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Waterproofing the Structure

By: Lucy Debenham BA (hons) - Updated: 19 Oct 2012 | comments*Discuss
Waterproof Foundation Waterproofing

Once you’ve got your garden building in place, you’ll want to make sure that it is kept in good condition for as long as possible. This is particularly important if your garden building is constructed using timber. The most common types of timber used in the frames of garden structures are softwoods like spruce and pine, which require regular upkeep to stop them from deteriorating too quickly.

The weather is a garden building’s most relentless adversary, especially rain and damp. To keep the exterior in good shape, and the interior free of problems, a good idea is to make sure that your garden structure is as waterproof as possible. There are a few jobs that you can undertake that will help to ensure that your building is kept dry, warm and free of damp or rot for years to come.

Starting at the Top

Many timber garden structures such as sheds, summerhouses, gazebos and potting sheds can suffer from leaky roofs. Once the rain penetrates a certain area of the roof, it can cause the timbers to swell, lift, crack or pull apart. This in turn leads to a larger area of the roof leaking, spreading the problem. These timbers should be replaced or repaired before waterproofing the roof.

Your shed might only have a thin layer of traditional rag based bitumen based felt laid across it, this being the only waterproof protection for the roof. However, there are a few steps that you can take to create a more robust protective waterproof roof.

If you have a ‘cold’ roof, i.e. non-insulated, then you must have enough through ventilation in the void space of the structure. Insulated roofs should have a vapour control layer, again to avoid a build up of trapped condensation.

Consider materials that can be used for your roofing – hardwoods such as ash, beech or oak will be much more resilient against the weather and rain. Redwood is also a good option. You could also look at specially treated boards such as marine plywood that are designed to be waterproof and long-lasting. Bear in mind that you can also apply waterproof sealants with a brush or spray to timber, boards and sheets. Furthermore, an application of marine preservative or an outdoor wood preservative will also give any timber that extra bit of protection that can last up to 5 years.

Felt can be thin and break easily, so laying a breathable membrane before you put your roofing material is a good idea. A high-performance polyester felt will also be more long lasting and tough than the traditional felt used. Again, you can apply a coat of waterproof water-based emulsion or solvent-based paint. Always use waterproof mastic sealer when sealing the edges of the felting or felt shingles.

If desired, you could replace the traditional timber and felting with slates or tiles. If you wanted to do this, firstly laying a waterproof membrane under the tiles or slates would help to waterproof the structure.

Finally, the addition of guttering around the roof leading to a waterbutt would help to control the flow of rainwater away from the foundations of the structure, avoiding any pooling or small pockets of flooding that could lead to problems in future.

Down Below

The foundations should ideally have been laid with waterproofing in mind. For instance, under the timbers flooring of your garden structure, you should have material such as concrete slabs that have been laid on gravel that aids drainage, particularly if the soil in your garden easily retains water like clay.

A breathable waterproof membrane can be laid in timber buildings to stop any damp rising; in brick buildings a damp-proof course should have been laid to avoid any problems in the foundations. If your garden structure has not already been built, consider applying a waterproof spray to the top of the footings prior to the foundation wall setting.

Siting the structure on high ground or a slope will also encourage rain to drain away and avoid any persistent problematic pooling of water. This is especially important with timber buildings.

There are many products available to help waterproof your structure. Take your time to investigate which are best for you, as the colour range, materials, price and design on offer can limit your choice. Why not consult your local building supplier for more information – they will be able to advise you according to your needs (and taste!).

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