Measuring the roof for metal panels

One of the most common problems in quoting a job accurately stems from measuring the roof for metal panels for re-roofing projects and then try to get the best results from their estimating software systems. Certainly, there are major variations in the way people do things for sure, so please don’t take the comments below personally.

The most common issue, especially when measuring a re-roof for metal panels and trim, is that the measurer measures for finished panel or trim size instead of measuring the roof.  

For example, they might measure the ridge length and then add a random amount because they want to be sure they have enough material when they order it. However, that is not the correct dimension for the length of the ridge geometry.

If your entire estimating, costing, and ordering process is a manual one, then this may be the best way, even if it is potentially very wasteful. However, if you employ estimating software that models the roof geometry in 3D, then that software doesn’t want panel sizes; it wants the actual roof measurements. In the simplest form, all the software needs are the perimeter measurements and the roof pitch to construct accurate roof geometry.

The reason is that software that does the job properly takes account of fabrication and installation allowances. That is to say, the overhang into the gutter, the set-back from ridges, hips, and valleys etc and on professional software, these allowances can be set up to suit different panel types and installation methods. So, we want to model the roof geometry ‘as built’ and then allow the software to take these into account.  

This is important because different panel types may require different allowances – a standing seam panel will most likely require a hem at the bottom, so this extra amount will be included in the allowance for the overhang into the gutter where a corrugated panel usually just hangs into the gutter.  

While the amounts for these adjustments may be small, they must be accounted for as you cannot stretch the panel if too short and cutting a couple of inches off every panel is time-consuming and wasteful if too long.

The other reason, and perhaps the most important reason for using actual roof measurements, is that the checking tools of the modelling software allow us the opportunity to check the measurements, verify the pitch etc. Once we know that the 3D model is correct, then we can count on the material and labour take-off also being correct. The software applies the fabrication and installation allowances for the selected roof system, so we know that the panel and trim cutting list will also be perfect for that job.  

We cannot guarantee that the materials are put onto the truck, but we can surely guarantee that the list is correct.

As can be seen in the illustration here, the geometry of the roof is NOT the same as the geometry of the panel and is potentially even more complicated if you consider the offset from the roof plane by the depth of purlin or battens, if installed (not usual on residential, but common on commercial structures).

So when you’re measuring your roof, be aware of how your information is going to be used. Our suggestion is that you always measure the roof geometry, then add or subtract from the required panel length based on the roof geometry. This is particularly relevant if you are using professional software where you can define the required offsets for different panel systems – for roofs or walls.

If you are going to save the installer a lot of work on-site, then it is relevant on jobs where there are hip/valley connections like the roof model example here.

It becomes immediately obvious when we try to model the roof in 3D with the dimensions provided – often, the roof simply doesn’t work, or the roof pitch is incorrect for the dimensions provided.

So, in my opinion, we should be very careful when we use the measurements provided by others when they apparently provide details of the “roof measurements”. They may not be roof but panel and trim measurements, and if we add further allowances, then the panel and trim cut list will be incorrect and expensive waste will be increased.  

Professional software will create the roof model from perimeter measurements and a pitch; then we add the fabrication and installation allowances.  

When you use someone else’s measurements, they become your measurements; right or wrong, they are now yours! Always ask the question – “What did you measure?”.