Are you looking to re-roof your home or working on a new build project? Knowing how many slates you need is one of the first and most important steps. Getting it right means less waste, fewer delays and a more accurate budget. If you get it wrong, you could face material shortages mid-project or overspend on surplus stock.
In this blog, we will explain how to calculate the number of roofing slates required based on your roof type, project size, roof pitch, and allowances for waste. Whether you’re a homeowner planning your first roofing project or an experienced tradesperson looking for a refresher, this guide will help you become more confident and make informed choices.
Key Factors that Affect Slate Quantity
The number of slates you’ll need isn’t a fixed figure; it depends on several factors. Let’s break them down so it is easier to understand.
1. Slate Size
The size of slate you choose affects how many are needed per square metre. Larger slates cover more area, so fewer are required. In comparison, smaller slates offer more detail but require a greater number to be used.
For example, 600mm x 300mm slates typically require approximately 13 slates per m², and 500mm x 250mm slates require closer to 20 per m².
2. Roof Pitch
Roof pitch plays a crucial role in determining the correct headlap, which is the amount by which each slate overlaps the one below. A steeper pitch typically requires less headlap, while shallower roofs need more to prevent water ingress.
Things to consider:
- Pitchers over 40° may need a headlap of 75mm
- Pitches between 30° could require a headlap of 100mm
- Anything below 27.5° often requires specialist advice.
- Pitches are subject to location variations and conditions, and is always best to check with a specialist first.
Knowing your roof pitch helps you choose the correct headlap, which in turn determines the number of slates you need.
3. Headlap
The headlap is crucial for water tightness. It also impacts how much of each slate is exposed. Once installed, it alters the number of slates needed per square metre. For example, if two roofers are using the same size slate but different headlap sizes, one uses a 75mm headlap, and the other uses a 100mm headlap, the roofer using the larger headlap will need more slates to cover the same area.
If you’re unsure how to calculate your headlap, our team is available to assist you and provide advice and recommendations. Alternatively, you can download a slate sizing and coverage guide for a quick reference.
How to Calculate Slate Coverage
Once you’ve chosen your roof slate size and confirmed your roof pitch and headlap, you can then start calculating how many slates you’ll need. Here is the method:
- Measure the roof area in square metres (length x width).
- Check the slate coverage rate (i.e., the number of slates per square meter) for your chosen size and headlap. This information is available on our website, and if you’re still unsure, make sure to contact one of our team members and we should be able to help you.
- Multiply the coverage rate by the roof area to get your total slate requirement.
So, for example, if your roof is 100 m² and your chosen slate requires 20 per m², you’ll need 2,000 slates.
Always Allow for Waste
It’s important to order slightly more slates than the exact number needed. You might be wondering why. Well, during installation, some slates may be lost due to cuts, breakages, and offcuts around roof features like valleys and chimneys.
We recommend around 5% extra on straightforward roofs and 10% extra for more complex designs. This ensures that you won’t be caught short during installation, especially if your slate is a natural product with colour variation, as matching batches later can be challenging.
Don’t Forget Underlay, Battens and Fixings
Slates are the main roofing material, a complete slate roof also includes:
These materials also impact how the slates are laid out, so it’s important to factor them into your planning. Our team are here to advise you on what’s needed for a complete and compliant installation.
Using The Right Tools
If you’re unsure where to start or confused about how much slate you need, as the calculation is too difficult, don’t worry, you’re not alone.
At UK Slate, we’ve supported many roofing projects, and we’re always happy to advise on the right slate for your project, quantities, fixings and more.
Why Choose Natural Slate from UK Slate
Natural slate is one of the most durable and aesthetically pleasing roofing materials available on the market, and at UK Slate, we supply only the highest-quality slates from trusted quarries across the UK and Europe. Whether you’re looking for Welsh slate, Spanish Slate, or any other type of slate, we offer a wide range of options to suit every project and budget.
If you need further help with your roofing project, we’re here to help. Contact us today to discuss your project and receive advice on the number of slates you’ll need. If you’re confident in your calculations, then browse our full range of roofing slate and order online today.