Flat Roofs - Part 2 - Built up Felt - Problems to look for

A well constructed and properly maintained flat roof should remain in serviceable use for approximately 15 – 20 years. Life expectancy can vary depending on a number of different factors however if not regularly inspected and maintained there are numerous problems that can occur

In my last article I provided a brief summary of typical UK flat roof construction together with an explanation of different roof coverings.  As a result of the design/form of a flat roof they are often more vulnerable than pitched (angled) roofs to problems due to their reduced ability to discharge rain and other surface water adequately. A well constructed and properly maintained flat roof should remain in serviceable use for approximately 15 – 20 years. Life expectancy can vary depending on a number of different factors however if not regularly inspected and maintained there are numerous problems that can occur.  Below I will discuss a number of typical problems that can be found when inspecting a flat roof and how these can be identified.

You do not need to be a surveyor or have any detailed knowledge of property or construction to realise that there are clearly issues with the flat roof shown in the images below.  Some of these images show us the extent that people will sometimes go to carry out temporary ‘repairs’, when it is clear that there is a much bigger problem. 

Source: http://grandvalleyservices.ca/

Source: http://www.diychatroom.com/

Source: http://www.showalterroofing.com/

Built up Felt Flat Roofs

Water Ponding – Water ponding can occur on a flat roof for a number of different reasons, for example the roof was not designed or constructed correctly in the first place and is not allowing water into guttering our outlets, there could be impact damage (possible people walking across) or heavy loadings such as plant or materials that have resulted in ‘depressions’ in the roof, outlets or guttering could be blocked etc.  In any event prolonged standing water on a flat roof can result in degradation of felt, stress and failure of joints, increased risk of condensation (as the internal roof surface temperature is reduced), and the build-up of moss and lichen.  Standing water will also freeze in sub-zero temperatures, further increasing the possibility of condensation as well as becoming a health & safety risk if there is a need to gain access to the roof at that time.

Source: Google Images

Although not the nicest of conditions, the best time to inspect a flat roof is in poor weather conditions particularly whilst it is raining.  This will highlight many of the problems with design, workmanship and general deterioration. Even in the warmer months when it may not have rained for a long time a surveyor will still look for visual clues that may indicate water ponding, such as a build-up of moss and debris, general staining of the roof covering or damaged coverings etc.  The image to the left provides an example of this.

Simple regular maintenance such as clearing gutters and outlets will help to reduce water ponding however this is something that is often overlooked by many property owners/occupiers.  This is likely to be because many flat roofs are out of sight (and therefore out of mind!) and unless the roof starts to exhibit problems many will choose to ignore it. Water ponding problems as a result of poor design or poor workmanship are often more problematic to deal with and consequently more expensive.  Again, many people will choose to live with these problems until of course the roof starts leaking and causing consequential damage to other parts of a building.

Source: http://thermalimages.ca/

Blistering of Felt – A fairly common issue identified by surveyors when inspecting a built up felt flat roof is blistering of the felt roof surface.  This occurs where water vapour pressure becomes trapped underneath the roof covering between layers of felt that have been inadequately bonded.  Any trapped water or moisture will evaporate and expand.  As it is trapped and cannot escape it will force the felt to form a blister. The source of any moisture will need to be addressed before any repairs are undertaken otherwise the problem will re-occur.  Blistering sometimes occurs when the felt covering is laid onto a substrate that is not fully dry, possible when roofing works are being undertaken either after or whilst it is raining.

Source: http://www.newcastlene1.co.uk/

Cracks and Tears - Cracks and tears to felt roof coverings can be caused by thermal or moisture movement, saturation of insulation or sagging of the roof deck. Cracks and tears can also occur when there is thermal or moisture movement between the roof substrate and membrane. In most cases the impact of thermal movement is not visible to  the naked eye, however even though we cannot see it, building materials expand and contract by varying amounts when they heat up and cool down. This is due to their differing coefficients of thermal movement. Materials expand because an increase in temperature leads to greater thermal vibration of the atoms in a material, and hence to an increase in the average separation distance of adjacent atoms. Due to the fact that roofs in the UK can experience a significant temperature difference due to the varied climate, the roofing felt material will be reacting to these ever changing conditions by expanding and contracting continually.  This can sometimes result in crack or tears appearing in the felt surface which can often be repaired by cutting back the existing felt and allowing the area to dry before applying a new layer of felt with a suitable overlap between the new and existing material.

Other issues such as de-bonding at up-stands can also be encountered by a surveyor during an inspection of a built up felt flat roof as well as numerous other problems that occur as a result of poor workmanship.  In my next article I will discuss problems associated with mastic asphalt flat roofs, where some similar issues to those described above will be explored together with some other problems that may occur as a result of using a different material.

