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Opening doors and windows will help to circulate air and combat damp in sheds.

Part 1 – Shed Damp

To understand how to damp-proof a wooden shed, first, we have to understand what damp is. Let’s get a definition of damp down, then we’ll move on to protecting your shed against it.

Damp is the presence of moisture, mildew, and mould in buildings. Damp can occur in sheds due to external (e.g. rain) or internal (e.g. condensation) factors. If left untreated it can damage your wooden shed and cause health problems.

When you don’t air out your shed, it invites moisture build-up. This, in turn, creates mould spores.

 

Mould is a fungus and can cause ‘black mould’ in cellulose-rich building materials (i.e. wood). 

 

open shed with man inside putting upturned bike back on shelving
Make sure to wipe damp bikes before putting them back in storage.

One of the main causes of damp in garden sheds is interior condensation. 

 

Factors that can affect dampness in a shed are:

 

  • Single-glazed windows - With direct contact to the outside air
  • Rising damp - From water absorbed by groundwork in a concrete base
  • Leaks - From guttering and/or roofing
  • Treated air - Either heated or air-conditioned inside air that is different to the outside temperature 

 

We’ll explain in parts 2 and 3 how to insulate and damp-proof your shed to combat these problems. Just know that dampness in a wooden shed can be made worse in winter. This is due to colder air outside as well as less regular aeration - with windows closed against the cold.

Aside from being potentially dangerous to your health, damp is a problem for outdoor garden buildings. 

 

Wood, or timber, is one of the most susceptible materials to damage by moist conditions. These conditions can also be made worse by bouts of wet weather and poor airflow in sheds.

 

The results of damp in a wooden shed can be:

 

  • Premature rot
  • Mould on walls and roof
  • Warping and sagging of timber

 

These results are not just bad for wooden garden sheds, they’re potentially dangerous too. Rot and sagging timber can make your outdoor shed structurally unsound!

 

Picture of a wet puppy on a sandy beach with the side out looking up to its owner
Avoid letting damp in your shed - no matter how tempting! Credit: @allaboutorla/instagram

 

It’s also smart to avoid storing damp items in your shed. This includes everything from rags to tools and bikes. Yep, even bikes! If you do have to store things in your outbuilding, make sure to dry them off before they go back in the shed. 

 

If you’ve bought a wooden shed with pressure-treated wood for extra protection, you shouldn’t need to treat the inside.

 

Although, if you want to prevent mould in your shed:

 

    • Keep it ventilated - Open doors and windows or install vents. If you regulate your shed’s temperature, moisture will be allowed to diffuse from high to low concentration (i.e. inside to out).
      Insulate it - Maintain your shed’s interior temp. And provide a layer to stop condensation from hitting surfaces
    • Seal gaps - Caulk your shed’s base. Use a draught excluder. Spray windows with expanding foam.
    • Guttering - Install adequate guttering for run-off and prevent moisture leaking in through the roof
    • Membrane - Think about installing a vapour membrane between your shed’s framing 
    • Get a hygrometer and dehumidifier - A hygrometer measures humidity and a dehumidifier will treat air and remove excess moisture.

 

Make sure you have an available plug socket for the last two items or buy battery-operated versions.

 

For suggestions on dehumidifiers, check out these reviews of the shed treatment and pesticide to the outside timber of your shed.

  • Leave the building to air.
  • Respray affected areas with a diluted vinegar spray. You can add a fragrance to combat the smell of vinegar.
  • (Optional) Leave open containers of baking soda or a dehumidifier in your shed.
  •  

    Glass window with condensation and water steaking down it
    Condensation and single-glazing can cause real problems for timber sheds.
    Cross profile of pink fibreglass insulation batts in a wall cavity
    Insulation can help regulate the temperature inside your shed.

    PIR insulation boards

    PIR is a type of rigid insulation board that’s popular for walls and even flooring. It’s also fibre free!

    PIR comes in foil-backed boards of varying thicknesses. Its popularity means that you should be able to fit suitable sizes to fit your wall cavity. Simply cut boards to size and slide them in.

    Wool

    If you want the insulation offered by fibreglass without the itchiness - simply go for thermal wool. This is also a more environmentally-friendly solution.

    Plus, wool offers stellar insulation and is safe to handle.

    You can also consider changing, or adding additional cladding to better insulate your shed.

    Plasterboard

    Plasterboard will offer effective wall covering. The problem is, it’s quite flimsy and won’t offer that much resistance against water and damp.

    Hardboard

    Hardboard sheets are a step up from plasterboard in terms of strength. But they still won’t offer much in the way of water resistance.

    Plywood

    The good thing about plywood is that it comes in varying thicknesses. Plywood will prove sufficient for wall-boarding. Plus, it’s tough and durable.

    Tongue and groove

    Tongue and groove wooden sheds equal a robust build with a sleek design. It’s also a lot easier to paint and treat timber due to there being no overlap. A tongue and groove design also means that sheds are weather-resistant and well-insulated.

