03 Jan How are manhole cover sizes measured?
How are manhole cover sizes measured?
It’s simple right? Surely you just measure the manhole you’re looking to cover and that’s the size you order.
Not quite! Here’s our Castings Services quick guide to making sure your manhole cover is the correct size.
Understanding manhole cover dimensions.
It’s counter-intuitive, but the key to measuring for a manhole cover is not to measure the existing lid itself. That’s because all manhole covers, regardless of who has manufactured them, are sold based on the size of the hole that they are covering, also known as the ‘clear opening size’. The clear opening is the internal dimension of the inspection chamber or manhole.
This actually makes a lot of sense. The inspection chamber below needs to be protected, so the cover itself needs to be larger than the inspection chamber to allow it to sit securely over the top. Not only that, but the frame of a manhole cover needs to be supported on all four sides to allow for even weight distribution, if a manhole cover and frame is too small, it may result in failure once trafficked.
Measuring the clear opening.
Whether your manhole is square or round, the same principal applies. You should measure the distance between the inside walls of the manhole. With a round manhole, this will be the internal wall to wall diameter. For rectangular manholes you’ll need both the width and the length. Dimensions should always be taken in millimetres.
Choosing your manhole cover
Once you have your clear opening dimensions, choosing a manhole cover is simple. Take our D400 ductile iron manhole cover for example. If you know your clear opening is 600mm x 600mm, our product data sheet will let you know which product corresponds to your measurements.
So, if we go back to our original question ‘how do I measure the size of a manhole cover’, you can see why it’s critical to get your clear opening dimension correct.
Replacing a cover
Although this guidance applies to sourcing new covers, the same process applies to replacing a cover that has been damaged. It’s worth considering that the manhole cover and frame work together. If one ends up damaged, there’s a good chance the other will be damaged too. For complete peace of mind, it’s worth replacing both the cover and the frame at the same time.
Throughout the last 40 years Castings Services have specialised in the supply of ductile iron products to the civil engineering sector, with galvanised mild steel coverings also available.
If you have any questions at all on ensuring you select the right product for your project, get in touch with our expert team today and source the most suitable manhole cover for your application. Talk to us on 0161 745 9095 or use our contact form.