Please feel free to share this article and other articles on this site with friends, family and colleagues who you think would be interested

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.

Flat Roofs – Part 1 – Different Types

I often find that the choice of flat roof construction is usually one of convenience and cost, however, this really is a false economy as the costs of maintenance, repairs and replacement of a flat roof, from a life cycle cost perspective, usually work out more expensive than constructing a pitched roof in the first place

Source: http://www.angleseyroofing.co.uk/gallery/

Flat roofs are commonplace throughout many parts of the World and are regularly used throughout the UK.  One of the primary functions of any roof is to keep the internal environment dry and it is essential that water is prevented from entering the internal environment.  Whereas pitched roofs readily allow rainwater to drain into gutters and downpipes due to their design, flat roofs tend to be more problematic particularly if not designed and constructed appropriately.  The use of flat roof construction in warmer parts of the World which experience limited rainfall is less problematic than in a varied climate such as that encountered in the UK, where the potential for extensive prolonged rainfall is highly likely.  I often find that the choice of flat roof construction is usually one of convenience and cost, however, this really is a false economy as the costs of maintenance, repairs and replacement of a flat roof, from a life cycle cost perspective, usually work out more expensive than constructing a pitched roof in the first place. The problem we seem to have in the UK is that property owners and occupiers do not think long term and most will not reap the benefits of longer term investment because people tend to move on a regular basis.

I am sure that many Building Surveyors (and others who inspect property on a regular basis) will agree that flat roofs are something that warrant a particularly close inspection and is an element where regular problems are identified.  In next week’s article I will focus on some typical defects/problems to look for when inspecting flat roofs however for the remainder of this article I want to focus on different types of flat roofs typically found in the UK.  When we refer to different types of flat roofs we can basically consider these in terms of both construction and coverings.  

Figure 1 - Source: Source: http://www.diynot.com/

A flat roof is defined in BS 6229 as having a pitch (gradient) of 10 degrees or less.  Therefore a flat roof is not actually completely flat and should be constructed with a subtle gradient which will allow any rainwater to find its way to gutters or outlets.  Even a small gradient of a few degrees will allow rainwater to become mobile by gravity, although clearly the greater the gradient the better.  There are a number of ways of achieving this subtle gradient on a flat roof such as the use of timber firing strips onto of joists (as seen in figure 1), however I have stumbled upon many examples when these have been incorrectly installed and in numerous cases completely omitted. In fact the vast majority of problems I encounter with flat roofs were a result of either poor workmanship during installation or lack of general maintenance.

In terms of flat roof construction there are two common types used in the UK, cold roof construction and warm roof construction (I will discuss flat roof coverings a little later). Buildingregs4plans.co.uk (online) provide a concise explanation of warm and cold roof construction which is used below;

Warm roof construction - In a warm deck roof the insulation is positioned above the structural deck and no ventilation is required. Throughout the course of the year the roof deck and all below it is kept at a temperature close to that of the inside of the building, therefore the roof structure is protected from extremes of hot and cold, lessening the potential for damage caused by thermal movement.

Figure 2 - Source: Source: http://www.buildingregs4plans.co.uk/

A warm deck also provides added protection from the dangers of condensation as the structure is kept warm, at a temperature above dewpoint, by the insulation above it. Therefore water vapour which enters the roof structure from the room below will not have a cold surface on which to condense. NHBC recommend that this type of roof be considered as the standard form of construction. (Figure 2)

There are two forms of warm deck roof, sandwich and inverted.  The sandwich warm deck roof is the most common type of flat roof. The insulation is placed below the waterproof covering and is either mechanically fixed or bitumen bonded on to the top of the deck.

Figure 3 - Source: Source: Chudley R. & Greeno R (2005), Building Construction   Handbook.  

The insulation boards in an inverted warm deck are laid over the structural deck and the waterproof covering. The insulation is secured by a layer of ballast or paving slabs to prevent wind uplift. The waterproofing membrane has the added protection of the insulation from foot traffic and degradation caused by exposure to solar radiation. However, it may be a more difficult to locate defects in the membrane (insert image 4)

Cold roof construction - In a cold roof the thermal insulation is laid between the joists below the structural deck. As the insulation is not required to take any loads, quilts and other loose fill materials can be used as well as rigid insulation. Because the structural elements of a cold roof are not protected by from the heat of the sun by a layer of insulation they are liable to suffer the damaging effects of thermal movement. Ventilation is required above the insulation in a cold roof to prevent the build-up of moisture vapour in the roof void. (Figure 4).

