FIVE ITEMS NOT FACTORED IN AT DESIGN STAGE OF A FLAT ROOF

Rules and conventions for the calculation of U-values include ways of adjusting the result to account for repeating thermal bridges like mortar joints in masonry and fixings through insulation layers. When calculations are performed at the design stage, these corrections aim to ensure that the result is a reflection of how the floor, wall or roof will perform once the building has been constructed.

However, there is a risk with any building element – and roofs in particular – that changes are made to the construction while work is progressing on site.

Fundamental changes to the specification, such as a different type of insulation, are likely to be factored in to as-built U-value and compliance calculations. This blog post looks at items that designers might specify for a roof, but for which their impact on a roof’s U-value may not be considered at design stage or picked up on at as-built stage.

1. Fall protection systems posts

A fall protection system is the perfect example of something that a designer or specifier allows for, but often only in basic detail; e.g., “Fall protection system by others”.

As a result, while they might be aware that posts or base plates need to be secured through the roof system (including insulation layer) to the structural deck, any U-value calculations are unlikely to allow for the effect of the fixings.

Depending on the nature of the roof and its access needs, post/base plate fixings could be responsible for a reasonable degree of thermal bridging. And yet, the worsening of the U-value that is likely to result probably won’t be accounted for.

2. Lightning conductor equipment

Securing earthing solutions to roofs can easily be an afterthought, leading to poor finishing. There are different ways of fixing earthing straps or conductor clips to roof systems. The choice depends on the type of waterproofing and whether the system is mechanically fixed, fully adhered, or uses a combination of fixing methods.

Most screw fixings we are familiar with are made from plastic and only penetrate part of the insulation layer, so they don’t create a thermal bridge. However, care should be taken to ensure that such fixings are appropriate for the insulation installed on the roof.

If a fixing is required that does go through the whole insulation layer, it should be included in U-value calculations.

3. Concrete plinths

Concrete plinths typically support rooftop plant, but may be used for other fixtures. They are specific to the size and weight of whatever they’re supporting, and so are not a repeating thermal bridge across the whole roof area.

As a result, at design stage their effect on a U-value is calculated by thermally modelling their chi value, or point thermal transmittance.

Because the need for concrete plinths is generally known in advance, it is comparatively rare that they won’t have been allowed for in design stage U-value calculations. However, it is possible that rooftop plant requirements could change between design and construction phases, leading to extra plinths through the insulation layer that weren’t factored into calculations originally.

4. Furniture for roof terraces

Similar to rooftop plant and concrete plinths discussed in the last point, rooftop furniture and any specific fixing requirements are likely to be known about in advance. But what if the furniture specification/provision changes and additional accommodation is needed for fixing it? Has the roof U-value calculation been updated to account for the changes?

5. Pipe work (including drainage outlets)

The 2018 revision to BS 6229 featured significant focus on achieving the correct falls for roofs, and avoiding negative falls that create ponding. One of the potential remedies suggested by the standard is additional drainage outlets to help remove rainwater.

A new rainwater outlet in the field area of a roof means creating a break in the insulation layer. In the context of the total surface area of the roof, the loss of thermal performance due to one rainwater outlet is minimal – but it is still a change to the thermal envelope and a loss of performance.

Depending on the building’s performance targets for airtightness and energy efficiency, creating a new rainwater outlet could negatively affect the outcome compared to the design stage calculations. Those calculations should therefore be reviewed and new thermal modelling carried out if necessary.

About Polyfoam XPS

What other rooftop items and fixtures are often thought about only after the design stage performance has been assessed? What could we include in a follow-up article to help try and ensure that building performance as-designed matches the building performance as-built?

For project support and U-value calculations for your warm and inverted flat roof constructions, contact us.