Roofing sheets: wastage allowance

This is more a general building question.
What wastage allowance do you guys use for roof sheets? Let’s take a hip and valley roof, for example. Once we ascertain the square metres of roof sheets required using plusspec, what percentage do you suggest for wastage?
I understand that this would vary regarding the complexity of the roof.
All advice is much appreciated.

Wastage on a metal roof depends on the size of the sheet. Generally a corrugated roof profile is 760mm effective cover. so teh exact answer is if you have a roof that works to suit the effective cover you do not need to allow any waste.
There is exactly the same amount of square meters in a gable roof as there is in a hip and valley roof, however there is more work involved cutting sheets onsite. I usually allow this in my capping price as basically it is giving the lineal measure of cuts.
Also remember the steeper the pitch the more safety requirements there will be. I usually use the Tag tool for this. You can associate a cost and it is great if you are sharing your model as it will highlight the risks.

I have been messing around with Sheet sizes and it will make PlusSpec when it is done.
Working out sheet roofs and wastage for estimating.jpg

We are doing a lot with rooves and fascia and gutter estimating as well. Right down to a cutting list that works to suit standard sizes.
fascia and gutter estimating and order lengths that suit manufacturers sizing.jpg

To answer your question I would allow 5% on a normal roof, the only time wastage is higher is when you create a roof that has a “bastard” hip or valley. No I am not swearing, a bastard Hip or Valley is where a valley or hip does not have an equally apposing side. This usually happens when changing pitch on one side of the roof or the supporting structure is not at 45, 90, 135, 180, 225 or 270 degrees to the adjacent. In this case allow 10% minimum wastage. If you really want to get technical you can simply draw a rectangle over the sheet according to effective cover and the apply the texture to the group. Tis iwll give you an exact sheet size, however it is time consuming. The quickest way is to add 1 m to every sheet in the bastard hip or valley.
Bastard Hip.jpg

I hope that helps. We are doing a lot of work on teh roof tool now, I wish this was available back in the day when I was estimating roofing.

Thanks a heap for that response,

Much appreciated.

Hello timc,

I’m tired of hearing Adjusters say “You’re the only roofing company doing this crap.” Dad, I thought I was the only one who got accused of doing that “crap”.:wink: I understand your concern over the waste issue but assuming only 25% will ultimately respond positively and award you for the small (not insignificant) additional dollar amount, is it worth it? I’d simply include an explanation of waste reality with the charge on every claim with the expectation that they will pay based on the real math, then, if necessary, confront them with the real math if they refuse to pay. At an average 41% profit per job I can’t worry too much about the small difference even though, on principle, I agree with your point.

“Adjusters get upset when a supplement is sent in after the roof has been completed.” Tough, I don’t expect them to like me, however, I do expect them to respect me and the insured and pay what they owe and the majority of the time, they do just that - per my price schedule, not theirs or some “preferred” contractor who just doesn’t get it.

About what type of wastage allowance you are talking ???