Roof Deck Construction Cost Philippines: Loads, Waterproofing, Permit and Price

A roof deck is a floor that happens to be outside, and the structure under it has to be built like one. Illustrative photo.

Short answer: a roof deck is not a roof. It's a concrete floor with people, tiles, plants and rainwater on it, and it can weigh several times what a GI roof weighs. On a new house designed for it from the start, it's a fairly ordinary slab. On an existing house, the first question isn't the price. It's whether the columns, beams and footings underneath were ever sized for a floor.

The checker below shows how much a deck adds per column compared with the light roof it replaces, and the minimum slab thickness NSCP 2015 allows for your bay. The cost calculator further down prices the slab, waterproofing, parapet, drains, stair and any strengthening.

What Load Does a Roof Deck Have to Carry?

NSCP 2015 treats the two cases differently, and this is where most "can I just pour a slab on top?" plans go wrong.

• A roof you only walk on for maintenance is a roof. Its live load comes from Table 205-3. For a flat roof (note 3 defines flat as a slope under 1 in 48, or 2%), Method 1 gives 1.00 kPa for a tributary area up to 20 sqm, 0.75 kPa from 20 to 60 sqm, and 0.60 kPa over 60 sqm.
• A roof deck you use is not. Table 205-1, item 18 lists roof decks as "same as area served or occupancy". In other words, it takes the load of whatever you'll do up there.

For a house, Table 205-1 item 15 (Residential) gives the numbers that usually apply:

Note 3 of Table 205-1 says assembly areas include "terraces and similar occupancies that are generally accessible to the public". A family roof deck isn't public, so 1.9 kPa is the code minimum for most houses. But Section 205.1 says live loads "shall be the maximum loads expected by the intended use", so if you're planning birthday parties for fifty people, tell your engineer, because that's closer to the assembly case. Note 4 on the deck and balcony rows also points to Section 205.4.4: roofs used for special purposes are designed for appropriate loads as approved by the building official.

The dead load is the bigger number

People worry about the live load. The slab and what sits on it weigh more. From NSCP 2015 Tables 204-1 and 204-2:

• Reinforced concrete: 23.6 kN/m³, so a 125 mm slab weighs 2.95 kPa on its own.
• Screed to fall: stone concrete fill at 0.023 kPa per mm. A screed averaging 40 mm adds 0.92 kPa.
• Ceramic or quarry tile (20 mm) on a 25 mm mortar bed: 1.10 kPa.
• Liquid-applied waterproofing: 0.05 kPa, the one layer that barely counts.
• Garden soil: damp clay is 17.3 kN/m³ in Table 204-1, so a 300 mm planter bed is about 5.19 kPa. That's more than two and a half times the residential deck live load, sitting there permanently.

Add a 150 mm CHB parapet as well: Table 204-2 gives 1.15 kPa for unplastered, ungrouted 150 mm hollow units of 16.5 kN/m³ density, plus 0.24 kPa for each plastered face. A plastered, 1.0 m high parapet is about 1.63 kN on every metre of beam around the edge. Our NSCP dead and live loads guide explains how these combine into design loads.

Can Your Existing House Carry a Roof Deck?

Run the checker's default case: a 3.5 m by 4.0 m column bay, a 125 mm slab, tiled finish and a 1.9 kPa family deck. The deck works out to 6.92 kPa of service load, against about 0.85 kPa for the GI roof it replaces. That's 8.1 times more, or roughly 96.9 kN arriving on an interior column instead of 11.9 kN. Factored (1.2D + 1.6L), it's 126.9 kN. The GI roof figure assumes a pitch between 1 in 3 and 1 in 1, which Table 205-3 gives 0.75 kPa of roof live load. A flatter GI roof takes 1.00 kPa, so the old load is 1.10 kPa (15.4 kN on the column) and the deck is about 6.3 times heavier instead. Either way, it's a big jump.

A house built with a GI roof on a ring beam was never designed for that, and the problems stack up in the same order as a second-floor addition:

• The footings. They were sized for a light roof, often on assumed soil. Multiplying the column load is the check that fails first, and it's the hardest to fix under a standing house.
• The roof beams. A ring beam that held trusses isn't a floor beam. Once a slab spans onto it, it's usually too shallow, and new or deeper beams follow.
• The columns and the bars in them. If the vertical bars stop at the roof beam and nothing was left for a future slab, every connection has to be drilled and anchored to a design.
• Seismic weight. A heavy slab sits at the top of the building, which is exactly where added mass does the most harm in an earthquake.

