Surveying A Flat Roof | Materials And Their Properties (Cont.)

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In this guide, we’ll look at the various insulation, structural decks and screed you will encounter when surveying a flat roof.

Insulating flat roofs

Under current Building Regulations, when more than 25% of a roof area is being refurbished or replaced, it is required that the insulation is upgraded to meet the current requirement.

Introducing insulation as part of a flat roof system provides many benefits. These include reducing heating bills, improving the environment within the building and reducing the risk of condensation. These features enhance both the performance and life expectancy of the new roof coverings.

Insulation is usually incorporated into the roofing system so that it sits on top of the roof structure, which keeps the roof deck and structure ‘warm’.  The insulation helps to maintain the roof structure at a regular temperature, which reduces the amount of thermal movement, and therefore the stress inflicted on the roofing membranes, increasing their life.

A ‘warm roof’ is when the insulation (a rigid urethane board) is laid on top of a vapour control layer, which in turn is laid on top of the roof decking. The build-up of membranes which form the main waterproofing layer are laid on top of the insulation, sandwiching the insulation within the system. The insulation in warm roofs can be designed and manufactured with a gradient to provide or improve falls and assist drainage. These insulation schemes are called ‘tapered’ or ‘cut to falls’.

An ‘inverted roof’ is when the insulation (a rigid Styrofoam board) is laid on top of the waterproofing. It is held down by ballast. In wet conditions, the insulation and the waterproofing become wet which reduces its thermal performance so that a greater depth of material is required to achieve the same thermal performance. Inverted roofs are common on new build projects but rarely used in roofing refurbishment projects. An advantage of inverted roofs is that the waterproofing membranes are protected from physical and thermal damage.

A ’cold roof’ is when insulation (fibre glass or mineral wood quilt) is inserted between the ceiling joists. Because of the risk of condensation forming in the roof void the space has to be ventilated. This is very rarely a practical option when replacing an existing flat roof.

Structural Decks

The structural deck provides the primary support for the waterproofing system and it must be able to resist the loads which will be imposed upon it, including storm loads. If the roof covering is being replaced, the suitability of the deck should be considered.

Decks should also provide a suitable fall for drainage, although this is often by modification, such as by using tapered or cut to falls insulation or the use of a screed. This should also be checked.

There are a great many different type of structural decks, and the following are common examples:

In-Situ Concrete Decks – These are sometimes cast in situ with a fall. Alternatively they can be laid level and the fall provided by a screed, timber firrings and decking or tapered/cut to falls insulation. In-situ concrete decks (and screeds) on existing buildings should have cured and dried out by the time the waterproofing needs to be replaced. However, remedial action may be necessary with saturated decks to remove moisture.

Pre-cast Concrete Decks – There are numerous systems available, generally consisting of planks or blocks set between concrete beams. The gaps between the blocks/planks are usually filled with mortar. The same methods are used to create falls as descried above and the same provisos apply.

Plywood & OSB – Both can be used for roof decks provided they meet the necessary standards. Be careful – not all are suitable. All edges should be supported and noggins should be used as well. The thickness of the boards is determined by the joist centres. Falls are usually created in the timber structure beneath the deck by using tapered ‘firring pieces’.

Chipboard – Although used in the past, chipboard is not considered suitable for use as a flat roof decking material. If you come across it, it should be replaced. Falls were usually created in the timber structure beneath by using tapered ‘firring pieces’ and these is usually suitable for re-use without adjustment.

Woodwoll Slabs – These boards are made from wood strands bound with a cement slurry. The boards were popular between the 1950s up to the 1990s. They were manufactured 600mm wide and in varying lengths. Shorter lengths had plain edges while longer boards (up to 4m and when formed with a trough, up to 6m) were reinforced with a metal channel on both sides. Woodwool slabs are now considered fragile and great care has to be taken when working with them. They should either be replaced or over-sheeted with a suitable board or panel. Any additional loading should be checked. They are even more dangerous when the roof has leaked and they have become wet. Risk assessments will be needed because of the fragile nature of the slabs.

Stramit/Straw Boards – Made of compressed straw with paper facings, these boards were popular in the 1950 and 1960s. There were three common depths; 50mm, 75mm and 100mm. The boards lose all structural integrity if they get wet and the removal of the bonded top surface will destroy the upper face – if you come across them they should be stripped up and replaced. Risk assessments will be needed because of the fragile nature of the slabs.

Profiled Metal Decks – As with other decks, these can be laid flat or laid to falls – this should be established during you survey.  Assuming they have been properly specified, supported and fixed, they should provide a good option and it should be possible to install a new roof system onto them. Because the sheets are corrugated, they do not provide a continually supported flat surface on which to bond the roofing system. Therefore, the crowns, to which the flat roof is to be laid, should represent at least 50% of the roof area. The VCL is laid across the crowns and care should be taken not to damage it by walking in the troughs. The insulation is laid over the VCL and it will then provide a continuous flat surface for the rest of the roofing system to be laid on top of.

Others Structural Decks –  There are other less common structural roof deck systems which you may come across from time to time, including timber panels, composite boards and many other oddities.


There are any number of different materials used as screeds on roof decks. Sometimes woodwool slabs are pre-screeded. In other cases, screeds are used to create the falls on the roof to assist drainage. When surveying the roof it is important to find out what the screed is and if it is being used to create the falls on the roof. This is especially important if the existing waterproofing is being removed.

If they are saturated all screeds are likely to require replacement. There are processes that can be used to ‘de-water’ screeds but they will not completely dry the screed. Alternatively (or as well as) it may be possible to provide vents to allow trapped moisture to escape over time by evaporation (such as the ‘screed vents’ used in some older buildings when they were constructed). After the trapped moisture has evaporated, the vents can be removed.

When replacing roof coverings, some screeds may have to be removed which can add considerably to the cost and the time the roof is exposed to the elements. It may also necessitate the introduction of a new screed or, more likely, a ‘cut to falls’ insulation scheme. Both have significant cost implications.

Sand:Cement Screeds – These are usually robust and once the old waterproofing has been removed, the new waterproofing can usually be installed on top of them.

Lightweight Screeds – There are many variants but they all consist of a lightweight, loose structured material (cementitious, perlite, expanded clay or sintered pulverised fuel ash). They are usually bonded with cement and have good insulation properties. The surface is usually topped with a thin screed, typically 13mm. There are variants which were bound with bitumen, which may be identified by a petroleum smell. All these systems are usually laid to falls. Because of the nature of these screeds, water drains through the material and is difficult to detect. If the roof has been leaking this can result in water being trapped within the system, usually on top of the deck, which can cause blistering of the new waterproofing membranes and other problems in the future. It is very difficult to strip existing waterproofing systems laid over lightweight screeds without damaging the screed beyond repair. In such cases the whole system has to be replaced. We’ve never managed to save one, so we recommend you allow to replace it.

Lightweight Aerated Screeds. These have the colour and texture of a breeze block. They consist of Portland cement, water and a foaming emulsion, which are combined to form a cellular material. They have good insulation properties. If the screed gets wet, it is difficult to remove moisture (see options for drying screeds in the introduction to this section). As with other lightweight screeds, great care has to be taken when removing the existing waterproofing (it may be sensible not to strip it if it is well-bonded) and assuming it is not damaged, it should provide a good surface for overlaying.

This article, as with all other articles we produce, is for guidance purposes only. It does not constitute formal advice and should not be relied upon as such. For bespoke, unbiased advice relating to your commercial roofing project please contact us and we would be pleased to assist.

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