Rebuilding a Coastal Roof: From Timber Structure to a Fibreglass Roof Terrace

There are some building projects where the location changes the way almost every detail needs to be considered.

This recent Calico project is a perfect example.

Sitting immediately beside the water, the existing roof structure needed to be completely rebuilt before receiving a new GRP fibreglass roof system.

The finished roof will do far more than simply keep the structure below dry. It is being designed as a usable waterside space, with designated anti-slip walking areas, room for a table and chairs and, eventually, a beautiful log cabin above.

The idea is to create somewhere that can be enjoyed throughout the year — outside when the weather is good and inside the future cabin when a little more shelter is needed.

Starting With What Was Already There

As with many renovation projects, the first challenge was understanding and working with the existing structure.

Once the old roof was removed, it became clear that the supporting walls were not all at the same height.

That is not something that can simply be ignored when trying to create a new roof with a controlled fall and a properly supported deck.

We have therefore had to correct those differences using a combination of timber work and brickwork, gradually bringing the structure to the levels required for the new roof.

Customer-Supplied Timber — Cheaper Doesn't Always Mean Quicker

The structural beams for the project were supplied directly by the customer.

They had managed to source them for a very competitive price, which initially looked like a useful saving.

However, building materials are not only about the purchase price.

The timbers were not as straight and consistent as we would normally want for this type of work, so we have had to spend additional time preparing them before installation.

To establish a proper straight reference, we have been snapping a chalk line along one side and cutting the timber true, before using that straight edge as the reference for the opposite side.

It takes longer than working with consistently straight material from the outset, but the alternative would be allowing those variations to work their way into the roof structure.

Once large sheets are laid across multiple joists, even small inaccuracies can become much more obvious.

That is why we would rather spend the extra time getting the structure right now.

Building the Fall Into the Roof

A flat roof is not actually supposed to be perfectly flat.

Water needs somewhere to go.

Now that the main roof timbers are installed, we are adding firring pieces to create approximately a 1:40 fall across the roof.

These tapered timbers gradually change the level of the deck and encourage rainwater to move in the direction we want rather than sitting on the finished roof surface.

In this case, the drainage is being designed to direct surface water toward the slipway side of the building, where new guttering will collect and manage the runoff.

It may look like a relatively small detail, but controlling water is one of the most important parts of a flat-roof installation.

Why We're Using 18mm BS EN 636 Class 3 Plywood

The next major stage is the roof deck.

Rather than simply using the easiest sheet material we could obtain locally, we have sourced 18mm plywood to BS EN 636 Class 3 specification.

This is a higher specification than the material readily available to us from the usual local suppliers and is better suited to the environment and roof system being created here.

The lower surface of the plywood has already been treated before installation, giving us the opportunity to protect an area that will become far more difficult to access once the roof is assembled.

The sheets will then be securely fixed across the prepared roof structure to create the continuous deck required for the fibreglass system.

Why Fibreglass Makes Sense Beside the Sea

Once the plywood deck is complete, the roof will receive its GRP fibreglass covering.

For this particular project, fibreglass makes a great deal of sense.

A properly installed GRP roof creates a seamless waterproof surface without the numerous joints found in some other roofing systems.

That is especially useful in a location exposed to sea air, wind and rain.

GRP also allows us to form details around edges, upstands and other areas of the roof as part of the overall waterproof system.

But this roof has another requirement.

It needs to be usable.

Creating a Roof You Can Actually Enjoy

This isn't going to be a roof that is installed and then forgotten about.

The intention is for the space above to become part of the property.

Once the fibreglass system is installed, we plan to incorporate additional grip into designated walking areas.

That will help create more secure routes around the roof and the future cabin, rather than leaving the whole surface with the same smooth finish.

We will also form toe rails around the appropriate edges, helping define and protect the usable roof area while directing water toward the planned drainage points.

Ultimately, there should be enough space for a table and chairs outside the cabin, creating a brilliant spot from which to enjoy the view across the water.

A Log Cabin With a View

The final stage will completely change how the roof is used.

A log cabin is planned for the upper level, providing a sheltered waterside retreat.

On a summer's day, the doors can be opened and the outside roof area enjoyed with a table and chairs.

When the temperature drops or the weather turns, there will still be somewhere to sit inside, protected from the elements while enjoying exactly the same view.

It should become a great place to sit with a cup of tea or coffee and look out over the water, even on the colder days.

Considering the position of this property, that view is very much part of the project.

The Details Beneath the Finished Roof Matter

One of the things we regularly talk about at Calico is the importance of the work you eventually won't be able to see.

When this project is complete, most people will notice the fibreglass finish, the cabin and the view.

They probably won't notice:

• The different original wall heights
• The brickwork alterations
• The timber used to correct levels
• The time spent straightening supplied beams
• The firrings creating the 1:40 fall
• The treated underside of the plywood
• The planning involved in directing rainwater toward the new guttering

But those details are what allow the visible parts of the project to work properly.

More Than Simply Replacing a Roof

This project is a good example of the way renovation work can evolve.

What starts as a roof rebuild can involve structural carpentry, masonry, drainage, specialist sheet materials, fibreglass roofing and eventually the creation of an entirely new recreational space.

It is also why we prefer to look at a project as a whole rather than treating every part as an isolated trade.

The joists affect the deck.

The deck affects the fibreglass.

The fall affects the drainage.

The finished roof affects how the future cabin and outside seating area can be used.

Everything is connected.

Following the Project Through to Completion

We will continue to share more of this project as the 18mm BS EN 636 Class 3 plywood deck, GRP fibreglass roof, anti-slip areas, guttering and final cabin begin to take shape.

With a waterside location like this one, we think the finished result will be worth following.