    Pallet board

    By de-nailing and sanding down old pallet boards, you can clad the outside of your shed and be on-trend. Re-purposing and pallet boards are all the rage at the moment. Simply stain and treat the boards and affix them to your shed’s vertical struts.

    metal shed built on paving stones with a step ladder and a pallet with coffee mug and drill on top
    Caulking your base and bolt holes is essential for metal sheds built on concrete bases.

    Most sheds are built with waterproof roofing materials. However, even felt and bitumen shingles can become water-damaged over time.

    For a metal shed, you might even consider anti-condensation roofing panels.

    Making sure that you seal around joins in your wooden shed and that your felt is fitted accurately will provide you with the best roofing. For an upgrade from felt shingles, you could also consider using weather-resistant EPDM.

    This is a synthetic rubber for shed roofs that provides waterproofing. You can find EPDM as an optional add-on for many of our wooden sheds.

    timber shed interior with walls and roof visible

    Check your shed walls and roof for any signs of leaks, damps, and holes.

    Part 3 – Damp-proofing a Shed

    By this stage in our ultimate guide, we’ve run through what damp is and how to insulate your shed. Now, let’s move on to damp-proofing your wooden shed.

    To damp proof a wooden shed, open doors and windows or install shed vents to circulate air. Reduce moisture levels in your garden shed by circulating air to combat damp. You can also use a dehumidifier in your outdoor shed to stop moisture buildup which causes damp.


    So if you’re asking yourself, ‘how do you damp proof an outbuilding?’, there are three main areas you need to consider.

    Roof

    To damp proof your shed, you need to protect it from moisture up high as well as down low.

    Check your roof for any gaps, holes, and leaks. Are these being caused by insufficient drainage? If so, you might want to think about upgrading your shed guttering system

    A guttering and water butt system can ensure that runoff is further directed away from your shed’s base. This, in turn, can combat water pooling and even rising damp. 

    apex shed roof with white vents installed to shed eaves
    Installing vents will help with airflow and heat regulation.

    To ventilate a small wooden shed, you’ll need some form of passive ventilation. This can be achieved by installing vents in your shed’s eaves or even just drilling some holes. Make sure to do this at both ends of your shed in the direction of steady airflow.

    Circulating airflow in your shed means that moisture-rich air won’t hang around. It can also help to regulate your shed’s indoor temperature versus outdoors.

    Think about what you need to use your shed for.

    If you’re utilising your garden building as a wooden office or workshop, ventilation is key. You don’t want tools and even electronics suffering from damp conditions. Installing a whirligig to sit on your shed’s roof, or an electric fan, if you have utilities hooked up, can help to regulate air.

    To read more about installing utilities in your wooden shed, check out this helpful guide. 

    Building a concrete shed base for your shed floor can lead to issues like rising damp. This is because concrete is porous and can absorb moisture from the surrounding ground. 

    Rising damp is when this water from the ground is absorbed through floor bearers into your shed’s floor.

    To build a damp-proof concrete shed base - make sure to put a damp-proof membrane under your shed. Also allow your base to dry sufficiently, and don’t build it much larger than your shed’s actual base.

    On top of this, grid system foundation base. This provides ground clearance and can be filled with gravel to allow for drainage. 

    Then, you can use laminate, plywood, vinyl, or even carpet to keep any chill off your wooden shed base.

    wall filled with various tools hung from it including axes and spanners
    Try to store tools in boxes and drawers with silica packets to avoid corrosion.

    Part 4 – Vapour Membranes for Damp-proofing a Shed

    One of the most common questions we hear when talking about damp proofing a wooden shed is – do I need a membrane?

    So, to finish off our guide on all things sheds and damp, we’re going to cover shed membranes once and for all!

    If you’ve read this guide then (firstly, well done!) you should have figured out how to damp proof a shed floor. We’ve also covered insulating and damp-proofing your shed’s roof

     

    But when do you need a membrane? Let's start small:

     

    What is a shed membrane?

     

    A breather membrane is a plastic sheet layer that goes between your shed’s walls and insulation to help with moisture. Shop Garden Sheds

    FAQs

    Treating the timber of your wooden shed can provide it with a weather-resistant exterior. Further insulation and vapour membranes can be used in a wooden shed’s walls to prevent moisture ingress. Any gaps around your shed’s windows, roof, and door should be sealed.

    To keep a shed-damp free make sure to store tools in tightly-sealed containers with silica packets. Don’t store damp items or open liquid containers in your storage shed. Make sure to ventilate or regularly aerate your shed to keep it damp-free.

    For concrete shed bases and metal sheds, sealing the base is essential. You might also decide to seal the base of your timber shed, though. Work around the outside first, following the line of your shed’s base with a silicone gun. Repeat on the inside. 

     

    Then, make sure to use a draught excluder for gaps that you can’t seal, like the base of your shed’s door.