None of this means "no". It means an engineer has to establish the real column sizes, rebar and footings before anyone prices the slab. Our structural damage warning signs guide covers what existing cracks can tell you before you add anything.

How Thick Should the Slab Be?

Most roof deck slabs on houses are two-way slabs supported by beams on all four sides. For that case, NSCP 2015 Section 408.3.1.2 and Table 408.3.1.2 set the minimum thickness. Where the beams are stiff (αfm greater than 2.0), it's the larger of ℓn(0.8 + fy/1400) ÷ (36 + 9β) and 90 mm, where ℓn is the clear span in the long direction and β is the long-to-short clear span ratio. Where the beams are more flexible (αfm from 0.2 to 2.0), the floor rises to 125 mm. At a discontinuous edge without an edge beam of αf at least 0.80, Section 408.3.1.2.1 adds at least 10%.

For the checker's 3.5 m × 4.0 m bay with 250 mm beams and Grade 60 bars, the formula gives 88.6 mm, so the 90 mm floor governs. That's a deflection minimum, not a design. Crack control matters more on a deck than inside a house, because a crack in the slab is a water path to the steel. Concrete cover, the bar layout, the fall to the drains and anything heavy you plan to put up there all push the real figure up. If your bay is more than twice as long as it is wide, the slab works one way; check it with our one-way slab thickness calculator instead. Square-ish bays can go through the full two-way slab thickness calculator.

Waterproofing and Slope to Drain

A roof deck leaks at the edges and at the drains, rarely in the middle. Three rules decide whether water leaves the slab.

• Drains at the low points, plus overflow. PD 1096 Section 1206(d) requires roof drains "at low points of the roof" adequate to discharge all tributary water, and where roof drains are required, adequate overflow drains (the paragraph is headed "Overflow Drains and Scuppers"). NSCP 2015 Section 210.1 adds that the flow capacity of the overflow drains or scuppers must not be less than the primary drains. Section 210.2 designs each part of the roof for the water that builds up to the overflow inlet if the primary drain is blocked, plus the extra depth that flows over that inlet. A single floor drain with no scupper through the parapet is the classic failure.
• Enough fall. NSCP 2015 Section 210.3 requires a ponding instability check for roofs with a slope less than 6 mm per 300 mm, which is 2%. A deck laid dead flat collects water in every low spot, and water weighs 9.8 kN/m³ (Table 204-1). Getting the fall into the slab or the screed is far cheaper than chasing puddles later.
• Flashing at every upstand. Section 1206(e) of PD 1096 requires flashing and counterflashing where the roof meets vertical surfaces. On a deck, that's the parapet base, the stair bulkhead wall, and every pipe and post that goes through the waterproofing.

For the coating itself, our waterproofing cost guide prices a roof deck at ₱620 to ₱840 per sqm with a two-part cementitious system, higher than a plain flat slab because of the extra detailing around drains, parapets and joints. On a deck people walk on, a third coat or a protective topcoat adds roughly 25 to 35% more.

Parapet and Railing Height

This is the part where the codes say less than people assume, so here is exactly what they say.

• PD 1096 Section 1209 (repeated in the Revised IRR) says railings "shall be provided for balconies, landings, or porches which are more than 750 millimeters above grade." A roof deck is well above that, so it needs a railing or parapet. The section doesn't give a height.
• The Fire Code IRR (2019), Section 10.2.5.4 paragraph H.5, sets guards at not less than 1.06 m high, and requires the space below to be filled with vertical balusters no more than 152.5 mm apart, intermediate rails, or equivalent mesh or grilles. But Section 10.2.14.6 says that for detached single and two-family dwellings "the only requirements applicable are those in this Section", and that Section brings in the stair width, riser and tread rules, not guards.

