Old House Journal - - Ohj -

Houses iden­ti­fied as Mid-cen­tury Mod­ern of­ten have flat or low-pitched roofs, which may lead to wa­ter pool­ing and drainage prob­lems. This list of roof­ing-ma­te­rial op­tions con­sid­ers their pros and cons.

BUILT-UP ROOF­ING Long the most com­mon sys­tem, and least ex­pen­sive, it uses mul­ti­ple lay­ers of as­phaltim­preg­nated roof­ing felt and mopped tar (bi­tu­men) topped with gravel for fire-re­tard­ing and UV pro­tec­tion. It pro­tects the roof against wind, snow, and pooled wa­ter, but is out of fa­vor as a pe­tro­leum prod­uct that gives off nox­ious fumes dur­ing ap­pli­ca­tion and be­yond. It can be dif­fi­cult to find a leak with this sys­tem. The black tar is not best for a hot cli­mate.

MOD­I­FIED BI­TU­MEN (MBR) roof­ing is re­lated to tar and gravel, but uses a sheet­ing ma­te­rial made of sev­eral lay­ers of poly­mer-mod­i­fied bi­tu­men. It’s roll roof­ing ap­plied with heat or liq­uid mas­tic, with a sur­face fin­ish of small rock gran­ules. The min­eral sur­fac­ing is fac­tory-ap­plied and a re­in­forced layer pro­vides bet­ter flex­i­bil­ity at low tem­per­a­tures. Durable and easy to re­pair.

RUB­BER or EPDM (ther­moset) roofs are made of re­cy­cled syn­thetic rub­ber. The mem­brane (black for UV pro­tec­tion or white for re­flec­tiv­ity) is chem­i­cal-, weather- and UVre­sis­tant. It must be in­stalled by a trained con­trac­tor, it’s some­what costly, and, while very durable, the roof­ing can be dam­aged by branches or foot traf­fic.

PVC MEMBRANES (ther­mo­plas­tic) are ap­plied in rolls, and the seams heat-welded. They are pli­able and durable with good punc­ture re­sis­tance— but PVC can­not be used with as­phalt (tar), which de­stroys the mem­brane. A sep­a­ra­tor goes over ex­ist­ing as­phalt.

SPRAY POLYURETHANE FOAM (SPF) roof sys­tems boast easy in­stal­la­tion. A liq­uid is sprayed over the cleaned, ex­ist­ing roof and ex­pands into a sur­face-con­form­ing foam. SPF de­liv­ers ther­mal, air, and mois­ture bar­ri­ers, re­sult­ing in a very high R-value per inch. With no seams, leaks are un­likely. A qual­i­fied, ex­pe­ri­enced con­trac­tor is a must, and the sys­tem is rel­a­tively ex­pen­sive.

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