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A Guide to Constructing a Flat Roofing System


4th April 2018

As with the construction of any type of roofing system, it's crucial to understand the correct process and essential criteria involved in flat roofing construction. While many will contact a professional installation company to complete the job, there are others who will feel confident enough in their own ability to install a flat roof onto the structure.

Read through our guide to constructing flat roofing to ensure you're aware of the potential costs, the materials required, and the process involved.

What is the Process of Building a Flat Roof?

There are key differences between constructing a flat roof compared to a pitched roof and it’s important to understand the basic criteria.

The starting point involves laying roofing joists across the two upright items, which in most cases will be the walls of the structure. Using the steel joists will provide more stability and these are usually found in one, if not both, of the walls.

The distance between the joists is usually around 400mm, although this will depend on the size of the roof and type of material that will be applied. This is to reduce the amount of pressure placed on top.

Flat-Roof-SIG - Flat Roofing Under Construction - Roof Stores
Flat Roofing Under Construction

Plywood decking is then used to cover the joists, with the width in-between also determined by the weight being placed upon them. Getting the balance right here is crucial, as too much weight will inevitably lead to an unstable roofing structure. A small incline must be built into the roof using a frame, which will enable drainage of collected rainwater.

Tongue and groove fitting is used to slot the decking and joists together to further enhance the strength and stability of the roof. This increases the integrity of the material to allow them to stand up to the outside elements and foot traffic over time.

The boards will need to be waterproofed with an additional layer of material, which is typically something like EPDM rubber, used in conjunction with a sealant to fully protect the inside of the building.

Remember, it's always best to consult a professional roofer for advice before considering a DIY approach.

How Do I Measure Flat Roofing?

Although measuring a pitched roof is a relatively straightforward process, finding the square meterage of a flat roof is even easier.

The width and length of each plane on the roof should be measured, including the dormers, before multiplying the two figures together. This will tell you the total area of each plane.

If you have more than one plane, simply add the two or more final calculations together and this will give you the size of the flat roof. This is invaluable in determining the amount of each material needed and the potential costs you will incur.

What is Required to Construct Flat Roofing?

There are two main types of flat roof systems to choose from. The reasons for selecting either one will depend on the requirements of the build, considering the level of insulation required, heat loss and building regulations.

  • Warm Flat Roofing: This approach uses insulation with no air gap either immediately above or below the roof deck. A vapour control layer must be used below the insulation. This option avoids the requirement to ventilate the roof.
  • Cold Flat Roofing: This is where the insulation is typically packed between the rafters. A 50mm air gap must be left above the insulation and allowance for venting must be made at roof perimeters.

Roofing-Felt-SIG - Flat Roofing - External Structure - Roof Stores
Flat Roofing - External Structure

Once you have determined which type you're going to construct, you can finalise the list of materials needed to build the roof. In addition, you will still need tools such as a measuring tape, drill and utility knife, along with the following:

  • Roofing Timber
  • Steel Joists
  • Plywood
  • Exterior Covering Such as EPDM or Modified Bitumen
  • Water-based Adhesive
  • Foam with Fibreglass Backing
  • Insulation (Warm Flat Roofing)

Which Flat Roofing Material Should I Choose?

There are several materials to choose from in flat roofing construction, each with their own advantages. Aesthetics are an important factor to the type of material you pick, as will durability if you expect high footfall. Below are some of the more popular options typically installed on flat roofs.

EPDM Roofing

EPDM is an easy-to-install option as you can usually apply the rubber in one complete layer. While it's is extremely lightweight, EPDM also tough enough to withstand heavy foot traffic due to its elasticity. This means that it can last anywhere up to 30 years, sometimes more for well-maintained roofs.

Torch-on Felt Roofing

A modern version of traditional felt roofing, this is possibly the most commonly used on flat roofs. As it's laid down across the surface, a hot torch creates a flame to mould the material with the adhesive underneath. It is then brushed over to secure its position. This can last anywhere between 20-30 years and sometimes even longer on roofs that enjoy regular checks and maintenance procedures.

GRP Fibreglass Roofing

The wide range of colours available is one of the reasons why many people go for this option. One or two layers at most are enough to produce a smooth, jointless finish on the roof once set. Although it can work out more expensive, it is extremely damage resistant and very easy to repair. A fibreglass roof is likely to last at least 25 years, although it can survive much longer if correctly maintained.


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Flat Roofing Price Guide

The cost of flat roofing construction will vary depending on the materials being used and the size of the project. However, it's always helpful to get an initial idea of cost, so you can assess where you stand budget-wise.

While some people will choose to hire an installation company to do the work, those with more DIY experience may decide to purchase their own materials for the job.

Below is a list of commonly used materials and their dimensions with links to their most up to date pricing.

Material Product Dimensions
EPDM FIX-R EPDM Membrane Available in widths of up to 15m and lengths of up to 30.5m
Torch-on Felt Cap Sheet Anderson Toptorch SBS Torch-on Mineral Felt Capsheet Roll Length: 8m
Roll Width: 1m
Thickness: 4mm
Torch-on Felt Underlay Anderson Toptorch SBS SB2 Torch-on Underlay Roll Length: 16m
Roll Width: 1m
Thickness: 1.5mm
GRP and Fibreglass CrysticROOF GRP Resin Supplied in 20kg and 225kg containers
GRP and Fibreglass CrysticROOF GRP Topcoat Supplied in 20kg and 225kg containers
Water-based Adhesive FIX-R EPDM Water Based Acrylic Deck Adhesive Available in 2.5l (7 – 10m² coverage), 5l (14 – 20m² coverage) and 15l pails (42 – 60m² coverage)
Insulation Powerdeck F Insulation Board 100mm Available in a range of thicknesses from 25mm – 160m and in a board size of 1200mm x 600mm

Costing a Professional Installation

When estimating the costs of flat roofing construction via a professional company it’s tricky to put together an accurate price.  This is due to the differing invoicing practices of commercial roofers. Labour and waste disposal costs will vary, and VAT will also have to be accounted for.

When you approach a company for a quote they should offer a free site consultation. This will break down every element of the cost. Always get at least three comparative quotes so you can assess which one offers more value. Never just opt for the cheapest.

References play an important role too. These will illustrate the company’s ability to do a satisfactory job.


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