The short answer
A spongy flat roof almost always means the timber deck under the covering has absorbed water and started to rot, or the sheet material has delaminated. The covering usually has to come off to repair it. A small soft patch can sometimes be cut out and replaced, while a deck that is soft across a wide area needs full replacement and a new covering on top.
What the sponginess is telling you
A sound flat roof deck feels firm, with no more give than a floor. When you press on a roof and it dips, crunches or rebounds, the boards or sheets beneath have lost their strength. Wet timber swells and softens. Plywood and chipboard sheets peel apart in layers once water gets into their edges. Once the fibres are rotting, the deck can no longer hold the fixings that keep the covering down, and the whole roof starts to move.
You may also notice a damp smell in the room below, brown stains on the ceiling, blistering of the surface, or a roof that seems to sag between the joists. None of these is an emergency in the way a flood is, but each points to moisture that has been reaching the boards for some while.
The sponginess often shows first near an outlet or a wall, because those are the places where water collects and lingers longest. Spots that feel firm on a dry day can soften after rain, so test the roof only from a ladder at the edge, never by walking across it, and note which areas worry you so the survey can start in the right place.
How the deck gets wet
Water reaches the deck in three common ways. A failed covering lets rain in through a split, a lifted seam or a hole left by an old aerial fixing. Standing water sits on the surface until something gives, which is why our page on the minimum fall for a flat roof matters here. And condensation forms inside the build-up when warm, damp air from the house meets a cold deck with no vapour control layer in between.
Edges are another weak spot. If the covering stops short at the trim or the wall junction, water wicks into the end grain of the boards and travels inwards. Rotten timber beside a gutter or a chimney upstand usually comes from that sort of slow seepage, well before the middle section softens.
Overhanging trees add to the problem, since wet leaves hold moisture against the covering and block the outlets, so water has nowhere to go. Rooflights and pipes that pass through the surface are further risks, because the flashing around them can open up and let a steady trickle reach the boards below, often for months before a stain shows indoors.
Repair or replace the deck?
We find out by lifting a small area of covering where the roof feels soft, and by inspecting the timbers from below, through a hatch or a lifted ceiling board where one exists. The aim is to learn how far the damage spreads and whether the joists are sound.
If the deck is soft in only one place, perhaps beside a leak, the boards can be cut out, replaced and the covering patched. If damage runs across the roof, a patch merely moves the weak point along, so we would suggest stripping to the joists, renewing the boards, adding insulation and a vapour layer, and laying a fresh covering. Because Part L sees fresh waterproofing as renovation of the thermal element, insulation gets reviewed in the same breath. New edges are finished with proper flat roof trims so water cannot creep back under the covering.
Photographs of the exposed boards explain the decision better than any description, and you will see them before you agree to anything. A clear picture of the damaged area, taken close up and from a distance, also helps when a neighbour, a freeholder or a lender needs to understand why the work is being done.
What it costs, in general terms
There is no useful national price for rotten decking alone, since the extent of the rot varies enormously from roof to roof. What national guides do publish is the cost of the new covering. MyBuilder's flat roof guide, updated on 21 August 2026, gives £70 to £100 per m² for bitumen felt, £90 to £130 for EPDM and £95 to £150 for GRP, while Checkatrade's October 2026 replacement guide gives £45 to £120 per m² for a flat roof, before VAT. Those figures cover coverings, not the timber under them.
Our flat roof cost page goes through what pushes a figure up or down. For your own roof, the survey settles how many boards are affected, whether any joists need strengthening, and which covering fits your budget, and our written quote is built on what we find.
Stay off it and get it looked at
Please do not keep walking across a soft roof to test it. Boards that have rotted may collapse underfoot, and dropping through into the room below is a serious injury. Our page on whether you can walk on a flat roof explains when foot traffic is acceptable, and a springy deck is not one of those situations.
Where rain is already getting through, a leak jumps the queue with us, and we stop it spreading before planning the lasting repair. A photograph of the stained ceiling helps, and you can send it by WhatsApp or the contact form, or simply ring 01793 626096. Rubbish is cleared as we finish, and the new roof is backed by a written ten-year workmanship guarantee. Anyone planning a larger job might also read about flat roof materials before the visit. More on the wider service is on our flat roofing page.