Figure 4 - Source: Source: http://www.buildingregs4plans.co.uk/

In addition to the construction types described above, flat roofs are also often referred to by the type of covering that is used.  In the UK, built up felt, mastic asphalt and single ply are the common types of coverings installed.  Some of the typical problems with flat roofs that I will discuss next week are a direct result of the type of covering selected.  It is therefore essential that an appropriate covering is considered during design and installation.

Built up Felt Roofing - involves the installation layers of tar impregnated roofing felt which are rolled out onto a roof.  Each layer overlaps the previous layer and in between the deck material and each layer of felt a layer of hot tar is applied over the surface.

Mastic Asphalt Roofing - premiermasticasphalt.co.uk (Online) define mastic asphalt as; comprises suitably graded aggregates bound together with an asphaltic cement (primarily refined bitumens) to produce a dense voidless material.  It cannot be compacted and is spread rather than rolled. As mastic asphalt is installed as a ‘hot liquid’, when it cures (cools down) it provides a continuous impervious membrane with no joints making it extremely waterproof.

Single Ply Membranes - everybodyneedsaroof.com (online) define Single-ply membranes as; factory-manufactured sheet membranes. which are generally catagorised as thermoplastic or thermoset. Thermoplastic materials can be repeatedly softened when heated and hardened when cooled. Thermoset materials solidify, or "set," irreversibly after heating. Single ply membranes commonly are referred to by their chemical acronyms, such as ethylene propylene diene terpolymer (EPDM). Single-ply membranes can be installed fully adhered, mechanically attached or held down with ballast. Most single-ply roof systems do not receive surfacings. 

The information above provides a brief introduction into UK flat roof construction and flat roof coverings and should be used as a reference point for the problems/defects associated with flat roofs that will be discussed next week.

Please feel free to share this article and other articles on this site with friends, family and colleagues who you think would be interested

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.

Septic Tanks – An alternative to mains drainage

Guest article from Duncan - www.propertyhive.org

Under the Water Resources Act 1991, it remains a criminal offence to discharge effluent that is not of a sufficient quality into a watercourse without the consent of the Environment Agency. This can result in a three month prison sentence or a fine of up to £100,000. Therefore it is important to ensure your installations are up to date.

Source: http://www.mtmdrains.co.uk/

Consideration of drainage is an important part of any development and in the vast majority of cases UK developments will enjoy the benefit of being able to connect to ‘main drainage’.  In rural or more isolated areas, where mains drainage is not generally available, alternative methods of controlling and disposing of drainage, particularly foul waste, need to be considered.  Septic tanks are one such option:

The first recorded septic tank was installed in 1896 by Donald Cameron who was the city Surveyor for Exeter. This septic tank served the entire town and was 20 x 6m to a depth 2.1m. Since then there has been some progress in the development of septic tanks, but the principles are basically the same.

Source: http://www.septicrepairny.com/

Septic tanks are covered in UK Building Regulations under Approved Document H, and it is an offence not to build new structures to the standards as set out in the Building Regulations under the Building Act 1990. However as long as the British Standards are followed or bettered this should not be a problem. See BS6297:2007 for further guidance. Building Regulations do not apply retrospectively and so septic tanks that pre-date the 1985 Building Regulations are not affected. This is despite the fact that many septic tanks are no longer fit for purpose due to the increase in water usage rates over the last fifty years. However under the Water Resources Act 1991, it remains a criminal offence to discharge effluent that is not of a sufficient quality into a watercourse without the consent of the Environment Agency. This can result in a three month prison sentence or a fine of up to £100,000. Therefore it is important to ensure your installations are up to date.

Treatment  - All foul drainage should enter a tank including water from W.C’s, showers, sinks, baths and domestic appliances such as washing machines etc. Washing machines and dishwashers are often discharged into rainwater pipes as it is easier, but this can have a significant impact on the local environment and should always be avoided.

A Septic Tank is a self contained installation that processes and treats raw sewage. It is a tank which stores the waste allowing enough time for the organic matter to decompose through natural processes. The sewage becomes liquefied, with a thick oily scum forming on the top and sludge that settles on the bottom. What is left is a central layer of clear liquid, which in older tanks is often emptied straight to a watercourse (which is illegal and harmful to the environment), but in new installations it is treated further before been discharged. The sludge at the bottom is made up of everything that is too heavy to remain in suspension and the oily layer that sits atop is the matter that can decompose naturally.

The secondary treatment usually comes in the form of a soakaway, where the water is drained into a hole formed underground, using plastic formwork that can be described as looking like milk crates, where the water will be soaked up slowly by the ground. Soakaways come with their own set of legal requirements.

