What does ‘wind and watertight’ mean?

In our last article ‘learning the lingo‘ we tackled timber frame terminology, explaining the typical terms, materials, and details that self-builders would benefit from knowing. Another phrase that self-builders often query is wind and watertight. This article aims to explain exactly what it means.

Wind and watertight (sometimes also referred to as weather-tight) is a popular term used in the construction industry to describe the point in the build process where the structure is sheltered from the weather. The term itself is perhaps a little misleading because a dwelling is not completely watertight at this stage of the project.

Wind and watertight is the first holy grail moment of a construction process, regardless of the construction method. It is that first significant moment in the project where the self-builder can take stock of all that has been achieved up until that moment, breathe again, and set a path towards completing the follow on (less risky and easier to manage) non-structural activities knowing that the structure is protected from the elements.

Until that moment, the self-builder has endured the trials and tribulations of the planning process and undertaken potentially the most critical stage of the process – coming out of the ground. Not to mention the intense early-stage activities of organising insurance, funding, scaffold, machinery, contractors, suppliers, schedules, payments, and more. It’s a real milestone.

Reaching the wind and watertight stage (W&WT) quickly has a massive impact on timescales and build schedules. Activity that is undertaken to get to W&WT is carried out through a unilateral process of trades. For example, the builder starts (and is the only contractor onsite) and completes the foundations. The builder leaves and the timber frame contractor arrives to erect the dwelling (and again is the only trade onsite). The two processes depend on each other so any delays on either side can have a significant impact on overall schedules as the activity is sequenced one after the other. However, when you get to W&WT onsite, activity can become multilateral; multiple parties can start working concurrently. Although their processes do impact each other, any delays to one particular party are less likely to impact the overall process as other works can still continue.

At the point of becoming W&WT, builders, joiners, plumbers, heating engineers and electricians can all commence their process on the dwelling.

At what point does a timber frame structure become wind and watertight?

A dwelling can be termed wind and watertight when it is wrapped from the elements. In the case of timber frame construction, this means the entire structural frame will be erected, windows and external doors will be installed and the structural roof cladding with membrane will be fitted.

 

(the image above shows a dwelling at the wind and watertight stage)

From starting the structural timber frame erection process to reaching W&WT would usually take around two weeks for a typical 250m2 4 bedroom house. In contrast, a masonry home of the same size could reach the same stage in one to two months.

As the term “wind and watertight” is a little misleading (due to the fact the dwelling is not completely watertight at this stage), there can be expectation issues when the stage is reached. It is important to note that a dwelling is not completely watertight until the finished roof and external cladding has been completed.

The following information details elements of the timber frame in their final state when described as wind and watertight in the Fleming Homes model.

Timber Frame Structure

All external panels are erected, clad with an external sheathing layer with a weatherproof breather membrane. All structural materials such as internal partitions, joists, and trusses are erected.

Structural Roof Cladding

There are two options we provide. If you are installing tiles as your finished roof cladding then the likely detail set over the structural roof members would be an OSB sarking board, non-breathable roof membrane, and vertical counter battens.

If you are installing slates then the likely detail set over the structural roof members would be a 150 x 22mm sawn sarking board with a breathable roof membrane. In both details, the roof membrane is lapped over the ridge and over roof valleys. Any water that falls onto the roof runs down the roof off the eaves. There are certain spots where water ingress is likely, such as a shallow pitched roof, roof junctions, dormer panel junctions, and so on. It is also possible for water to blow up underneath the breather membrane laps in high winds.

 

 

External Joinery (Windows and External Doors)

The external joinery product is set within the structural timber frame aperture (external joinery opening) with the recommended supplied fixings. For Rationel products this entails setting the windows and external doors with cranked stainless steel battens. Other suppliers set their windows with timber ‘pinch’ battens. The window and external door products are supplied with a size tolerance so that they can be set within the structural opening and appropriately leveled and squared. The tolerance is usually 10mm to 20mm, so once the external joinery product is set within the structural frame there is a gap between the unit and the frame. This gap is closed externally by either an expanding perimeter tape or by the timber pinch battens.

               

(the images above illustrate the window and external door fitting details)

Further work is required to the timber frame structure, structural roof cladding, and external joinery to make the dwelling completely watertight. The external cladding is wrapped around the structure, the finished roof cladding is fitted to the structural roof and the external joinery products are completed relative to the applied detail instructed by the product supplier.

Wind and watertight timber frame structural packages are very popular not least due to the speed of achieving the weathertight milestone and the significant build schedule benefits. Our erection service is a great solution for getting your timber frame to the W&WT stage with the help of just one supplying company.

For more information, contact us and a member of the team would be happy to discuss our wind and watertight packages with you.