So for a house, the code requires a railing but no specific height. We design house roof deck guards to at least 1.06 m anyway, measured from the finished deck, with gaps no wider than 152.5 mm. That's our recommendation, not a legal minimum for dwellings, and it's the figure a building official is likely to recognise. The structural side is in NSCP 2015 Table 205-2, item 8: a balcony railing or guardrail other than an exit facility takes 0.30 kN/m horizontally at the top rail, infill components take 1.2 kPa (note 8), and handrails must resist 890 N in any direction (note 10). A tall CHB parapet also needs vertical bars and a proper tie to the slab, because it's a cantilever wall in an earthquake.

One more parapet detail: the IRR defines building height to the top of the parapet where there is one, so a tall parapet counts toward any height limit on your lot.

Stair Access and Roof Structures

A deck you can only reach by ladder won't get used. A proper stair has to meet PD 1096 Section 1207(e): rise no more than 200 mm, run at least 250 mm, headroom of at least 2.00 m, and handrails 800 to 900 mm above the tread nosing. For dwellings, Section 10.2.14.6 of the Fire Code IRR also applies its own width, riser and tread rows, which are stricter: 180 mm maximum riser and 280 mm minimum tread for new stairs. Our stair dimensions guide works out the steps for your floor height.

The roofed box over the top of the stair is a "penthouse" in PD 1096 terms. Section 1210 allows penthouses and roof structures only to shelter mechanical equipment or vertical shaft openings such as the stair, caps their combined area at one third of the supporting roof, and says a penthouse used for anything else "shall conform to the requirements of this Code for an additional storey." Turn that stair housing into a bar or bedroom and you've built a third floor, with everything that comes with it. On the plus side, the IRR's building bulk guidelines exclude uncovered roof decks from gross floor area (GFA), although they still count toward total gross floor area (TGFA).

Will a Roof Deck Make the House Hotter?

It changes the heat pattern. A GI roof gets hot fast and cools fast, and a ventilated ceiling space under it can carry some of that heat away. A concrete slab soaks up sun all afternoon and gives the heat back into the rooms below in the evening, right when people want to sleep. The fixes are the same ones we rank in why is my house so hot, applied to a slab:

• A light, reflective top surface. A pale coating or light tiles absorb less of the sun than bare grey concrete.
• Shade over part of the deck. Keep it open and light. A roofed shelter is a roof structure, so check it against Section 1210 with your designer before building it.
• Insulation under the slab. An insulated ceiling below keeps the stored heat out of the rooms.

If the main reason for the deck is a cooler house, it's usually the wrong project. A deck is for using the space; roof insulation and ventilation are the cheaper cure for heat.

Do You Need a Permit?

Yes. Section 301 of PD 1096 says no one shall "erect, construct, alter, repair, move, convert or demolish any building or structure" without a building permit. The Revised IRR's exemptions only cover repair works "not affecting or involving any structural member", and a concrete roof deck is a new structural floor. Expect to submit structural plans and computations signed and sealed by a licensed civil engineer, plus the ancillary permits for any drains, lights or outlets on the deck. Our building permit fee calculator estimates the fees for your area; the calculator below only carries an allowance.

What a Roof Deck Costs per Sqm

The spread is wide because two very different projects both get called "a roof deck".

Worked example 1, a house built to take it (from the calculator): 40 sqm of deck, coating finish, 26 m of CHB parapet and two drains, no stair or strengthening. That comes to ₱288,208 to ₱466,656, or about ₱7,205 to ₱11,666 per sqm. The slab is ₱140,000-220,000, screed ₱14,000-24,000, waterproofing ₱24,800-33,600, parapet ₱72,800-109,200, drains ₱6,000-12,000, design and plans ₱20,608-47,856, the permit allowance ₱5,000-15,000 and the assessment ₱5,000.

Worked example 2, converting a GI roof on a house that needs work: the same 40 sqm and parapet, plus stripping the old roof, a tile finish, a new steel stair, 16 m of new beams and four jacketed columns. That comes to ₱471,808 to ₱838,496, about ₱11,795 to ₱20,962 per sqm. Add widened footings and it climbs again, by ₱15,000-45,000 for each one before design fees.

Two things aren't in either figure: the house's existing condition (cracks, corroded bars or a leaking old slab all get fixed first) and the cost of doing without the room under the deck while the slab is shored and curing. Our roofing cost guide puts a complete metal roof at ₱1,200-2,600 per sqm, which is a useful comparison if you're choosing between re-roofing and a deck.

Frequently Asked Questions