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.

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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.

Rising Damp – A Salty Problem - An update for 2015

Guest Article - Joe Malone - Principal: Malone Associates Ltd

My academic research into rising damp led me to the conclusion that most text books are wrong in their description of rising damp since they state that it is caused by capillary action.

Following on from my articles written in 2013 I thought it was time to provide a further update on one problem in particular relating to the diagnosis of rising damp. In my update for 2013 I wrote that, ‘you need to confirm that three conditions are present to definitively confirm a case of rising damp.’ These are:

1.  You must have a rising damp moisture profile. That is a profile that is wetter at the wall base but gradually decreases with height to a theoretical maximum height of circa 1.5m.

2.  You must prove that moisture is present at depth in the masonry and it is not enough to take surface readings from the plasterwork. You will need deep wall probes or a calcium carbide (speedy) meter to confirm this on site.

3.  You will need to confirm that nitrates are present in the damp apex of your moisture profile. This will involve doing on site analysis or sending a sample off to the labs. You might have noted that I've ignored chloride salts because these can be present in tap water or building materials. A positive test for nitrates confirms that the moisture has leached up from the soil.

My academic research into rising damp led me to the conclusion that most text books are wrong in their description of rising damp since they state that it is caused by capillary action. Since we know that the major moisture pathway for rising damp is the mortar perps and since we also know that the moisture transfer mechanism in mortar is diffusion then clearly the generally accepted cause by capillary action is incorrect. It is probably worth reprinting my own updated description at this point…

‘Rising damp is an upward migration of groundwater in masonry walls. It will act in combination on the masonry units and their separating mortar joints or it will act primarily on the mortar joints. The moisture transfer mechanism in masonry is capillary action whilst the moisture transfer mechanism within mortar is diffusion. The major moisture pathway for rising damp is the mortar perps so it can be stated that there are dual moisture transfer mechanisms for rising damp, diffusion and capillary action’

The purpose of this article is to right another wrong with regard to the academically accepted principle for salts analysis. If you note again the requirement to prove the third condition, the need to prove that Nitrates are present in the damp apex of your moisture profile. This requirement stems from the fact that Nitrates are present in the soil so if moisture is leached up from the soil then it stands to reason that the moisture contains Nitrates. Personally I have always used the chloride test very little since it has extremely limited value in the course of most damp investigations and it is a test that has always been of zero value for the diagnosis of rising damp, simply because we know that chlorides are present in tap water so a positive test for chlorides does not help us determine the source of moisture.

The Nitrates Anomaly

When testing for Nitrates we do so on the assumption  that Nitrates are not present in tap water and therefore a positive result moves us to conclude that moisture in the masonry has been drawn from the ground. I’m only aware of one salts analysis kit sold by Protimeter and indeed this is the one I use. Interestingly, Protimeter do not make their own salts analysis tablets and these are sourced from a company called Palintest.

Figure 1. Protimeter salts analysis kit.                   Source: surveyexpress.co.uk

However, we know that Nitrates can be present at very low levels. World Health Organisation guidelines stipulate a guideline for 50mg/l or less. So this raises a key question… Do we know that the tablets supplied by Palintest for the Protimeter salts analysis kit are discriminatory enough to only give a positive test result when  Nitrates levels are above and below 50 milligrams per litre? If not, then the source of moisture cannot be determined using this test since a positive result may also be obtained when the source of the wall base damp is a leaking incoming water main containing Nitrates at low levels of 50mg/l or less? If you have used the Protimeter kit you will know that a positive test for Nitrates turns the water cherry red and there is no colour chart to match against your sample to help determine the approximate quantity of Nitrates present. The practice of matching the sample colour obtained against a colour chart is a principle that may be familiar to a lot of freshwater fish enthusiasts since they have to regularly check Nitrate levels in their fish tanks to ensure levels do not become so high as to become dangerous for their fish.

Figure 2. Colour card familiar to freshwater fish keepers. Source: www.cichlid-forum.com

The Protimeter test for Nitrates appears to be less discriminatory than the test used by freshwater fish enthusiasts since it will turn cherry red and give a simple positive result with no clue as to the level of Nitrates present.

Figure 3. A positive nitrates test using Protimeter kit with Palintest tablets

I was uncomfortable with this and decided to contact the technical team at Palintest to ask them how discriminatory the Nitrates test was? They were incredibly helpful in providing the following response…

‘Your question is a good one.  I'm afraid you won't ever be able to be certain that the nitrates don't come from the tap water.  The test doesn't know the source of the water, it'll just react to any nitrates present.

You're also correct that the limit for nitrates is 50mg/l.

What I would recommend is to test the tap water on the site to test the 'baseline' nitrate level.  If your 'real test' is higher than this, you can be sure that some nitrate is coming from the rising damp.  If it’s less or equal to the baseline though, you aren't going to be able to tell.

You could expand your test protocol to include other minerals (sulphate, chloride etc) which would be able to give you some more confidence that the water is from the ground as opposed to the tap but you would need more equipment and would be a bit more complex than your current method.’

Before I even asked the question of Palintest I was already experimenting with a Nitrates test kit manufactured by Salifert. I chose this particular kit since I’d read on most forums that this was generally thought to be the most accurate kit. The kit is supplied with a colour match chart and is significantly cheaper than the Protimeter test kit.

Figure 4. Alternative Nitrates Test Kit  Source: Authors own

In fact I had been doing precisely what was recommended by Palintest… I was and am testing the tap water to establish a baseline Nitrates level according to my Salifert colour chart before I take a sample from the wall. The advice given by Palintest is good but of course ignores the issue relating to the absence of a colour chart in the Protimeter kit. It would seem that I was being advised to use another kit since the Protimeter kit can not discriminate and will simply, ‘just react to Nitrates present.’

In figure 5 you can see where I have obtained a positive test result for Nitrates at a level of around 25mg/l but this was for a tap water sample.

Figure 5. Positive nitrates test result in tap water.   Source: Authors Own

My trials and research with this alternative test method are still ongoing since rising damp is incredibly rare and to date I have only obtained negative results from walls and positive results from tap water.  I will need substantially more time to evaluate the effectiveness of my alternative Nitrates test but what is clear is that an alternative test is needed or some refinement is needed in the Nitrates test kit currently used by most building surveyors or damp investigators.  

Joe Malone - Principal: Malone Associates Ltd

First Published: 16/2/15 at   www.buildingdefectanalysis.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.