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Roofing Contractors

This guide provides a full review on the tips and considerations to make when procuring a roofing contractor. The guide broken down into sections: Tender Analysis, Value Engineering, Programming.

The Tender Analysis

When you receive a quotation from several Subcontractors, it’s unlikely they’ll all be like-for-like, but it’s important that you’re comparing Apples with Apples. Look out for these important cross-checks:

1. Roof type

There are different options for flat roofs. The following systems are the most common: single ply membrane; liquid applied systems (includes cold applied and hot melt); torch on felt system. There are pros and cons to each, and build environment and use will dictate which system is most suitable. From a tender analysis perspective, make sure you’re comparing against the same systems. A liquid system is suitable in areas where a single ply membrane system isn’t. Therefore you need to determine which system is right for the area in question and ensure that each tender provides a price for the selected system.

2. Flashings & details

The volume of weathering and detail required on a roof has a significant impact on the cost and time to complete. A large area of 500m2 with few upstands or penetrations can be straight forward, whilst an area of 150m2 with intricate detailing can be costly and overseen at design stage. At tender analysis, you need to ensure that all relevant areas on the roof that require detail and flashings has been included in every quotation provided.

3. Warranty

Warranties vary from manufacturer to manufacturer and contractor to contractor, and as such so do prices. Ensure you have a clear understanding on your exact requirements for a warranty. Do you need a single point warranty that covers design, materials and workmanship? Or, do you need an insurance backed 10 year workmanship warranty and nothing else? Make sure you understand what form of warranty you require and ensure each quotation prices for that exact form.

4. System

Some contractors may price for an alternative system manufacturer than the one specified. This isn’t necessarily a problem, but if you’re working on a standard JCT Building Contract, you need to ensure the architect is open to a change or has allowed for equal approved in the specification.

5. Product

Where slate, tile, lead or zinc is concerned, check that the exact product is named and priced in the quotations. If you require Lead Code 6 on a certain roof, ensure every contractor has explicitly priced for lead. If you require Welsh slates, ensure all prices are for Welsh slates. Prices vary wildly in these areas and whilst it’s good to explore value engineering options later on, at tender you the aim is to compare apples with apples. Also check that recommended laps are specified as this can have a bearing on price.

6. Access

All roofing work requires a certain level of access, but some roof types require more than others. For example, with a green roof you need to take into consideration the requirements to lift one tonne bags of soil substrate from the ground onto the roof. For hot melt systems, large equipment needs to be lifted onto the roof. Who is responsible for the cost? Does each quotation specifiy? This is a large cost to consider and will make a big difference in the tender analysis. Make sure you understand the full costs associated with any access requirements, and whether these will be included within the roofing package, before reviewing the tenders.

Value Engineering

This next section identifies five simple, intelligent solutions to Value Engineer a roofing package…

1. Thermal insulation at balconies

Vacuum Insulated Panels (VIP) insulation is often specified on balconies to meet the required U-Values. The product provides an improved thermal performance compared to other alternative insulations, and therefore requires a thinner thickness on a balcony where thresholds and insulation heights are a factor. However, VIP is also extremely expensive and whilst you can reduce insulation thickness by approximately 65%, you’ll also increase cost by around 400%. If you enter into dialogue with roofing contractors and manufacturers at an early stage, you may be able to design a new roof build up at balconies that eliminates the need for a VIP insulation and saves hundreds of pounds per square metre on balcony waterproofing.

2. Slate selection

Slates vary wildly in price, but not so much in appearance. To the untrained eye, there’s little difference between a Welsh and Spanish slate, but a Welsh slate costs approximately four times as much and the lead time is likely eight times longer. If you’re in Central London or other heritage spots, you may be obliged to use Welsh slate. However, if the choice is driven by the client’s preference, request a sample of Spanish slate to present along with a breakdown of shared savings. Also, the larger the slate or tile, the cheaper the installation costs (larger slates means fewer to install). So, where possible, go with a larger slate.