The other type of secondary treatment comes in the form of a bio-filter. These have sweeper arms that drip the effluent over clinker beds. Please note that a septic tank is not the same thing as a cess pit, which is a container that stores sewage until it can be emptied and treated elsewhere. Older tanks are often square in shape with brick walls, although modern installations are often pre-fabricated uPVC units that can be square or circular. The septic tank is made of either two separate tanks or one tank split into two

Capacity - A septic tank should be 180 litres in size for every person it serves, with an additional 2000 litres of capacity regardless of the number of people using it.

Litres = 180P + 2000

Where P = the amount of persons served by the tank.

The above calculation should treat children as adults and allows for emptying on an annual basis.

This is so the sewage can be stored long enough that the decomposition of the waste can occur. Anything smaller than this will mean untreated sewage is discharged and solids may cause blockages in the pipework, or if installed the bio-filter. If untreated sewage is allowed to drain into the local environment, there is likely to be an increase in disease, rat infestation and damage to the local environment.

End Product - One of the end products of a septic tank is sludge. This is raw sewage that remains in the tank, and then emptied periodically. The other is water that consists of nitrates and tiny fragments of sewage.

General Problems - An excess of detergent will inhibit separation of the three layers. An excess of disinfectants will kill of the bacteria involved in the treatment process. Day to day domestic use should not cause any problems in a well designed and maintained system. Water softeners however produce salts which in excess can kill of the bacteria required to make the system work.

De-sludging - How often to de-sludge depends largely upon the occupancy of the household and size of tank, but for an average household of 4 people it would be reasonable to assume an annual de-sludging would be sufficient.

Mains Drainage - If there is a nearby sewer you have the right to connect to it. Just contact the local sewerage company and they will give you further details on how to arrange for a connection.

Construction Requirements - Reference should be made to BS6297:2007, which contains recommended materials for use. Rain and groundwater must be prevented from entering the Septic tank, as clean water should not be contaminated. Therefore the tank should be covered, which also helps with the safety issues of having an open tank. Access must be maintained with an access hatch of at least 600mm square. This access should allow for inspection of the inlets and outlets, and should be large enough to allow rodding. Tanks should be situated;
1. Not within 15 metres of the dwelling in question.
2. Not within 10 metres of a watercourse.
3. Not on land that it is regularly flooded, or land that has a high water table.
4. So that a tanker can access for emptying (e.g within 30 metres of vehicle access).
5. Downwind of nearby dwellings if at all possible.

For more information on a variety of property and construction related articles please visit www.propertyhive.org

Please feel free to share this article and other articles on this site with friends, family and colleagues who you think would be interested

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.

What is BIM?

Guest article from Danny McGough BSc (Hons)  ACIAT – Assistant Lecturer at Coventry University

Building Information Modelling (BIM) is a design methodology, which enables all of the design details, decisions and characteristics to be held within a collaborative digital model and information package. Having all of the information centralised in one core model will inevitably lead to improved design and document efficiency. It is this added value that takes BIM beyond that of a simple 3D visual model.

I’m going to open with a few statistics but don’t be too frightened, 4 stats and I’m done!

Designed by and Property of Danny McGough

The knowledge of BIM within the construction industry is on the rise, the recent NBS National BIM Report 2013 states that only ‘6% of the industry were neither aware nor using BIM’ in 2012, compare this to previous years of 21% and 43% in 2011 and 2010 respectively it is clear that the awareness of BIM amongst construction professionals is rising. Looking at these particular statistics it does paint a promising future for BIM.  However following on from this, in the same NBS BIM Report 2013, the statistic that ‘74% of the industry is not clear enough on what BIM is yet’. Does this mean that we are not completely past the ‘What’ stage just yet for all construction professionals? It is with this in mind that has led me to write this week’s article, ‘What is BIM?’

In the context of Architecture, Building Information Modelling (BIM) is a design methodology, which enables all of the design details, decisions and characteristics to be held within a collaborative digital model and information package.  For the wider range of parties involved the concentration may be more on the 'Information' sharing aspects of BIM.  A predominate aspect of BIM which continually needs to be expressed and driven home is that the 'Information' side of BIM is just as important as any design model. Working within a BIM environment, adopting BIM information sharing protocols, collating the data in an interoperable format, and utilising documents such as BuildingSMART's 'Project Execution Plan' into a project, aims to improve how information is shared amongst all varying parties involved in an efficient interoperable manner, regardless of whether you’re the Architect or the Contractor.

