Reroofing with IKO Cambridge

Reroofing over an existing shingle roof: Some need-to-know facts

Photo: Reroofing “The Westin Turtle Bay Resort & Spa, Mauritius” with IKO Cambridge 53

You want to renovate? or
Change the look of your property? or
The lifespan of your shingle roof is nearing its end

It is probably time for an important, big decision on what to do next. The decision must be made whether to remove the old shingles or apply new shingles directly over the existing layer.

Applying new shingles directly over the existing layer is called reroofing, which is the process of recovering or replacing an existing roof covering.

Except asphalt shingles, no other roof cover for sloped roofs allows you the process of recovering, directly over the existing roof cover.
Using shingles you save time, money and manage easier and faster the renovation process.
If you look a bit further in the future using bitumen shingles for your roof reduce the impact on environment.  

Difference between reroofing and roof replacement

Roofing over an existing roof is also called reroofing or an overlay. It is the process where roofers install a new roof over your existing one, i.e. add a new layer of shingles, which means that the old roof will not be removed/teared off.

Roof replacement is the process of installing a completely new roof that includes the tear-off of the old one. Roofers will tear everything off, down to the deck of the roof, and basically start from scratch with the new roof, from putting an underlay e.g. Armourbase Pro Plus and then installing the new shingles.

When do I put new shingles over an existing roof?

When you look for a roof improvement or your roof starts to fail, but its underlying structures and wooden deck are in a generally good condition, a reroofing might be the best choice for you. It means that your shingles may be losing granules and are worn out due to age or weather, but they are not causing leaks, they are not curling or missing. These are all good conditions for installing a new layer of new shingles over the existing layer.

Reroofing can also be done when you simply want or need a change. If you are completely renovating your home and your roof needs to match the new style, adding a different design or color to your roof can completely mix up the look and feel of your home. Installing a new shingle layer will give your roof depth and even a three-dimensional appearance, as it is slightly raised due to the existing layer of shingles beneath. The best IKO shingles for this are the Cambridge Xtreme 9,5° or Cambridge Xpress.

When it comes to the financial aspect of reroofing, it is undoubtedly less expensive than a complete roof replacement. The main reason for that is that there is no need for a tear off of the existing shingles and the underlying structures. This means that there is significantly less time and labor needed and therefore the costs are lower. Your roofer can immediately start with the installation of the new roof shingles, without spending time on shingle removal and disposal. Without a tear above all you don’t impact on the environment, as this must become a significant concern for all of us!

When is it not recommended to do a reroofing?

After their roof inspection, a professional roofer will come to you with his report or a recommendation, explaining why your roof is or isn’t suitable for reroofing. You can ask for a few roof estimates from different contractors just to be sure that you are doing the right thing. That way you can choose the best roofer for yourself. One of the main and most straightforward reasons a roof isn’t suitable for reroofing is curling shingles, as you can only install a new shingles layer if the existing shingle roof is completely flat and level. A second reason can be the presence of existing damage to the wooden structure underneath, such as rot or sagging.

Step by step application of reroofing

When using IKO bitumen roof shingles in combination with the best techniques, the renovations of a worn out shingle roof becomes a mostly simple and inexpensive job. Before starting off any project, first check out whether the existing roof shingle layer could remain in place or has to be removed. Depending on local building codes at least three layers of roof shingles can be applied before stripping off the roof shingles is necessary. 

Some important tips:

  • The condition of the existing roof structure must be strong enough to support the additional load of roof shingles.
  • The overall appearance of the roof must be flat. If sagging is visible, there might be structural problems needing to be fixed.
  • Split and nail flat all buckled, raised tabs and curling roof shingles.
  • Nail warped decking material if necessary.
  • The fastening anchorage for the nails in the new roof shingle layer must be tested.
  • The existing surface has to be reasonably smooth.
  • Nail down accidentally curled roof shingle edges.
  • Replace rotten wood trim or worn out chimney and rake flashings or add eaves flashing.
  • A uniform and straight edge on the drip edge helps prevent water infiltration under the old roof shingle edges.
  • Additional ventilation should be added.

Before & after: Reroofing over existing roof with Cambridge Xpress 53:

Application requirements

Inspect the complete roof construction looking for defects before starting to reroof. The existing shingles roof should be dry before reroofing as trapped moisture can shorten the lifespan of your roof structure. Clear the roof from debris and fungus before application. Ridge and hip covering has to be removed. New hips and ridges have to be installed after the new layer has been applied. Make sure the nails are long enough to assure sufficient anchorage. To nail down new hip and ridge cap shingles even longer nails are needed. Consider exchanging old metal flashing for new ones. All existing roof penetrations should be taken out of the old layer and have to be reinstalled in the new layer.


Application techniques

There are 2 reroofing application techniques we want to highlight: bridging and nesting.

Bridging method

The bridging method allows the new course of roof shingles to overlay 2 or 3 courses of the existing roof shingles. This method usually allows for the proper exposure of the roof shingles and is commonly used when applying metric-sized roof shingles over English-sized or over wood shingles. Installation using this method may lead to “telegraphing” of the underlying material. 
Telegraphing (or telemarking) means that the old layer of old shingles can be visible in the new layer of shingles installed.


Nesting method

The nesting method usually minimizes the “telegraphing” noted above and is commonly used when applying English or metric-sized roof shingle over like-sized roof shingles. Nesting minimizes the uneven appearance of the roof when compared with bridging over existing roof shingles. Installation using this method may shorten the roof shingles exposure and uses more roof shingles.

The nesting method butts the top edge of the new roof shingle to the bottom edge of the existing roof shingle. This method sometimes is also called “Butt and run” technique.


Try fitting the new roof shingles into the existing layers prior to deciding whether to use this technique.  The existing exposure (the exposed part of the roof shingles) mainly determines what the new visual part will be.


Reroofing is a great option for those who have a generally good roof which is at the end of its life cycle or a roof that is slowly losing its function. As opposed to a roof replacement, it does not involve tear off, which means that it reduces both time and cost for the project. It is always a good idea to consult with a roofer you trust, who can do an inspection, give you an accurate estimate, and who will advise you on what the best option is for you.