3. Flat roof manufacturer

Flat roof systems can vary significantly between manufacturers. At times, this is justified. Not all systems are equal, and some manufacturers are more rigorous than others when it comes to site audits and contractor inspections. However, of the few manufacturers that offer insurance supported single point warranties on design, workmanship and materials, quality and support doesn’t particularly differ, but cost does. As mentioned above, you need to ensure that you’re using the appropriate system for the roof environment. Torch on felt systems are robust, but they’re not great for tight areas and they come with an open flame, which may be unsuitable on your site. When you’re confident that you have the correct system after discussions with the architect and roofing contractor, look at the warranty required and system specified and request prices from a handful of system manufacturers. You’ll find big price differences, even across the bigger manufacturers all offering the same performance and warranty.

4. Green Roof finish

At times you’ll find that a green roof system has been specified, and it’s part of the brand specified for the roof finish. However, hardly any roofing manufacturer’s have their own nursery to grow sedum and provide substrate (we only know of one). The reality is the green roof finish has little impact on the roof warranty or performance. Let me qualify that, the design is important and it’s good practice to stick with a design provided by the flat roof manufacturer. However, the material itself, the growing medium and substrate, do not form part of the warranty. Unless the manufacturer or contractor offers a maintenance service, they won’t offer a warranty on the look and performance of the green roof finish. From a value engineering perspective, it makes sense to specify the design (e.g. 80mm sedum mat), but allow the contractor choice of supplier. This will provide a saving.

5. Choice of flashing

 This very much depends on the roof type you’re working with, but on many occasions, there are value engineering opportunities when it comes to flashings. Some manufacturer’s specify lead flashings, which are expensive, whilst others are happy to specify lead alternative flashings (Ubiflex for example) or simply alternative weathering methods which are much cheaper (e.g. cut and chase into wall). Provided you’re getting the correct warranty that you require, it makes sense to go with the lead alternative as the manufacturer is ultimately backing the performance through a design warranty.

6. Creating the fall

Tapered insulation is a common method of designing a fall on a flat roof. It’s also the most expensive method. From a cost perspective, a fall should always be created either through a screed or timber firrings. You’ll save approximately 15-20% on premium cost charged for tapered insulation (rather than flat insulation).


Roofing is a package of works that fall on the critical path of the project. Follow these four tips for a snag free handover…

1. Design 

Design is so often overlooked on roofing packages. Details are often missed by architects and can get missed until the roofer starts work. At which point you’ve missed any value engineering opportunities and you may slow down works. Meet with the roofing contractor and system manufacturer at an early stage to identify every area of the roofing works that requires design consideration and approval. Any penetrations in the roof require weathering. Upstands, capping, drip edges, etc. all require an approved design. A green roof requires pebble borders around penetrations to act as a fire stop, are they in your design? Get everything sign off early, preferably during the tender process or before. This is also where you need to look at the practically of installing the specified system. Architect’s often specify a system based on performance but fail to consider the practicalities of installing the system. A torch on felt system is specified for a 1.5m2 lightwell. Can the roofer install the system in such a tight area? A green roof has been specified on an area of the roof that doesn’t get any sunlight. Does your client really want a dead patch within 6 weeks of moving in?

2. Green roofs are best installed in Spring and Autumn

A green roof is a living thing. Just as you would try to avoid laying turf in January, same goes for sedum on a roof. If possible organise to have the green roof installed in either Autumn or Spring. If not, accept the fact that any harsh weather conditions could have a detrimental effect on the medium at a time when it’s at it’s most delicate.

3. Product lead times

Certain roofing products have long lead times, and you need to factor this into the programme. Welsh slates and certain tiling accessories can have lead times up to 12 weeks in peak season. At the time of writing this, global insulation production has been hit due to a manufacturing crisis in China, meaning that lead-time on certain types can be 8 weeks. 

4. Co-ordination meeting

Several other trades work on the roof. Hold a co-ordination meeting early on with all trades working on the roofing areas. Identify who needs what when. For example, what does your fall protection contractor need to fix into? When is he installing and can the roofer weather the fall protection posts as soon as they’re in. If you have cladding contractors on the roof or electrical contractors working on PV panels, each cross over and you need to make that as soon as possible to avoid any down time between trades which can potentially cost you money and delays.