A core feature of working within a BIM environment is the drive towards encouraging multi-disciplinary collaboration from the very outset of a project. The benefits of all disciplines working together within one core BIM environment are multiple. A major issue that is experienced within non BIM design processes is the matter of conflicting design issues. The ethos of having a core central BIM model is to facilitate a smoother transition through these issues by identifying conflicts as early as possible in the project stages, thus reducing the negative effects on schedule and costs. From an early stage projects can be visualised allowing the Client and Designer alike to gain an appreciation of how the design is going to materialise. This allows for important design decisions and alterations to be made at an early stage where the cost repercussions are minimal or sometimes even zero. The efficiency of the effects of changes within documentation or design is greatly improved as any changes made which are linked to the main BIM package will be carried through and updated to all corresponding linked documents and models automatically.

Below you can see a graphical example of an 'Independent Separated Design Environment'. This simple graphical representation expresses the chaos when all parties are working independently of the others:

Designed by Danny McGough

Having the design process completed within a BIM environment using 3D BIM models with a core 3D BIM model at the centre of the project leads to multiple benefits post model creation. The models can be analysed allowing for a multitude of model interrogations to take place including; energy analysis, structural analysis, accurate schedules and quantity take-offs to name a few. It is argued that by using BIM processes for building projects it will improve the energy efficiency, improve the scheduling, and facilitate a reduction of waste and possibly paramount to this, a reduction in costs.

The diagram to the right  is another graphical representation , of a  'BIM Collaborative Design Environment' showing how a BIM core model and digital information package can assist the project team to deliver a fluid fully collaborative project.

BIM for Architectural Design and Modelling

As many are aware BIM models can be used to allow the designer to present and communicate 3D designs in a clear, easily accessible way for all to see. BIM models and information analysis packages provides a platform for multiple discipline teams to analyse, interrogate and navigate the project further, beyond the limitations of 2D design. Once the information is data dropped to the core model further clash detection analysis can take place, reducing issues and conflicts. As discussed earlier having all of the information centralised in one core model will inevitably lead to improved design and document efficiency. It is these added values that take BIM beyond that of a simple 3D visual model.

BIM for Structural Modelling and Analysis

BIM software can assist the Structural Designer in their analysis of the structural performance of a structure. Employing one core structural model means that there is no need for multiple models to be created for each different structural analysis that is needed. Time is saved through not having to continually transcribe information from one design package to another. All the related information to the project can then be easily shared and accessed by multiple project disciplines.

BIM for MEP Modelling, Detailing and Energy Analysis

Creating MEP details in a BIM model allows Building Service Engineers' and Architectural Designers alike to be able to visually appreciate how the services within a design relate to the building as a whole, ensuring that clashes and issues are identified early on. Using MEP and energy analysis software many analytical programmes can be run to test the overall efficiency of the design. This should result in a circle of design – analysis – redesign, with an aim at establishing the most energy efficient design at as early stage as possible.

BIM for Programming and Scheduling (4D BIM)

BIM 3D models can be utilised to assist Contractors in the programming and scheduling of BIM projects. This is achieved by adding programming and time data to a BIM project, once the data is married to the building project then the 4D programming schedule can be established. The 4D programme can then be used to assist Contractors and Designers to improve and refine the schedule of the project.

BIM for Quantity Schedules and Costing Information (5D BIM)

Quantity Surveyors and Designers are able to produce accurate quantity schedules and cost information for building projects. Standardised data can also be integrated into BIM models ensuring that all the building components meet the required Building Regulations. You are probably picking up on the theme here.... all of this information can then be stored and accessed within the BIM core model by all those involved in the project.

BIM for Facilities and Asset Management (6D BIM)

The management processes and time that it takes for a Facilities Management Team to continually assess and maintain their asset stock can be considerable.  For instance if you think of a University Estates Team, with tens of buildings to manage trying to find a detailed specification of a fixture or fitting in a building that was built 40 years previously, you can imagine this being a tough task. Having all of the information available within one core model could be invaluable, at the click of a button the details, manufacturer, performance criteria and cost could be accessed almost instantaneously.

In summary, through carrying out all of this pre-construction design analysis and interrogation the result will be a reduction in conflicts and changes made during the construction phase which usually will have a detrimental effect on a project in terms of wastage, quality, time and costs. At the same time the stringent energy analysis that can take place in the early stages of a BIM project aims to improve the performance of a project in regards to low impact design. And finally post project completion, the BIM model can continue to be utilised by the FM team to assist in the management of their assets in an environmentally conscious manner.

For more BIM and Architectural Design articles you can visit Danny’s personal blog; Architecture, Technology & BIM - http://architecturetechnologybim.blogspot.co.uk/

Please feel free to share this article and other articles on this site with friends, family and colleagues who you think would be interested

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.