First time lean-to shed build - sealing query

by Iain

Firstly, great site - I am looking to build my first shed and have gleaned a lot of useful information and guidance from here.

Secondly, my question: I am considering my first build to be a lean-to sharing a wall with the house in order to make the most of the narrow piece of land to the side of my house. How do I ensure that I will successfully seal the shed to the house and not compromise the house or shed with regards to water tightness etc. As a side note I intend to consult with my local council to fully understand any restrictions/guidelines associated with this (as has been discussed on other threads).

The shed will be a pent roof type and plan to seal the top edge against the house with overlapping flashing, my real area of unknown is the vertical joints where the walls join the house. Is it really as simple as filling the void with putty/sealant? I'm sure there is a more comprehensive method to do this and any advice would be gratefully received.

On a separate point, there will be very little space (circa 100mm) between the outer wall and the neighboring property, should I be concerned regarding a damp atmosphere in this gap? My intent is to fit guttering to this side of the roof to collect rain water running off the roof, but I imagine the air will be still a lot of the time and so may remain moist. NOTE: the wall on this side will be pre-treated with a long term coating (TBC) so will not require access to re-apply whilst I am in the property - that's the plan at least!

Sorry for the wordy post. If you need more information please ask.

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Aug 07, 2014
Sealing around junction of lean to shed
by: John


Thank you for your detailed question.

The way that many ‘lean-to’ sheds are built is that they are a complete shed in themselves in that they have their own rear wall so that the shed is structurally independent of the main structure. The flashing that is installed along the roof /wall junction is to stop the rain that runs down the side of the house getting in the void between the two structures. So the vertical junctions where the two structures meet can indeed be sealed with putty/silicone sealant as you are just trying to stop any wind driven rain getting in this junction and causing damp problems.

The above solution of two separate structures would be the ideal situation where you require the inside of the shed to be as dry and draught free as possible. Many sheds are built without the roof flashing or side sealant when items such as garden tools and bikes that are fairly resilient are to be stored.

With this sort of situation you may have to cope with the occasional leak or retrofit some weather proofing as you discover the direction of the prevailing winds that blow rain into any weak spots.

With regard to the gap between your neighbours house and the side of the shed I think that it is something that he should be concerned about. Not so much due to general dampness in the air but what happens when there is very heavy rain and the gutter overflows, or the gutter gets blocked. At this point you will be discharging water on to his wall. Definitely something worth considering and discussing with him.

Best of luck with your project and please let me know how your project progresses.

All the best


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