Damp proofing a summerhouse

6 Jan 2009
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United Kingdom
Hi All,

I am planning to build a summerhouse in my garden on the foundation of my old garage which I took down at the end of last summer. The foundation itself is concrete and I would say that it is structurally solid and suitable for the project but I am concerned about damp proofing. I wouldn’t think that there would be any damp proofing under this type of foundation being a garage initially.

Am I right in thinking that I could just lay out a damp proof membrane over the foundation and construct a treated timer floor frame (50x150 joists) on top of this? If so how do I fix the frame to the floor without making a hole on the damp proof membrane?

I would be grateful for any advice and guidance with this issue.

Kind Regards,
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Do I need to do something to generate a response?

This is all very new to me.
Yes you need to type in capital letters ;) :)

Is the concrete footing from the ex-walls lower than the concrete base in the middle?
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Thanks for a response.

At the moment all I have is a level concrete base where the garage used to stand. It is also level is level with my lawn.

I will attach a photo when I get home of the area so you can see.

There's many ways of doing this, if it was me, I would use 3 course of engineer bricks (may get away with 2 courses)all round including air-vent bricks providing you know the footing is okay then dpc with 4" x 2" timber treated wallplate bedded on top of the dpc then use joists hangers for the floor joists + 18mm exterior plywood for the flooring but do the flooring after the cills is put down. Again, many ways of doing this, you could put wooden cills all round on top of the wallplate screw down and start building your summerhouse.
Hi Masona,

What do you mean by putting a cills down? Do you have any diagrams you could share?

Is this roughly what you mean with a dpc under the timber?

View media item 7677

What is wrong in just using the treated timber as the base like this?

View media item 7678
Sorry for all the questions but I am trying to understand why. Thank you for your patience.

nothing wrong with doing it as a timber floor supported off of posts or piers like a deck, as shown in the pictures. It will get plenty of air circulation underneath it
breezer, I thought the picture feature a summerhouse foundation as the guy is boarding it off as if he's going to insulate?

Thermo, Can the timber frame go straight on to the concrete as long as it's treated timber?

Cheers for the replies.
if your doing it as a timber base like in the last pictures then it needs to be set off of the floor. Your trying to avoid the timber touching the base to stop water transferring up into the timber.
if your doing it as a timber base like in the last pictures then it needs to be set off of the floor.

thats the picture i was looking at since it was the last one.

would raising it allow for ventilation?
If it was built as a normal deck type frame then there would be plenty of ventilation.

Having said that, as there is a concrete base already and its a self build jobby, id go with masonas suggestion
I think I'm starting to understand but correct me if I'm wrong. Ventilation is the key and by putting the timber frame direct on the concrete wouldn't allow any ventilation as floor would be sealed, unlike a deck which has gaps for ventilation.

I suppose I'm just reluctant to do the bricks as I am not a confident brick layer.

What other options do I have?

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