difference.

The recommended Type X has a denser core and contains glass fibers that keep it from crumbling in the heat. We assume that weather for lack of knowledge or because Type X is slightly more expensive (about 75 to 90 cents more per sheet at the time that this article was written) it was not used in the subject wall.

There is a possibility that the subject wall was built before more stringent code came about. We recommend an intrusive inspection by removing all the layers of gypsum board on the subject areas and installation of up-to-code type X, 5/8” gypsum boards because the local building code requires it on all walls separating different units in multi-family dwellings.

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Demising walls - continuation of inspection recommendations:

I’d also recommend a more intrusive inspection on the other items that are required by code to be fire-rated in the assemblies that were built after the original certificate of occupancy was issued.

Items that require a close look for compliance, include but are not limited to, doors to close off all the other pathways for fire to travel, as per doorways that were created to separate one penthouse into two units, possible exterior walls without fire blocking and other code-related issues that the subject wall, according to our sample, shows that it does not follow current local codes, this could be pointing to other serious building violations on the other assemblies built after the original certificate of occupancy was issued.

We recommend, at minimum, the installation of Type X sheetrock and fiberglass R-19 insulation, not only because it is the minimum required current code, but it is the minimum improvement to reduce the noise transmission that was another reason why this report was requested. It was pointed out by the resident that noises could be heard and conversations could be clearly understood in between the walls in question; therefore, the recommended improvement would also help in the quality of life of the residents of both sides of the wall in question.

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6. Sources:

This demising wall report relied on the following information:

National Gypsum resources – website:

7. Attachments

Demising walls location graphic representations:

Attachment “A” – Floor plans of subject area

8. qualifications of inspector and producer of this report:

XXXXX XXXXXXX is a General Contractor since 1996, and a member of the California State License Board number XXXXXX and Master Builders Association since 2004 – member number XXXXX. 

Mr. XXXXXXX has been practicing his profession actively in the Southern California region for the entire time that his license is in effect. 

He is able to collect samples and determine even under oat the extent that the materials collected are, or are not, complying with current code.

His qualifications for this report are not as important as to determining that the actual codes and minimum requirements for the assemblies in question, for which, Mr. XXXXXXXXX  is simply indicating the materials collected do not conform to current standard building practices and can be corroborated by anyone with the same building knowledge, as building officials and other building contractors.

Samples will be retained for one year from the date on this report and will be disposed after this period has passed.

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