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Waterproofing a Store Room Beneath a Patio on Holtspur Top Lane, Beaconsfield HP9
A store room beneath a patio on Holtspur Top Lane in Beaconsfield was letting water in through its ceiling, even though the patio had been laid over a membrane. The owner wanted the stored goods protected without lifting the paving.
Project at a Glance
The Brief
The store room sits underneath a terrace that had been membraned before the flagstones were laid, but water was still finding its way through. The owner asked for the room to be assessed and for a cost effective way to keep the stored goods dry.
The Survey
The patio acts as both a walking surface and the roof of the store room. The survey found that the waterproofing layer beneath the patio, installed by others, had failed, so water was getting past it and becoming trapped on the structural deck.
The deck is a block and beam floor: concrete beams with infill blocks. Unlike a solid concrete slab it is full of joints, so once water passes the patio membrane it travels across the deck until it finds a joint and drips into the room below.
The Solution
Lifting the paving to replace the patio waterproofing would have been a large and costly job, so the report proposed an internal approach instead:
- A studded cavity drain membrane fixed to the underside of the block and beam floor and to the walls, acting as an umbrella that catches water coming through the joints.
- Collected water diverted to the edges of the room rather than dripping onto stored goods.
The report was clear that this is a remedial water management measure rather than a full system to BS 8102. A fully compliant Type C system would collect water in a maintained perimeter channel and remove it with a sump pump or gravity drain. The owner chose the budget approach knowing the floor itself could still become damp. For below ground spaces that need to be fully dry, see our cavity drain membrane page.
The Work
The cavity membrane system was fixed to the specified walls and the soffit, with plugs, seals and joint tapes, and the work was completed in April 2026. A written guarantee covers the membrane installation.
The Result
Water coming through the ceiling joints is now caught by the membrane and kept off the stored goods, without any disruption to the patio above. See our page on damp proofing in Beaconsfield.
Frequently Asked Questions
Why does water leak through a patio into the room below?
When the waterproofing under a patio fails, water gets trapped on the deck below and finds its way through any joints, such as those in a block and beam floor.
Can a leaking patio roof be fixed without lifting the paving?
Yes. A cavity membrane fixed to the ceiling and walls inside can catch and divert the water, although it manages the water rather than stopping it at the surface.
Is a membrane on its own a full waterproofing system?
Not to BS 8102. A full Type C system also needs a perimeter channel and a sump pump or drain so water is removed in a controlled way.
Water getting into a basement or store room?
Our CSSW and CSRT trained surveyors find the real cause before recommending any work. Non-invasive survey with a clear written report. No obligation.
Survey carried out by Michael Young at Advanced Damp. Project completed in Beaconsfield, HP9.
Sam Fitzgerald is the founder of Advanced Damp and a working damp and structural waterproofing surveyor diagnosing and treating moisture problems in residential and commercial properties across London and the South East. He is FODCS certified (a Federation of Damp Certified Surveyor) and a member of the Federation of Damp. His hands-on work covers rising damp, penetrating damp, condensation, basement waterproofing to BS 8102, and thermal imaging surveys, and he has personally completed over 400 damp surveys across properties of every type and age. Everything he writes draws directly from jobs he has carried out. He does not write about treatments, products or causes he has not dealt with first-hand in the field.
