Flat Roof Checklist Part 5: Buildability – The Flat Roof On Site

How can appropriate details be installed in the right order to ensure that the terms of the warranty can be met?

In Part 4 we discussed the importance of interfaces to the effective design of a flat roof. Thresholds are a classic example of how the incorrect sequencing of works can have severe consequences not only on the programme but also on the ability of your roofing installer to correctly install the roof system. If a door frame and threshold are installed before the threshold is waterproofed, the contractor may have very little access to carry out his work.

Sometimes things will take an unexpected turn on site, and it is wise to have a relationship with your roofing specialist which includes full technical support and monitoring on site to identify issues and deal with them if they arise. This is why our service takes you right through to the issuing of warranties; so that in the unlikely event that good preparation is not enough, the team is on hand to resolve it.

A good example of this, albeit on a large scale, was the huge spliced trusses on the roof at 5 Broadgate in the City of London, where the bolts at the bottom of the trusses were unexpectedly cast into the concrete slab. A bespoke, hybrid waterproofing solution was developed with the architect, Ben Stuart of Make Architects, which provided a much more cost effective solution than breaking out the concrete. You can read about it, including all about waterproofing the spliced truss, in the case study here.

Speak to your roofing specialist to help identify any likely sequencing risk areas in your project, and include consideration of these in the specification. If possible you will want to make compromises now, to avoid more costly compromises on site.

Aligned to the issue of sequencing is that of protection. A large area of flat roof can be used as a working platform for later stages in the work, or it may simply become an unintended magnet for materials, tools and storage. Which areas of your flat roof will need protection after the waterproofing has been installed?

You should also consider areas of the roof that will be visited for maintenance of either the roof itself or any plant, rainwater goods and other elements. How frequently will the roof be visited? Might it be vulnerable to unauthorised visitors?

Some materials are more vulnerable to damage after installation than others, and if damage occurs it can lead to the need for repair or replacement, with consequent impacts on aesthetics, bringing us right back to the start of our checklist.

Identify if your roof is likely to become an intended or unintended working platform, so that the specification can include the right protection measures and ensure the roof material is suitable for use in this way.

Now we’ve looked at the risks of the commissioning of the roofing project, we can consider the cost of any extended programme, additional prelims, protection and sequencing. Are there likely additional cost implications we can identify? This is the second consideration of cost in our checklist, and it enables us to ask: is that which has been specified, capable of being delivered? And if not, what changes do we need to make?