Working out Gauge for Slating


Hi, i wonder if i could get a bit of advice from the experts on here.

Next week im undertaking a Slate job. Last time i did any slating was a good few years ago and at the time i was just leraning the trade, so im not fully experienced in slate roofing.

Now, working the gauge out is not a problem , slate length - headlap divided by 2. My question really is can this gauge be adjusted slightly to account for the roof space, for example, say my set gauge leaves me 5 inch short at the top of the roof, can i adjust it slightly to fit. If not is the best method to “cut” down the top course of slates and top half slates?

Also, im not sure if the slates will be pre holed or not, if theyre not, it souldnt be a problem. But if they do come pre-holed whats the best way of coming to the correct gauge for the laths, if the normal gauge formula leaves me miles away from the drilled holes.


Sounds like the slate are on the job site? They most likely are pre-punched they may work if not you will have to punch holes to make them work,or even install some extra lath,…you could of ordered the right size slate and had them punched to fit the existing lath layout(just needed to measure the existing slate and ordered the same size)…say a 20" slate you would layout the wood lath- 1x4(3.5") at 8.5" to 9"on center( 9"gives a little more space to adjust slate) --start with a pc. of lath centered at the top of the first full slate ( close to the same layout as slate with a 3" headlap)
… for the nailing pattern would have the slate punched at 7 3/4" to 8" down from the top edge of slate-want the nail close to the center of lath and just above the top edge of lower row of slate --do not want to double nail slate,…,to adjust layout of slate it is easier to work it out so the top of a full slate ends at the peak,but at times it may be to far off and it may compromise the proper headlap,then you just have to cut the slate to fit…shrinking the rows is the best way this gives you a little more headlap,but you can also raise them if the roof is steep enough , just have to always be aware of the proper headlap ect…you can get away with 2" headlap if the roof is 12/12 and up…Also on the lath layout and installation once you fiqure out the math you can cut spacers to fit your layout so you do not have to measure each row… but it sounds like your job is an existing roof…good luck.


to find slate exposure its minus headlap 3" is prefered divided by two. you can chaulk line accordingly and adjust slightly over several courses to hit your desired mark. lath installation is extreamly rare in new england for slate. They are usually installed over solid sheathing with 30lb felt or you find on really old repairs rosin felt. They should be prepunched.


Don’t forget to take into account the size and coverage of the ridge cap which can also be adjusted.


Thanks for the replys, now i know you can gradually reduce the gauge it shouldn`t be a problem.

Not sure on the pitch of the roof yet, but yes, ill take the headlap into consideration there


We have quite a few spaced sheathing(lath) roofs in the Pittsburgh area with slate on them…Just remember you do not have a whole lot of room when adjusting the layout on sheathing that is only 3.5"wide because you have to get the top of the slate on the sheathing plus be able to nail the next row of slate without nailing thru the lower row of slate–your sheathing may be wider,easier to adjust rows…and if you do end up cutting a short row at peak,install wood lath( 1/4"x 1.5"-2") along the peak and have the top of the short row of slate rest on it,if not that row of slate will rock-the bottoms will lift when the cap or finish row is installed


headlap should never go under 2" but is ideal at 3".
it could be 4" on a low pitch or in a round vally. Always remember a slate roof should last a century or more. It really is the worlds best roofing material.